# 4.2. Command-line options

At run time, LAMMPS recognizes several optional command-line switches which may be used in any order. Either the full word or a one-or-two letter abbreviation can be used:

For example, the lmp_mpi executable might be launched as follows:

mpirun -np 16 lmp_mpi -v f tmp.out -l my.log -sc none -i in.alloy
mpirun -np 16 lmp_mpi -var f tmp.out -log my.log -screen none -in in.alloy


-echo style

Set the style of command echoing. The style can be none or screen or log or both. Depending on the style, each command read from the input script will be echoed to the screen and/or logfile. This can be useful to figure out which line of your script is causing an input error. The default value is log. The echo style can also be set by using the echo command in the input script itself.

-help

Print a brief help summary and a list of options compiled into this executable for each LAMMPS style (atom_style, fix, compute, pair_style, bond_style, etc). This can tell you if the command you want to use was included via the appropriate package at compile time. LAMMPS will print the info and immediately exit if this switch is used.

-in file

Specify a file to use as an input script. This is an optional switch when running LAMMPS in one-partition mode. If it is not specified, LAMMPS reads its script from standard input, typically from a script via I/O redirection; e.g. lmp_linux < in.run. I/O redirection should also work in parallel, but if it does not (in the unlikely case that an MPI implementation does not support it), then use the -in flag. Note that this is a required switch when running LAMMPS in multi-partition mode, since multiple processors cannot all read from stdin.

-kokkos on/off keyword/value …

Explicitly enable or disable KOKKOS support, as provided by the KOKKOS package. Even if LAMMPS is built with this package, as described in Speed kokkos, this switch must be set to enable running with the KOKKOS-enabled styles the package provides. If the switch is not set (the default), LAMMPS will operate as if the KOKKOS package were not installed; i.e. you can run standard LAMMPS or with the GPU or USER-OMP packages, for testing or benchmarking purposes.

Additional optional keyword/value pairs can be specified which determine how Kokkos will use the underlying hardware on your platform. These settings apply to each MPI task you launch via the “mpirun” or “mpiexec” command. You may choose to run one or more MPI tasks per physical node. Note that if you are running on a desktop machine, you typically have one physical node. On a cluster or supercomputer there may be dozens or 1000s of physical nodes.

Either the full word or an abbreviation can be used for the keywords. Note that the keywords do not use a leading minus sign. I.e. the keyword is “t”, not “-t”. Also note that each of the keywords has a default setting. Examples of when to use these options and what settings to use on different platforms is given on the Speed kokkos doc page.

• d or device
• g or gpus
• n or numa
device Nd


This option is only relevant if you built LAMMPS with CUDA=yes, you have more than one GPU per node, and if you are running with only one MPI task per node. The Nd setting is the ID of the GPU on the node to run on. By default Nd = 0. If you have multiple GPUs per node, they have consecutive IDs numbered as 0,1,2,etc. This setting allows you to launch multiple independent jobs on the node, each with a single MPI task per node, and assign each job to run on a different GPU.

gpus Ng Ns


This option is only relevant if you built LAMMPS with CUDA=yes, you have more than one GPU per node, and you are running with multiple MPI tasks per node (up to one per GPU). The Ng setting is how many GPUs you will use. The Ns setting is optional. If set, it is the ID of a GPU to skip when assigning MPI tasks to GPUs. This may be useful if your desktop system reserves one GPU to drive the screen and the rest are intended for computational work like running LAMMPS. By default Ng = 1 and Ns is not set.

Depending on which flavor of MPI you are running, LAMMPS will look for one of these 3 environment variables

SLURM_LOCALID (various MPI variants compiled with SLURM support)
MV2_COMM_WORLD_LOCAL_RANK (Mvapich)
OMPI_COMM_WORLD_LOCAL_RANK (OpenMPI)


which are initialized by the “srun”, “mpirun” or “mpiexec” commands. The environment variable setting for each MPI rank is used to assign a unique GPU ID to the MPI task.

threads Nt


This option assigns Nt number of threads to each MPI task for performing work when Kokkos is executing in OpenMP or pthreads mode. The default is Nt = 1, which essentially runs in MPI-only mode. If there are Np MPI tasks per physical node, you generally want Np*Nt = the number of physical cores per node, to use your available hardware optimally. This also sets the number of threads used by the host when LAMMPS is compiled with CUDA=yes.

numa Nm


This option is only relevant when using pthreads with hwloc support. In this case Nm defines the number of NUMA regions (typically sockets) on a node which will be utilized by a single MPI rank. By default Nm = 1. If this option is used the total number of worker-threads per MPI rank is threads*numa. Currently it is always almost better to assign at least one MPI rank per NUMA region, and leave numa set to its default value of 1. This is because letting a single process span multiple NUMA regions induces a significant amount of cross NUMA data traffic which is slow.

