variable command

Syntax

variable name style args ...
  • name = name of variable to define

  • style = delete or index or loop or world or universe or uloop or string or format or getenv or file or atomfile or python or internal or equal or vector or atom

    delete = no args
    index args = one or more strings
    loop args = N
      N = integer size of loop, loop from 1 to N inclusive
    loop args = N pad
      N = integer size of loop, loop from 1 to N inclusive
      pad = all values will be same length, e.g. 001, 002, ..., 100
    loop args = N1 N2
      N1,N2 = loop from N1 to N2 inclusive
    loop args = N1 N2 pad
      N1,N2 = loop from N1 to N2 inclusive
      pad = all values will be same length, e.g. 050, 051, ..., 100
    world args = one string for each partition of processors
    universe args = one or more strings
    uloop args = N
      N = integer size of loop
    uloop args = N pad
      N = integer size of loop
      pad = all values will be same length, e.g. 001, 002, ..., 100
    string arg = one string
    format args = vname fstr
      vname = name of equal-style variable to evaluate
      fstr = C-style format string
    getenv arg = one string
    file arg = filename
    atomfile arg = filename
    python arg = function
    internal arg = numeric value
    equal or vector or atom args = one formula containing numbers, thermo keywords, math operations, group functions, atom values and vectors, compute/fix/variable references
      numbers = 0.0, 100, -5.4, 2.8e-4, etc
      constants = PI, version, on, off, true, false, yes, no
      thermo keywords = vol, ke, press, etc from thermo_style
      math operators = (), -x, x+y, x-y, x*y, x/y, x^y, x%y,
                       x == y, x != y, x < y, x <= y, x > y, x >= y, x && y, x || y, x |^ y, !x
      math functions = sqrt(x), exp(x), ln(x), log(x), abs(x),
                       sin(x), cos(x), tan(x), asin(x), acos(x), atan(x), atan2(y,x),
                       random(x,y,z), normal(x,y,z), ceil(x), floor(x), round(x)
                       ramp(x,y), stagger(x,y), logfreq(x,y,z), logfreq2(x,y,z),
                       stride(x,y,z), stride2(x,y,z,a,b,c),
                       vdisplace(x,y), swiggle(x,y,z), cwiggle(x,y,z)
      group functions = count(group), mass(group), charge(group),
                        xcm(group,dim), vcm(group,dim), fcm(group,dim),
                        bound(group,dir), gyration(group), ke(group),
                        angmom(group,dim), torque(group,dim),
                        inertia(group,dimdim), omega(group,dim)
      region functions = count(group,region), mass(group,region), charge(group,region),
                        xcm(group,dim,region), vcm(group,dim,region), fcm(group,dim,region),
                        bound(group,dir,region), gyration(group,region), ke(group,reigon),
                        angmom(group,dim,region), torque(group,dim,region),
                        inertia(group,dimdim,region), omega(group,dim,region)
      special functions = sum(x), min(x), max(x), ave(x), trap(x), slope(x), gmask(x), rmask(x), grmask(x,y), next(x)
      feature functions = is_active(category,feature,exact), is_defined(category,id,exact)
      atom value = id[i], mass[i], type[i], mol[i], x[i], y[i], z[i], vx[i], vy[i], vz[i], fx[i], fy[i], fz[i], q[i]
      atom vector = id, mass, type, mol, x, y, z, vx, vy, vz, fx, fy, fz, q
      compute references = c_ID, c_ID[i], c_ID[i][j], C_ID, C_ID[i]
      fix references = f_ID, f_ID[i], f_ID[i][j], F_ID, F_ID[i]
      variable references = v_name, v_name[i]
    

Examples

variable x index run1 run2 run3 run4 run5 run6 run7 run8
variable LoopVar loop $n
variable beta equal temp/3.0
variable b1 equal x[234]+0.5*vol
variable b1 equal "x[234] + 0.5*vol"
variable b equal xcm(mol1,x)/2.0
variable b equal c_myTemp
variable b atom x*y/vol
variable foo string myfile
variable foo internal 3.5
variable myPy python increase
variable f file values.txt
variable temp world 300.0 310.0 320.0 ${Tfinal}
variable x universe 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
variable x uloop 15 pad
variable str format x %.6g
variable x delete

Description

This command assigns one or more strings to a variable name for evaluation later in the input script or during a simulation.

Variables can thus be useful in several contexts. A variable can be defined and then referenced elsewhere in an input script to become part of a new input command. For variable styles that store multiple strings, the next command can be used to increment which string is assigned to the variable. Variables of style equal store a formula which when evaluated produces a single numeric value which can be output either directly (see the print, fix print, and run every commands) or as part of thermodynamic output (see the thermo_style command), or used as input to an averaging fix (see the fix ave/time command). Variables of style vector store a formula which produces a vector of such values which can be used as input to various averaging fixes, or elements of which can be part of thermodynamic output. Variables of style atom store a formula which when evaluated produces one numeric value per atom which can be output to a dump file (see the dump custom command) or used as input to an averaging fix (see the fix ave/chunk and fix ave/atom commands). Variables of style atomfile can be used anywhere in an input script that atom-style variables are used; they get their per-atom values from a file rather than from a formula. Variables of style python can be hooked to Python functions using code you provide, so that the variable gets its value from the evaluation of the Python code. Variables of style internal are used by a few commands which set their value directly.

Note

As discussed on the Commands parse doc page, an input script can use “immediate” variables, specified as $(formula) with parenthesis, where the formula has the same syntax as equal-style variables described on this page. This is a convenient way to evaluate a formula immediately without using the variable command to define a named variable and then evaluate that variable. See below for a more detailed discussion of this feature.

In the discussion that follows, the “name” of the variable is the arbitrary string that is the 1st argument in the variable command. This name can only contain alphanumeric characters and underscores. The “string” is one or more of the subsequent arguments. The “string” can be simple text as in the 1st example above, it can contain other variables as in the 2nd example, or it can be a formula as in the 3rd example. The “value” is the numeric quantity resulting from evaluation of the string. Note that the same string can generate different values when it is evaluated at different times during a simulation.

Note

When an input script line is encountered that defines a variable of style equal or vector or atom or python that contains a formula or Python code, the formula is NOT immediately evaluated. It will be evaluated every time when the variable is used instead. If you simply want to evaluate a formula in place you can use as so-called. See the section below about “Immediate Evaluation of Variables” for more details on the topic. This is also true of a format style variable since it evaluates another variable when it is invoked.