-log file

Specify a log file for LAMMPS to write status information to. In one-partition mode, if the switch is not used, LAMMPS writes to the file log.lammps. If this switch is used, LAMMPS writes to the specified file. In multi-partition mode, if the switch is not used, a log.lammps file is created with hi-level status information. Each partition also writes to a log.lammps.N file where N is the partition ID. If the switch is specified in multi-partition mode, the hi-level logfile is named “file” and each partition also logs information to a file.N. For both one-partition and multi-partition mode, if the specified file is “none”, then no log files are created. Using a log command in the input script will override this setting. Option -plog will override the name of the partition log files file.N.

-mpicolor color

If used, this must be the first command-line argument after the LAMMPS executable name. It is only used when LAMMPS is launched by an mpirun command which also launches another executable(s) at the same time. (The other executable could be LAMMPS as well.) The color is an integer value which should be different for each executable (another application may set this value in a different way). LAMMPS and the other executable(s) perform an MPI_Comm_split() with their own colors to shrink the MPI_COMM_WORLD communication to be the subset of processors they are actually running on.

Currently, this is only used in LAMMPS to perform client/server messaging with another application. LAMMPS can act as either a client or server (or both). More details are given on the Howto client/server doc page.

Specifically, this refers to the “mpi/one” mode of messaging provided by the message command and the CSlib library LAMMPS links with from the lib/message directory. See the message command for more details.

-nocite

Disable writing the log.cite file which is normally written to list references for specific cite-able features used during a LAMMPS run. See the citation page for more details.

-package style args ….

Invoke the package command with style and args. The syntax is the same as if the command appeared at the top of the input script. For example “-package gpu 2” or “-pk gpu 2” is the same as package gpu 2 in the input script. The possible styles and args are documented on the package doc page. This switch can be used multiple times, e.g. to set options for the USER-INTEL and USER-OMP packages which can be used together.

Along with the “-suffix” command-line switch, this is a convenient mechanism for invoking accelerator packages and their options without having to edit an input script.

-partition 8x2 4 5 …

Invoke LAMMPS in multi-partition mode. When LAMMPS is run on P processors and this switch is not used, LAMMPS runs in one partition, i.e. all P processors run a single simulation. If this switch is used, the P processors are split into separate partitions and each partition runs its own simulation. The arguments to the switch specify the number of processors in each partition. Arguments of the form MxN mean M partitions, each with N processors. Arguments of the form N mean a single partition with N processors. The sum of processors in all partitions must equal P. Thus the command “-partition 8x2 4 5” has 10 partitions and runs on a total of 25 processors.

Running with multiple partitions can be useful for running multi-replica simulations, where each replica runs on on one or a few processors. Note that with MPI installed on a machine (e.g. your desktop), you can run on more (virtual) processors than you have physical processors.

To run multiple independent simulations from one input script, using multiple partitions, see the Howto multiple doc page. World- and universe-style variables are useful in this context.

-plog file

Specify the base name for the partition log files, so partition N writes log information to file.N. If file is none, then no partition log files are created. This overrides the filename specified in the -log command-line option. This option is useful when working with large numbers of partitions, allowing the partition log files to be suppressed (-plog none) or placed in a sub-directory (-plog replica_files/log.lammps) If this option is not used the log file for partition N is log.lammps.N or whatever is specified by the -log command-line option.

-pscreen file

Specify the base name for the partition screen file, so partition N writes screen information to file.N. If file is none, then no partition screen files are created. This overrides the filename specified in the -screen command-line option. This option is useful when working with large numbers of partitions, allowing the partition screen files to be suppressed (-pscreen none) or placed in a sub-directory (-pscreen replica_files/screen). If this option is not used the screen file for partition N is screen.N or whatever is specified by the -screen command-line option.