Variables of style equal and vector and atom can be used as inputs to various other commands which evaluate their formulas as needed, e.g. at different timesteps during a run.

Variables of style internal can be used in place of an equal-style variable, except by commands that set the value stored by the internal-style variable. Thus any command that states it can use an equal-style variable as an argument, can also use an internal-style variable. This means that when the command evaluates the variable, it will use the value set (internally) by another command.

Variables of style python can be used in place of an equal-style variable so long as the associated Python function, as defined by the python command, returns a numeric value. Thus any command that states it can use an equal-style variable as an argument, can also use such a python-style variable. This means that when the LAMMPS command evaluates the variable, the Python function will be executed.

Note

When a variable command is encountered in the input script and the variable name has already been specified, the command is ignored. This means variables can NOT be re-defined in an input script (with two exceptions, read further). This is to allow an input script to be processed multiple times without resetting the variables; see the jump or include commands. It also means that using the command-line switch -var will override a corresponding index variable setting in the input script.

There are two exceptions to this rule. First, variables of style string, getenv, internal, equal, vector, atom, and python ARE redefined each time the command is encountered. This allows these style of variables to be redefined multiple times in an input script. In a loop, this means the formula associated with an equal or atom style variable can change if it contains a substitution for another variable, e.g. $x or v_x.

Second, as described below, if a variable is iterated on to the end of its list of strings via the next command, it is removed from the list of active variables, and is thus available to be re-defined in a subsequent variable command. The delete style does the same thing.


The Commands parse doc page explains how occurrences of a variable name in an input script line are replaced by the variable’s string. The variable name can be referenced as $x if the name “x” is a single character, or as ${LoopVar} if the name “LoopVar” is one or more characters.

As described below, for variable styles index, loop, file, universe, and uloop, which string is assigned to a variable can be incremented via the next command. When there are no more strings to assign, the variable is exhausted and a flag is set that causes the next jump command encountered in the input script to be skipped. This enables the construction of simple loops in the input script that are iterated over and then exited from.

As explained above, an exhausted variable can be re-used in an input script. The delete style also removes the variable, the same as if it were exhausted, allowing it to be redefined later in the input script or when the input script is looped over. This can be useful when breaking out of a loop via the if and jump commands before the variable would become exhausted. For example,

label       loop
variable    a loop 5
print       "A = $a"
if          "$a > 2" then "jump in.script break"
next        a
jump        in.script loop
label       break
variable    a delete

This section describes how all the various variable styles are defined and what they store. Except for the equal and vector and atom styles, which are explained in the next section.

Many of the styles store one or more strings. Note that a single string can contain spaces (multiple words), if it is enclosed in quotes in the variable command. When the variable is substituted for in another input script command, its returned string will then be interpreted as multiple arguments in the expanded command.

For the index style, one or more strings are specified. Initially, the 1st string is assigned to the variable. Each time a next command is used with the variable name, the next string is assigned. All processors assign the same string to the variable.

Index style variables with a single string value can also be set by using the command-line switch -var.

The loop style is identical to the index style except that the strings are the integers from 1 to N inclusive, if only one argument N is specified. This allows generation of a long list of runs (e.g. 1000) without having to list N strings in the input script. Initially, the string “1” is assigned to the variable. Each time a next command is used with the variable name, the next string (“2”, “3”, etc) is assigned. All processors assign the same string to the variable. The loop style can also be specified with two arguments N1 and N2. In this case the loop runs from N1 to N2 inclusive, and the string N1 is initially assigned to the variable. N1 <= N2 and N2 >= 0 is required.

For the world style, one or more strings are specified. There must be one string for each processor partition or “world”. LAMMPS can be run with multiple partitions via the -partition command-line switch. This variable command assigns one string to each world. All processors in the world are assigned the same string. The next command cannot be used with equal style variables, since there is only one value per world. This style of variable is useful when you wish to run different simulations on different partitions, or when performing a parallel tempering simulation (see the temper command), to assign different temperatures to different partitions.

For the universe style, one or more strings are specified. There must be at least as many strings as there are processor partitions or “worlds”. LAMMPS can be run with multiple partitions via the -partition command-line switch. This variable command initially assigns one string to each world. When a next command is encountered using this variable, the first processor partition to encounter it, is assigned the next available string. This continues until all the variable strings are consumed. Thus, this command can be used to run 50 simulations on 8 processor partitions. The simulations will be run one after the other on whatever partition becomes available, until they are all finished. Universe style variables are incremented using the files “tmp.lammps.variable” and “tmp.lammps.variable.lock” which you will see in your directory during such a LAMMPS run.

The uloop style is identical to the universe style except that the strings are the integers from 1 to N. This allows generation of long list of runs (e.g. 1000) without having to list N strings in the input script.

For the string style, a single string is assigned to the variable. Two differences between this this and using the index style exist: a variable with string style can be redefined, e.g. by another command later in the input script, or if the script is read again in a loop. The other difference is that string performs variable substitution even if the string parameter is quoted.

For the format style, an equal-style variable is specified along with a C-style format string, e.g. “%f” or “%.10g”, which must be appropriate for formatting a double-precision floating-point value. The default format is “%.15g”. This variable style allows an equal-style variable to be formatted precisely when it is evaluated.

If you simply wish to print a variable value with desired precision to the screen or logfile via the print or fix print commands, you can also do this by specifying an “immediate” variable with a trailing colon and format string, as part of the string argument of those commands. This is explained on the Commands parse doc page.

For the getenv style, a single string is assigned to the variable which should be the name of an environment variable. When the variable is evaluated, it returns the value of the environment variable, or an empty string if it not defined. This style of variable can be used to adapt the behavior of LAMMPS input scripts via environment variable settings, or to retrieve information that has been previously stored with the shell putenv command. Note that because environment variable settings are stored by the operating systems, they persist beyond a clear command.

For the file style, a filename is provided which contains a list of strings to assign to the variable, one per line. The strings can be numeric values if desired. See the discussion of the next() function below for equal-style variables, which will convert the string of a file-style variable into a numeric value in a formula.

When a file-style variable is defined, the file is opened and the string on the first line is read and stored with the variable. This means the variable can then be evaluated as many times as desired and will return that string. There are two ways to cause the next string from the file to be read: use the next command or the next() function in an equal- or atom-style variable, as discussed below.