-reorder

This option has 2 forms:

-reorder nth N
-reorder custom filename


Reorder the processors in the MPI communicator used to instantiate LAMMPS, in one of several ways. The original MPI communicator ranks all P processors from 0 to P-1. The mapping of these ranks to physical processors is done by MPI before LAMMPS begins. It may be useful in some cases to alter the rank order. E.g. to insure that cores within each node are ranked in a desired order. Or when using the run_style verlet/split command with 2 partitions to insure that a specific Kspace processor (in the 2nd partition) is matched up with a specific set of processors in the 1st partition. See the Speed tips doc page for more details.

If the keyword nth is used with a setting N, then it means every Nth processor will be moved to the end of the ranking. This is useful when using the run_style verlet/split command with 2 partitions via the -partition command-line switch. The first set of processors will be in the first partition, the 2nd set in the 2nd partition. The -reorder command-line switch can alter this so that the 1st N procs in the 1st partition and one proc in the 2nd partition will be ordered consecutively, e.g. as the cores on one physical node. This can boost performance. For example, if you use “-reorder nth 4” and “-partition 9 3” and you are running on 12 processors, the processors will be reordered from

0 1 2 3 4 5 6 7 8 9 10 11


to

0 1 2 4 5 6 8 9 10 3 7 11


so that the processors in each partition will be

0 1 2 4 5 6 8 9 10
3 7 11


See the “processors” command for how to insure processors from each partition could then be grouped optimally for quad-core nodes.

If the keyword is custom, then a file that specifies a permutation of the processor ranks is also specified. The format of the reorder file is as follows. Any number of initial blank or comment lines (starting with a “#” character) can be present. These should be followed by P lines of the form:

I J


where P is the number of processors LAMMPS was launched with. Note that if running in multi-partition mode (see the -partition switch above) P is the total number of processors in all partitions. The I and J values describe a permutation of the P processors. Every I and J should be values from 0 to P-1 inclusive. In the set of P I values, every proc ID should appear exactly once. Ditto for the set of P J values. A single I,J pairing means that the physical processor with rank I in the original MPI communicator will have rank J in the reordered communicator.

Note that rank ordering can also be specified by many MPI implementations, either by environment variables that specify how to order physical processors, or by config files that specify what physical processors to assign to each MPI rank. The -reorder switch simply gives you a portable way to do this without relying on MPI itself. See the processors out command for how to output info on the final assignment of physical processors to the LAMMPS simulation domain.

-restart2data restartfile [remap] datafile keyword value …

Convert the restart file into a data file and immediately exit. This is the same operation as if the following 2-line input script were run:

read_restart restartfile [remap]
write_data datafile keyword value ...


The specified restartfile and/or datafile name may contain the wild-card character “*”. The restartfile name may also contain the wild-card character “%”. The meaning of these characters is explained on the read_restart and write_data doc pages. The use of “%” means that a parallel restart file can be read. Note that a filename such as file.* may need to be enclosed in quotes or the “*” character prefixed with a backslash (“”) to avoid shell expansion of the “*” character.

Following restartfile argument, the optional word “remap” may be used. This has the same effect like adding it to a read_restart command, and operates as explained on its doc page. This is useful if reading the restart file triggers an error that atoms have been lost. In that case, use of the remap flag should allow the data file to still be produced.

The syntax following restartfile (or remap), namely

datafile keyword value ...


is identical to the arguments of the write_data command. See its doc page for details. This includes its optional keyword/value settings.

-restart2dump restartfile [remap] group-ID dumpstyle dumpfile arg1 arg2 …

Convert the restart file into a dump file and immediately exit. This is the same operation as if the following 2-line input script were run:

read_restart restartfile [remap]
write_dump group-ID dumpstyle dumpfile arg1 arg2 ...


Note that the specified restartfile and dumpfile names may contain wild-card characters (“*”,”%”) as explained on the read_restart and write_dump doc pages. The use of “%” means that a parallel restart file and/or parallel dump file can be read and/or written. Note that a filename such as file.* may need to be enclosed in quotes or the “*” character prefixed with a backslash (“”) to avoid shell expansion of the “*” character.

Note that following the restartfile argument, the optional word “remap” can be used. This has the effect as adding it to the read_restart command, as explained on its doc page. This is useful if reading the restart file triggers an error that atoms have been lost. In that case, use of the remap flag should allow the dump file to still be produced.

The syntax following restartfile (or remap), namely

group-ID dumpstyle dumpfile arg1 arg2 ...


is identical to the arguments of the write_dump command. See its doc page for details. This includes what per-atom fields are written to the dump file and optional dump_modify settings, including ones that affect how parallel dump files are written, e.g. the nfile and fileper keywords. See the dump_modify doc page for details.

-screen file

Specify a file for LAMMPS to write its screen information to. In one-partition mode, if the switch is not used, LAMMPS writes to the screen. If this switch is used, LAMMPS writes to the specified file instead and you will see no screen output. In multi-partition mode, if the switch is not used, hi-level status information is written to the screen. Each partition also writes to a screen.N file where N is the partition ID. If the switch is specified in multi-partition mode, the hi-level screen dump is named “file” and each partition also writes screen information to a file.N. For both one-partition and multi-partition mode, if the specified file is “none”, then no screen output is performed. Option -pscreen will override the name of the partition screen files file.N.

-suffix style args

Use variants of various styles if they exist. The specified style can be cuda, gpu, intel, kk, omp, opt, or hybrid. These refer to optional packages that LAMMPS can be built with, as described in Accelerate performance. The “gpu” style corresponds to the GPU package, the “intel” style to the USER-INTEL package, the “kk” style to the KOKKOS package, the “opt” style to the OPT package, and the “omp” style to the USER-OMP package. The hybrid style is the only style that accepts arguments. It allows for two packages to be specified. The first package specified is the default and will be used if it is available. If no style is available for the first package, the style for the second package will be used if available. For example, “-suffix hybrid intel omp” will use styles from the USER-INTEL package if they are installed and available, but styles for the USER-OMP package otherwise.

Along with the “-package” command-line switch, this is a convenient mechanism for invoking accelerator packages and their options without having to edit an input script.

As an example, all of the packages provide a pair_style lj/cut variant, with style names lj/cut/gpu, lj/cut/intel, lj/cut/kk, lj/cut/omp, and lj/cut/opt. A variant style can be specified explicitly in your input script, e.g. pair_style lj/cut/gpu. If the -suffix switch is used the specified suffix (gpu,intel,kk,omp,opt) is automatically appended whenever your input script command creates a new atom, pair, fix, compute, or run style. If the variant version does not exist, the standard version is created.

For the GPU package, using this command-line switch also invokes the default GPU settings, as if the command “package gpu 1” were used at the top of your input script. These settings can be changed by using the “-package gpu” command-line switch or the package gpu command in your script.

For the USER-INTEL package, using this command-line switch also invokes the default USER-INTEL settings, as if the command “package intel 1” were used at the top of your input script. These settings can be changed by using the “-package intel” command-line switch or the package intel command in your script. If the USER-OMP package is also installed, the hybrid style with “intel omp” arguments can be used to make the omp suffix a second choice, if a requested style is not available in the USER-INTEL package. It will also invoke the default USER-OMP settings, as if the command “package omp 0” were used at the top of your input script. These settings can be changed by using the “-package omp” command-line switch or the package omp command in your script.

For the KOKKOS package, using this command-line switch also invokes the default KOKKOS settings, as if the command “package kokkos” were used at the top of your input script. These settings can be changed by using the “-package kokkos” command-line switch or the package kokkos command in your script.

For the OMP package, using this command-line switch also invokes the default OMP settings, as if the command “package omp 0” were used at the top of your input script. These settings can be changed by using the “-package omp” command-line switch or the package omp command in your script.

The suffix command can also be used within an input script to set a suffix, or to turn off or back on any suffix setting made via the command line.

-var name value1 value2 …

Specify a variable that will be defined for substitution purposes when the input script is read. This switch can be used multiple times to define multiple variables. “Name” is the variable name which can be a single character (referenced as $x in the input script) or a full string (referenced as${abc}). An index-style variable will be created and populated with the subsequent values, e.g. a set of filenames. Using this command-line option is equivalent to putting the line “variable name index value1 value2 …” at the beginning of the input script. Defining an index variable as a command-line argument overrides any setting for the same index variable in the input script, since index variables cannot be re-defined.

See the variable command for more info on defining index and other kinds of variables and the Commands parse page for more info on using variables in input scripts.

Note

Currently, the command-line parser looks for arguments that start with “-” to indicate new switches. Thus you cannot specify multiple variable values if any of them start with a “-“, e.g. a negative numeric value. It is OK if the first value1 starts with a “-“, since it is automatically skipped.