The rules for formatting the file are as follows. A comment character “#” can be used anywhere on a line; text starting with the comment character is stripped. Blank lines are skipped. The first “word” of a non-blank line, delimited by white space, is the “string” assigned to the variable.

For the atomfile style, a filename is provided which contains one or more sets of values, to assign on a per-atom basis to the variable. The format of the file is described below.

When an atomfile-style variable is defined, the file is opened and the first set of per-atom values are read and stored with the variable. This means the variable can then be evaluated as many times as desired and will return those values. There are two ways to cause the next set of per-atom values from the file to be read: use the next command or the next() function in an atom-style variable, as discussed below.

The rules for formatting the file are as follows. Each time a set of per-atom values is read, a non-blank line is searched for in the file. A comment character “#” can be used anywhere on a line; text starting with the comment character is stripped. Blank lines are skipped. The first “word” of a non-blank line, delimited by white space, is read as the count N of per-atom lines to immediately follow. N can be be the total number of atoms in the system, or only a subset. The next N lines have the following format

ID value

where ID is an atom ID and value is the per-atom numeric value that will be assigned to that atom. IDs can be listed in any order.

Note

Every time a set of per-atom lines is read, the value for all atoms is first set to 0.0. Thus values for atoms whose ID does not appear in the set, will remain 0.0.

For the python style a Python function name is provided. This needs to match a function name specified in a python command which returns a value to this variable as defined by its return keyword. For example these two commands would be self-consistent:

variable foo python myMultiply
python myMultiply return v_foo format f file funcs.py

The two commands can appear in either order so long as both are specified before the Python function is invoked for the first time.

Each time the variable is evaluated, the associated Python function is invoked, and the value it returns is also returned by the variable. Since the Python function can use other LAMMPS variables as input, or query interal LAMMPS quantities to perform its computation, this means the variable can return a different value each time it is evaluated.

The type of value stored in the variable is determined by the format keyword of the python command. It can be an integer (i), floating point (f), or string (s) value. As mentioned above, if it is a numeric value (integer or floating point), then the python-style variable can be used in place of an equal-style variable anywhere in an input script, e.g. as an argument to another command that allows for equal-style variables.

For the internal style a numeric value is provided. This value will be assigned to the variable until a LAMMPS command sets it to a new value. There are currently only two LAMMPS commands that require internal variables as inputs, because they reset them: create_atoms and fix controller. As mentioned above, an internal-style variable can be used in place of an equal-style variable anywhere else in an input script, e.g. as an argument to another command that allows for equal-style variables.


For the equal and vector and atom styles, a single string is specified which represents a formula that will be evaluated afresh each time the variable is used. If you want spaces in the string, enclose it in double quotes so the parser will treat it as a single argument. For equal-style variables the formula computes a scalar quantity, which becomes the value of the variable whenever it is evaluated. For vector-style variables the formula must compute a vector of quantities, which becomes the value of the variable whenever it is evaluated. The calculated vector can be on length one, but it cannot be a simple scalar value like that produced by an equal-style compute. I.e. the formula for a vector-style variable must have at least one quantity in it that refers to a global vector produced by a compute, fix, or other vector-style variable. For atom-style variables the formula computes one quantity for each atom whenever it is evaluated.

Note that equal, vector, and atom variables can produce different values at different stages of the input script or at different times during a run. For example, if an equal variable is used in a fix print command, different values could be printed each timestep it was invoked. If you want a variable to be evaluated immediately, so that the result is stored by the variable instead of the string, see the section below on “Immediate Evaluation of Variables”.

The next command cannot be used with equal or vector or atom style variables, since there is only one string.

The formula for an equal, vector, or atom variable can contain a variety of quantities. The syntax for each kind of quantity is simple, but multiple quantities can be nested and combined in various ways to build up formulas of arbitrary complexity. For example, this is a valid (though strange) variable formula:

variable x equal "pe + c_MyTemp / vol^(1/3)"

Specifically, a formula can contain numbers, constants, thermo keywords, math operators, math functions, group functions, region functions, atom values, atom vectors, compute references, fix references, and references to other variables.

Number 0.2, 100, 1.0e20, -15.4, etc
Constant PI, version, on, off, true, false, yes, no
Thermo keywords vol, pe, ebond, etc
Math operators (), -x, x+y, x-y, x*y, x/y, x^y, x%y, x == y, x != y, x < y, x <= y, x > y, x >= y, x && y, x || y, x |^ y, !x
Math functions sqrt(x), exp(x), ln(x), log(x), abs(x), sin(x), cos(x), tan(x), asin(x), acos(x), atan(x), atan2(y,x), random(x,y,z), normal(x,y,z), ceil(x), floor(x), round(x), ramp(x,y), stagger(x,y), logfreq(x,y,z), logfreq2(x,y,z), stride(x,y,z), stride2(x,y,z,a,b,c), vdisplace(x,y), swiggle(x,y,z), cwiggle(x,y,z)
Group functions count(ID), mass(ID), charge(ID), xcm(ID,dim), vcm(ID,dim), fcm(ID,dim), bound(ID,dir), gyration(ID), ke(ID), angmom(ID,dim), torque(ID,dim), inertia(ID,dimdim), omega(ID,dim)
Region functions count(ID,IDR), mass(ID,IDR), charge(ID,IDR), xcm(ID,dim,IDR), vcm(ID,dim,IDR), fcm(ID,dim,IDR), bound(ID,dir,IDR), gyration(ID,IDR), ke(ID,IDR), angmom(ID,dim,IDR), torque(ID,dim,IDR), inertia(ID,dimdim,IDR), omega(ID,dim,IDR)
Special functions sum(x), min(x), max(x), ave(x), trap(x), slope(x), gmask(x), rmask(x), grmask(x,y), next(x)
Atom values id[i], mass[i], type[i], mol[i], x[i], y[i], z[i], vx[i], vy[i], vz[i], fx[i], fy[i], fz[i], q[i]
Atom vectors id, mass, type, mol, x, y, z, vx, vy, vz, fx, fy, fz, q
Compute references c_ID, c_ID[i], c_ID[i][j], C_ID, C_ID[i]
Fix references f_ID, f_ID[i], f_ID[i][j], F_ID, F_ID[i]
Other variables v_name, v_name[i]

Most of the formula elements produce a scalar value. Some produce a global or per-atom vector of values. Global vectors can be produced by computes or fixes or by other vector-style variables. Per-atom vectors are produced by atom vectors, compute references that represent a per-atom vector, fix references that represent a per-atom vector, and variables that are atom-style variables. Math functions that operate on scalar values produce a scalar value; math function that operate on global or per-atom vectors do so element-by-element and produce a global or per-atom vector.

A formula for equal-style variables cannot use any formula element that produces a global or per-atom vector. A formula for a vector-style variable can use formula elements that produce either a scalar value or a global vector value, but cannot use a formula element that produces a per-atom vector. A formula for an atom-style variable can use formula elements that produce either a scalar value or a per-atom vector, but not one that produces a global vector. Atom-style variables are evaluated by other commands that define a group on which they operate, e.g. a dump or compute or fix command. When they invoke the atom-style variable, only atoms in the group are included in the formula evaluation. The variable evaluates to 0.0 for atoms not in the group.


Numbers, constants, and thermo keywords

Numbers can contain digits, scientific notation (3.0e20,3.0e-20,3.0E20,3.0E-20), and leading minus signs.

Constants are set at compile time and cannot be changed. PI will return the number 3.14159265358979323846; on, true or yes will return 1.0; off, false or no will return 0.0; version will return a numeric version code of the current LAMMPS version (e.g. version 2 Sep 2015 will return the number 20150902). The corresponding value for newer versions of LAMMPS will be larger, for older versions of LAMMPS will be smaller. This can be used to have input scripts adapt automatically to LAMMPS versions, when non-backwards compatible syntax changes are introduced. Here is an illustrative example (which will not work, since the version has been introduced more recently):

if $(version<20140513) then "communicate vel yes" else "comm_modify vel yes"

The thermo keywords allowed in a formula are those defined by the thermo_style custom command. Thermo keywords that require a compute to calculate their values such as “temp” or “press”, use computes stored and invoked by the thermo_style command. This means that you can only use those keywords in a variable if the style you are using with the thermo_style command (and the thermo keywords associated with that style) also define and use the needed compute. Note that some thermo keywords use a compute indirectly to calculate their value (e.g. the enthalpy keyword uses temp, pe, and pressure). If a variable is evaluated directly in an input script (not during a run), then the values accessed by the thermo keyword must be current. See the discussion below about “Variable Accuracy”.


Math Operators

Math operators are written in the usual way, where the “x” and “y” in the examples can themselves be arbitrarily complex formulas, as in the examples above. In this syntax, “x” and “y” can be scalar values or per-atom vectors. For example, “ke/natoms” is the division of two scalars, where “vy+vz” is the element-by-element sum of two per-atom vectors of y and z velocities.

Operators are evaluated left to right and have the usual C-style precedence: unary minus and unary logical NOT operator “!” have the highest precedence, exponentiation “^” is next; multiplication and division and the modulo operator “%” are next; addition and subtraction are next; the 4 relational operators “<”, “<=”, “>”, and “>=” are next; the two remaining relational operators “==” and “!=” are next; then the logical AND operator “&&”; and finally the logical OR operator “||” and logical XOR (exclusive or) operator “|^” have the lowest precedence. Parenthesis can be used to group one or more portions of a formula and/or enforce a different order of evaluation than what would occur with the default precedence.

Note

Because a unary minus is higher precedence than exponentiation, the formula “-2^2” will evaluate to 4, not -4. This convention is compatible with some programming languages, but not others. As mentioned, this behavior can be easily overridden with parenthesis; the formula “-(2^2)” will evaluate to -4.

The 6 relational operators return either a 1.0 or 0.0 depending on whether the relationship between x and y is TRUE or FALSE. For example the expression x<10.0 in an atom-style variable formula will return 1.0 for all atoms whose x-coordinate is less than 10.0, and 0.0 for the others. The logical AND operator will return 1.0 if both its arguments are non-zero, else it returns 0.0. The logical OR operator will return 1.0 if either of its arguments is non-zero, else it returns 0.0. The logical XOR operator will return 1.0 if one of its arguments is zero and the other non-zero, else it returns 0.0. The logical NOT operator returns 1.0 if its argument is 0.0, else it returns 0.0.

These relational and logical operators can be used as a masking or selection operation in a formula. For example, the number of atoms whose properties satisfy one or more criteria could be calculated by taking the returned per-atom vector of ones and zeroes and passing it to the compute reduce command.


Math Functions

Math functions are specified as keywords followed by one or more parenthesized arguments “x”, “y”, “z”, each of which can themselves be arbitrarily complex formulas. In this syntax, the arguments can represent scalar values or global vectors or per-atom vectors. In the latter case, the math operation is performed on each element of the vector. For example, “sqrt(natoms)” is the sqrt() of a scalar, where “sqrt(y*z)” yields a per-atom vector with each element being the sqrt() of the product of one atom’s y and z coordinates.

Most of the math functions perform obvious operations. The ln() is the natural log; log() is the base 10 log.

The random(x,y,z) function takes 3 arguments: x = lo, y = hi, and z = seed. It generates a uniform random number between lo and hi. The normal(x,y,z) function also takes 3 arguments: x = mu, y = sigma, and z = seed. It generates a Gaussian variate centered on mu with variance sigma^2. In both cases the seed is used the first time the internal random number generator is invoked, to initialize it. For equal-style and vector-style variables, every processor uses the same seed so that they each generate the same sequence of random numbers. For atom-style variables, a unique seed is created for each processor, based on the specified seed. This effectively generates a different random number for each atom being looped over in the atom-style variable.

Note

Internally, there is just one random number generator for all equal-style and vector-style variables and another one for all atom-style variables. If you define multiple variables (of each style) which use the random() or normal() math functions, then the internal random number generators will only be initialized once, which means only one of the specified seeds will determine the sequence of generated random numbers.

The ceil(), floor(), and round() functions are those in the C math library. Ceil() is the smallest integer not less than its argument. Floor() if the largest integer not greater than its argument. Round() is the nearest integer to its argument.

The ramp(x,y) function uses the current timestep to generate a value linearly interpolated between the specified x,y values over the course of a run, according to this formula:

value = x + (y-x) * (timestep-startstep) / (stopstep-startstep)

The run begins on startstep and ends on stopstep. Startstep and stopstep can span multiple runs, using the start and stop keywords of the run command. See the run command for details of how to do this.

The stagger(x,y) function uses the current timestep to generate a new timestep. X,y > 0 and x > y are required. The generated timesteps increase in a staggered fashion, as the sequence x,x+y,2x,2x+y,3x,3x+y,etc. For any current timestep, the next timestep in the sequence is returned. Thus if stagger(1000,100) is used in a variable by the dump_modify every command, it will generate the sequence of output timesteps:

100,1000,1100,2000,2100,3000,etc

The logfreq(x,y,z) function uses the current timestep to generate a new timestep. X,y,z > 0 and y < z are required. The generated timesteps are on a base-z logarithmic scale, starting with x, and the y value is how many of the z-1 possible timesteps within one logarithmic interval are generated. I.e. the timesteps follow the sequence x,2x,3x,…y*x,x*z,2x*z,3x*z,…y*x*z,x*z^2,2x*z^2,etc. For any current timestep, the next timestep in the sequence is returned. Thus if logfreq(100,4,10) is used in a variable by the dump_modify every command, it will generate this sequence of output timesteps:

100,200,300,400,1000,2000,3000,4000,10000,20000,etc

The logfreq2(x,y,z) function is similar to logfreq, except a single logarithmic interval is divided into y equally-spaced timesteps and all of them are output. Y < z is not required. Thus, if logfreq2(100,18,10) is used in a variable by the dump_modify every command, then the interval between 100 and 1000 is divided as 900/18 = 50 steps, and it will generate the sequence of output timesteps:

100,150,200,...950,1000,1500,2000,...9500,10000,15000,etc

The stride(x,y,z) function uses the current timestep to generate a new timestep. X,y >= 0 and z > 0 and x <= y are required. The generated timesteps increase in increments of z, from x to y, i.e. it generates the sequence x,x+z,x+2z,…,y. If y-x is not a multiple of z, then similar to the way a for loop operates, the last value will be one that does not exceed y. For any current timestep, the next timestep in the sequence is returned. Thus if stride(1000,2000,100) is used in a variable by the dump_modify every command, it will generate the sequence of output timesteps:

1000,1100,1200, ... ,1900,2000

The stride2(x,y,z,a,b,c) function is similar to the stride() function except it generates two sets of strided timesteps, one at a coarser level and one at a finer level. Thus it is useful for debugging, e.g. to produce output every timestep at the point in simulation when a problem occurs. X,y >= 0 and z > 0 and x <= y are required, as are a,b >= 0 and c > 0 and a < b. Also, a >= x and b <= y are required so that the second stride is inside the first. The generated timesteps increase in increments of z, starting at x, until a is reached. At that point the timestep increases in increments of c, from a to b, then after b, increments by z are resumed until y is reached. For any current timestep, the next timestep in the sequence is returned. Thus if stride2(1000,2000,100,1350,1360,1) is used in a variable by the dump_modify every command, it will generate the sequence of output timesteps:

1000,1100,1200,1300,1350,1351,1352, ... 1359,1360,1400,1500, ... ,2000

The vdisplace(x,y) function takes 2 arguments: x = value0 and y = velocity, and uses the elapsed time to change the value by a linear displacement due to the applied velocity over the course of a run, according to this formula:

value = value0 + velocity*(timestep-startstep)*dt

where dt = the timestep size.

The run begins on startstep. Startstep can span multiple runs, using the start keyword of the run command. See the run command for details of how to do this. Note that the thermo_style keyword elaplong = timestep-startstep.

The swiggle(x,y,z) and cwiggle(x,y,z) functions each take 3 arguments: x = value0, y = amplitude, z = period. They use the elapsed time to oscillate the value by a sin() or cos() function over the course of a run, according to one of these formulas, where omega = 2 PI / period:

value = value0 + Amplitude * sin(omega*(timestep-startstep)*dt)
value = value0 + Amplitude * (1 - cos(omega*(timestep-startstep)*dt))

where dt = the timestep size.

The run begins on startstep. Startstep can span multiple runs, using the start keyword of the run command. See the run command for details of how to do this. Note that the thermo_style keyword elaplong = timestep-startstep.


Group and Region Functions

Group functions are specified as keywords followed by one or two parenthesized arguments. The first argument ID is the group-ID. The dim argument, if it exists, is x or y or z. The dir argument, if it exists, is xmin, xmax, ymin, ymax, zmin, or zmax. The dimdim argument, if it exists, is xx or yy or zz or xy or yz or xz.

The group function count() is the number of atoms in the group. The group functions mass() and charge() are the total mass and charge of the group. Xcm() and vcm() return components of the position and velocity of the center of mass of the group. Fcm() returns a component of the total force on the group of atoms. Bound() returns the min/max of a particular coordinate for all atoms in the group. Gyration() computes the radius-of-gyration of the group of atoms. See the compute gyration command for a definition of the formula. Angmom() returns components of the angular momentum of the group of atoms around its center of mass. Torque() returns components of the torque on the group of atoms around its center of mass, based on current forces on the atoms. Inertia() returns one of 6 components of the symmetric inertia tensor of the group of atoms around its center of mass, ordered as Ixx,Iyy,Izz,Ixy,Iyz,Ixz. Omega() returns components of the angular velocity of the group of atoms around its center of mass.

Region functions are specified exactly the same way as group functions except they take an extra final argument IDR which is the region ID. The function is computed for all atoms that are in both the group and the region. If the group is “all”, then the only criteria for atom inclusion is that it be in the region.


Special Functions

Special functions take specific kinds of arguments, meaning their arguments cannot be formulas themselves.

The sum(x), min(x), max(x), ave(x), trap(x), and slope(x) functions each take 1 argument which is of the form “c_ID” or “c_ID[N]” or “f_ID” or “f_ID[N]” or “v_name”. The first two are computes and the second two are fixes; the ID in the reference should be replaced by the ID of a compute or fix defined elsewhere in the input script. The compute or fix must produce either a global vector or array. If it produces a global vector, then the notation without “[N]” should be used. If it produces a global array, then the notation with “[N]” should be used, when N is an integer, to specify which column of the global array is being referenced. The last form of argument “v_name” is for a vector-style variable where “name” is replaced by the name of the variable.

These functions operate on a global vector of inputs and reduce it to a single scalar value. This is analogous to the operation of the compute reduce command, which performs similar operations on per-atom and local vectors.

The sum() function calculates the sum of all the vector elements. The min() and max() functions find the minimum and maximum element respectively. The ave() function is the same as sum() except that it divides the result by the length of the vector.

The trap() function is the same as sum() except the first and last elements are multiplied by a weighting factor of 1/2 when performing the sum. This effectively implements an integration via the trapezoidal rule on the global vector of data. I.e. consider a set of points, equally spaced by 1 in their x coordinate: (1,V1), (2,V2), …, (N,VN), where the Vi are the values in the global vector of length N. The integral from 1 to N of these points is trap(). When appropriately normalized by the timestep size, this function is useful for calculating integrals of time-series data, like that generated by the fix ave/correlate command.

The slope() function uses linear regression to fit a line to the set of points, equally spaced by 1 in their x coordinate: (1,V1), (2,V2), …, (N,VN), where the Vi are the values in the global vector of length N. The returned value is the slope of the line. If the line has a single point or is vertical, it returns 1.0e20.

The gmask(x) function takes 1 argument which is a group ID. It can only be used in atom-style variables. It returns a 1 for atoms that are in the group, and a 0 for atoms that are not.

The rmask(x) function takes 1 argument which is a region ID. It can only be used in atom-style variables. It returns a 1 for atoms that are in the geometric region, and a 0 for atoms that are not.

The grmask(x,y) function takes 2 arguments. The first is a group ID, and the second is a region ID. It can only be used in atom-style variables. It returns a 1 for atoms that are in both the group and region, and a 0 for atoms that are not in both.

The next(x) function takes 1 argument which is a variable ID (not “v_foo”, just “foo”). It must be for a file-style or atomfile-style variable. Each time the next() function is invoked (i.e. each time the equal-style or atom-style variable is evaluated), the following steps occur.

For file-style variables, the current string value stored by the file-style variable is converted to a numeric value and returned by the function. And the next string value in the file is read and stored. Note that if the line previously read from the file was not a numeric string, then it will typically evaluate to 0.0, which is likely not what you want.

For atomfile-style variables, the current per-atom values stored by the atomfile-style variable are returned by the function. And the next set of per-atom values in the file is read and stored.

Since file-style and atomfile-style variables read and store the first line of the file or first set of per-atoms values when they are defined in the input script, these are the value(s) that will be returned the first time the next() function is invoked. If next() is invoked more times than there are lines or sets of lines in the file, the variable is deleted, similar to how the next command operates.


Feature Functions

Feature functions allow to probe the running LAMMPS executable for whether specific features are either active, defined, or available. The functions take two arguments, a category and a corresponding argument. The arguments are strings thus cannot be formulas themselves (only $-style immediate variable expansion is possible). Return value is either 1.0 or 0.0 depending on whether the function evaluates to true or false, respectively.

The is_active() function allows to query for active settings which are grouped by categories. Currently supported categories and arguments are:

  • package (argument = cuda or gpu or intel or kokkos or omp)
  • newton (argument = pair or bond or any)
  • pair (argument = single or respa or manybody or tail or shift)
  • comm_style (argument = brick or tiled)
  • min_style (argument = any of the compiled in minimizer styles)
  • run_style (argument = any of the compiled in run styles)
  • atom_style (argument = any of the compiled in atom styles)
  • pair_style (argument = any of the compiled in pair styles)
  • bond_style (argument = any of the compiled in bond styles)
  • angle_style (argument = any of the compiled in angle styles)
  • dihedral_style (argument = any of the compiled in dihedral styles)
  • improper_style (argument = any of the compiled in improper styles)
  • kspace_style (argument = any of the compiled in kspace styles)

Most of the settings are self-explanatory, the single argument in the pair category allows to check whether a pair style supports a Pair::single() function as needed by compute group/group and others features or LAMMPS, respa allows to check whether the inner/middle/outer mode of r-RESPA is supported. In the various style categories, the checking is also done using suffix flags, if available and enabled.

Example 1: disable use of suffix for pppm when using GPU package (i.e. run it on the CPU concurrently to running the pair style on the GPU), but do use the suffix otherwise (e.g. with USER-OMP).

pair_style lj/cut/coul/long 14.0
if $(is_active(package,gpu)) then "suffix off"
kspace_style pppm

Example 2: use r-RESPA with inner/outer cutoff, if supported by pair style, otherwise fall back to using pair and reducing the outer time step

timestep $(2.0*(1.0+*is_active(pair,respa))
if $(is_active(pair,respa)) then "run_style respa 4 3 2 2  improper 1 inner 2 5.5 7.0 outer 3 kspace 4" else "run_style respa 3 3 2  improper 1 pair 2 kspace 3"

The is_defined() function allows to query categories like compute, dump, fix, group, region, and variable whether an entry with the provided name or id is defined.

The is_available(category,name) function allows to query whether a specific optional feature is available, i.e. compiled in. This currently works for the following categories: command, compute, fix, pair_style and feature. For all categories except command and feature also appending active suffixes is tried before reporting failure.

The feature category is used to check the availability of compiled in features such as GZIP support, PNG support, JPEG support, FFMPEG support, and C++ exceptions for error handling. Corresponding values for name are gzip, png, jpeg, ffmpeg and exceptions.

This enables writing input scripts which only dump using a given format if the compiled binary supports it.

if "$(is_available(feature,png))" then "print 'PNG supported'" else "print 'PNG not supported'"

if "$(is_available(feature,ffmpeg)" then "dump 3 all movie 25 movie.mp4 type type zoom 1.6 adiam 1.0"

Atom Values and Vectors

Atom values take an integer argument I from 1 to N, where I is the atom-ID, e.g. x[243], which means use the x coordinate of the atom with ID = 243. Or they can take a variable name, specified as v_name, where name is the name of the variable, like x[v_myIndex]. The variable can be of any style except vector or atom or atomfile variables. The variable is evaluated and the result is expected to be numeric and is cast to an integer (i.e. 3.4 becomes 3), to use an an index, which must be a value from 1 to N. Note that a “formula” cannot be used as the argument between the brackets, e.g. x[243+10] or x[v_myIndex+1] are not allowed. To do this a single variable can be defined that contains the needed formula.

Note that the 0 < atom-ID <= N, where N is the largest atom ID in the system. If an ID is specified for an atom that does not currently exist, then the generated value is 0.0.

Atom vectors generate one value per atom, so that a reference like “vx” means the x-component of each atom’s velocity will be used when evaluating the variable.

The meaning of the different atom values and vectors is mostly self-explanatory. Mol refers to the molecule ID of an atom, and is only defined if an atom_style is being used that defines molecule IDs.

Note that many other atom attributes can be used as inputs to a variable by using the compute property/atom command and then specifying a quantity from that compute.


Compute References

Compute references access quantities calculated by a compute. The ID in the reference should be replaced by the ID of a compute defined elsewhere in the input script. As discussed in the doc page for the compute command, computes can produce global, per-atom, or local values. Only global and per-atom values can be used in a variable. Computes can also produce a scalar, vector, or array.

An equal-style variable can only use scalar values, which means a global scalar, or an element of a global or per-atom vector or array. A vector-style variable can use scalar values or a global vector of values, or a column of a global array of values. Atom-style variables can use global scalar values. They can also use per-atom vector values, or a column of a per-atom array. See the doc pages for individual computes to see what kind of values they produce.

Examples of different kinds of compute references are as follows. There is typically no ambiguity (see exception below) as to what a reference means, since computes only produce either global or per-atom quantities, never both.

c_ID global scalar, or per-atom vector
c_ID[I] Ith element of global vector, or atom I’s value in per-atom vector, or Ith column from per-atom array
c_ID[I][J] I,J element of global array, or atom I’s Jth value in per-atom array

For I and J indices, integers can be specified or a variable name, specified as v_name, where name is the name of the variable. The rules for this syntax are the same as for the “Atom Values and Vectors” discussion above.

One source of ambiguity for compute references is when a vector-style variable refers to a compute that produces both a global scalar and a global vector. Consider a compute with ID “foo” that does this, referenced as follows by variable “a”, where “myVec” is another vector-style variable:

variable a vector c_foo*v_myVec

The reference “c_foo” could refer to either the global scalar or global vector produced by compute “foo”. In this case, “c_foo” will always refer to the global scalar, and “C_foo” can be used to reference the global vector. Similarly if the compute produces both a global vector and global array, then “c_foo[I]” will always refer to an element of the global vector, and “C_foo[I]” can be used to reference the Ith column of the global array.

Note that if a variable containing a compute is evaluated directly in an input script (not during a run), then the values accessed by the compute must be current. See the discussion below about “Variable Accuracy”.


Fix References

Fix references access quantities calculated by a fix. The ID in the reference should be replaced by the ID of a fix defined elsewhere in the input script. As discussed in the doc page for the fix command, fixes can produce global, per-atom, or local values. Only global and per-atom values can be used in a variable. Fixes can also produce a scalar, vector, or array. An equal-style variable can only use scalar values, which means a global scalar, or an element of a global or per-atom vector or array. Atom-style variables can use the same scalar values. They can also use per-atom vector values. A vector value can be a per-atom vector itself, or a column of an per-atom array. See the doc pages for individual fixes to see what kind of values they produce.

The different kinds of fix references are exactly the same as the compute references listed in the above table, where “c_” is replaced by “f_”. Again, there is typically no ambiguity (see exception below) as to what a reference means, since fixes only produce either global or per-atom quantities, never both.

f_ID global scalar, or per-atom vector
f_ID[I] Ith element of global vector, or atom I’s value in per-atom vector, or Ith column from per-atom array
f_ID[I][J] I,J element of global array, or atom I’s Jth value in per-atom array

For I and J indices, integers can be specified or a variable name, specified as v_name, where name is the name of the variable. The rules for this syntax are the same as for the “Atom Values and Vectors” discussion above.

One source of ambiguity for fix references is the same ambiguity discussed for compute references above. Namely when a vector-style variable refers to a fix that produces both a global scalar and a global vector. The solution is the same as for compute references. For a fix with ID “foo”, “f_foo” will always refer to the global scalar, and “F_foo” can be used to reference the global vector. And similarly for distinguishing between a fix’s global vector versus global array with “f_foo[I]” versus “F_foo[I]”.

Note that if a variable containing a fix is evaluated directly in an input script (not during a run), then the values accessed by the fix should be current. See the discussion below about “Variable Accuracy”.

Note that some fixes only generate quantities on certain timesteps. If a variable attempts to access the fix on non-allowed timesteps, an error is generated. For example, the fix ave/time command may only generate averaged quantities every 100 steps. See the doc pages for individual fix commands for details.


Variable References

Variable references access quantities stored or calculated by other variables, which will cause those variables to be evaluated. The name in the reference should be replaced by the name of a variable defined elsewhere in the input script.

As discussed on this doc page, equal-style variables generate a single global numeric value, vector-style variables generate a vector of global numeric values, and atom-style and atomfile-style variables generate a per-atom vector of numeric values. All other variables store one or more strings.

The formula for an equal-style variable can use any style of variable including a vector_style or atom-style or atomfile-style. For these 3 styles, a subscript must be used to access a single value from the vector-, atom-, or atomfile-style variable. If a string-storing variable is used, the string is converted to a numeric value. Note that this will typically produce a 0.0 if the string is not a numeric string, which is likely not what you want.

The formula for a vector-style variable can use any style of variable, including atom-style or atomfile-style variables. For these 2 styles, a subscript must be used to access a single value from the atom-, or atomfile-style variable.

The formula for an atom-style variable can use any style of variable, including other atom-style or atomfile-style variables. If it uses a vector-style variable, a subscript must be used to access a single value from the vector-style variable.

Examples of different kinds of variable references are as follows. There is no ambiguity as to what a reference means, since variables produce only a global scalar or global vector or per-atom vector.

v_name global scalar from equal-style variable
v_name global vector from vector-style variable
v_name per-atom vector from atom-style or atomfile-style variable
v_name[I] Ith element of a global vector from vector-style variable
v_name[I] value of atom with ID = I from atom-style or atomfile-style variable

For the I index, an integer can be specified or a variable name, specified as v_name, where name is the name of the variable. The rules for this syntax are the same as for the “Atom Values and Vectors” discussion above.


Immediate Evaluation of Variables:

If you want an equal-style variable to be evaluated immediately, it may be the case that you do not need to define a variable at all. See the Commands parse doc page for info on how to use “immediate” variables in an input script, specified as $(formula) with parenthesis, where the formula has the same syntax as equal-style variables described on this page. This effectively evaluates a formula immediately without using the variable command to define a named variable.

More generally, there is a difference between referencing a variable with a leading $ sign (e.g. $x or ${abc}) versus with a leading “v_” (e.g. v_x or v_abc). The former can be used in any input script command, including a variable command. The input script parser evaluates the reference variable immediately and substitutes its value into the command. As explained on the Commands parse doc page, you can also use un-named “immediate” variables for this purpose. For example, a string like this $((xlo+xhi)/2+sqrt(v_area)) in an input script command evaluates the string between the parenthesis as an equal-style variable formula.

Referencing a variable with a leading “v_” is an optional or required kind of argument for some commands (e.g. the fix ave/chunk or dump custom or thermo_style commands) if you wish it to evaluate a variable periodically during a run. It can also be used in a variable formula if you wish to reference a second variable. The second variable will be evaluated whenever the first variable is evaluated.

As an example, suppose you use this command in your input script to define the variable “v” as

variable v equal vol

before a run where the simulation box size changes. You might think this will assign the initial volume to the variable “v”. That is not the case. Rather it assigns a formula which evaluates the volume (using the thermo_style keyword “vol”) to the variable “v”. If you use the variable “v” in some other command like fix ave/time then the current volume of the box will be evaluated continuously during the run.

If you want to store the initial volume of the system, you can do it this way:

variable v equal vol
variable v0 equal $v

The second command will force “v” to be evaluated (yielding the initial volume) and assign that value to the variable “v0”. Thus the command

thermo_style custom step v_v v_v0

would print out both the current and initial volume periodically during the run.

Note that it is a mistake to enclose a variable formula in double quotes if it contains variables preceded by $ signs. For example,

variable vratio equal "${vfinal}/${v0}"

This is because the quotes prevent variable substitution (explained on the Commands parse doc page), and thus an error will occur when the formula for “vratio” is evaluated later.


Variable Accuracy:

Obviously, LAMMPS attempts to evaluate variables containing formulas (equal and atom style variables) accurately whenever the evaluation is performed. Depending on what is included in the formula, this may require invoking a compute, either directly or indirectly via a thermo keyword, or accessing a value previously calculated by a compute, or accessing a value calculated and stored by a fix. If the compute is one that calculates the pressure or energy of the system, then these quantities need to be tallied during the evaluation of the interatomic potentials (pair, bond, etc) on timesteps that the variable will need the values.

LAMMPS keeps track of all of this during a run or energy minimization. An error will be generated if you attempt to evaluate a variable on timesteps when it cannot produce accurate values. For example, if a thermo_style custom command prints a variable which accesses values stored by a fix ave/time command and the timesteps on which thermo output is generated are not multiples of the averaging frequency used in the fix command, then an error will occur.

An input script can also request variables be evaluated before or after or in between runs, e.g. by including them in a print command. In this case, if a compute is needed to evaluate a variable (either directly or indirectly), LAMMPS will not invoke the compute, but it will use a value previously calculated by the compute, and can do this only if it was invoked on the current timestep. Fixes will always provide a quantity needed by a variable, but the quantity may or may not be current. This leads to one of three kinds of behavior:

(1) The variable may be evaluated accurately. If it contains references to a compute or fix, and these values were calculated on the last timestep of a preceding run, then they will be accessed and used by the variable and the result will be accurate.

(2) LAMMPS may not be able to evaluate the variable and will generate an error message stating so. For example, if the variable requires a quantity from a compute that has not been invoked on the current timestep, LAMMPS will generate an error. This means, for example, that such a variable cannot be evaluated before the first run has occurred. Likewise, in between runs, a variable containing a compute cannot be evaluated unless the compute was invoked on the last timestep of the preceding run, e.g. by thermodynamic output.

One way to get around this problem is to perform a 0-timestep run before using the variable. For example, these commands

variable t equal temp
print "Initial temperature = $t"
run 1000

will generate an error if the run is the first run specified in the input script, because generating a value for the “t” variable requires a compute for calculating the temperature to be invoked.

However, this sequence of commands would be fine:

run 0
variable t equal temp
print "Initial temperature = $t"
run 1000

The 0-timestep run initializes and invokes various computes, including the one for temperature, so that the value it stores is current and can be accessed by the variable “t” after the run has completed. Note that a 0-timestep run does not alter the state of the system, so it does not change the input state for the 1000-timestep run that follows. Also note that the 0-timestep run must actually use and invoke the compute in question (e.g. via thermo or dump output) in order for it to enable the compute to be used in a variable after the run. Thus if you are trying to print a variable that uses a compute you have defined, you can insure it is invoked on the last timestep of the preceding run by including it in thermodynamic output.

Unlike computes, fixes will never generate an error if their values are accessed by a variable in between runs. They always return some value to the variable. However, the value may not be what you expect if the fix has not yet calculated the quantity of interest or it is not current. For example, the fix indent command stores the force on the indenter. But this is not computed until a run is performed. Thus if a variable attempts to print this value before the first run, zeroes will be output. Again, performing a 0-timestep run before printing the variable has the desired effect.

(3) The variable may be evaluated incorrectly and LAMMPS may have no way to detect this has occurred. Consider the following sequence of commands:

pair_coeff 1 1 1.0 1.0
run 1000
pair_coeff 1 1 1.5 1.0
variable e equal pe
print "Final potential energy = $e"

The first run is performed using one setting for the pairwise potential defined by the pair_style and pair_coeff commands. The potential energy is evaluated on the final timestep and stored by the compute pe compute (this is done by the thermo_style command). Then a pair coefficient is changed, altering the potential energy of the system. When the potential energy is printed via the “e” variable, LAMMPS will use the potential energy value stored by the compute pe compute, thinking it is current. There are many other commands which could alter the state of the system between runs, causing a variable to evaluate incorrectly.

The solution to this issue is the same as for case (2) above, namely perform a 0-timestep run before the variable is evaluated to insure the system is up-to-date. For example, this sequence of commands would print a potential energy that reflected the changed pairwise coefficient:

pair_coeff 1 1 1.0 1.0
run 1000
pair_coeff 1 1 1.5 1.0
run 0
variable e equal pe
print "Final potential energy = $e"

Restrictions

Indexing any formula element by global atom ID, such as an atom value, requires the atom style to use a global mapping in order to look up the vector indices. By default, only atom styles with molecular information create global maps. The atom_modify map command can override the default, e.g. for atomic-style atom styles.

All universe- and uloop-style variables defined in an input script must have the same number of values.