fix ID group-ID style args
ID = user-assigned name for the fix
group-ID = ID of the group of atoms to apply the fix to
style = one of a long list of possible style names (see below)
args = arguments used by a particular style
fix 1 all nve fix 3 all nvt temp 300.0 300.0 0.01 fix mine top setforce 0.0 NULL 0.0
Set a fix that will be applied to a group of atoms. In LAMMPS, a “fix” is any operation that is applied to the system during timestepping or minimization. Examples include updating of atom positions and velocities due to time integration, controlling temperature, applying constraint forces to atoms, enforcing boundary conditions, computing diagnostics, etc. There are hundreds of fixes defined in LAMMPS and new ones can be added; see the Modify doc page for details.
Fixes perform their operations at different stages of the timestep. If 2 or more fixes operate at the same stage of the timestep, they are invoked in the order they were specified in the input script.
The ID of a fix can only contain alphanumeric characters and underscores.
Fixes can be deleted with the unfix command.
The unfix command is the only way to turn off a fix; simply specifying a new fix with a similar style will not turn off the first one. This is especially important to realize for integration fixes. For example, using a fix nve command for a second run after using a fix nvt command for the first run, will not cancel out the NVT time integration invoked by the “fix nvt” command. Thus two time integrators would be in place!
If you specify a new fix with the same ID and style as an existing fix, the old fix is deleted and the new one is created (presumably with new settings). This is the same as if an “unfix” command were first performed on the old fix, except that the new fix is kept in the same order relative to the existing fixes as the old one originally was. Note that this operation also wipes out any additional changes made to the old fix via the fix_modify command.
The fix modify command allows settings for some fixes to be reset. See the doc page for individual fixes for details.
Some fixes store an internal “state” which is written to binary restart files via the restart or write_restart commands. This allows the fix to continue on with its calculations in a restarted simulation. See the read_restart command for info on how to re-specify a fix in an input script that reads a restart file. See the doc pages for individual fixes for info on which ones can be restarted.
Some fixes calculate one of three styles of quantities: global, per-atom, or local, which can be used by other commands or output as described below. A global quantity is one or more system-wide values, e.g. the energy of a wall interacting with particles. A per-atom quantity is one or more values per atom, e.g. the displacement vector for each atom since time 0. Per-atom values are set to 0.0 for atoms not in the specified fix group. Local quantities are calculated by each processor based on the atoms it owns, but there may be zero or more per atoms.
Note that a single fix can produce either global or per-atom or local quantities (or none at all), but not both global and per-atom. It can produce local quantities in tandem with global or per-atom quantities. The fix doc page will explain.
Global, per-atom, and local quantities each come in three kinds: a single scalar value, a vector of values, or a 2d array of values. The doc page for each fix describes the style and kind of values it produces, e.g. a per-atom vector. Some fixes produce more than one kind of a single style, e.g. a global scalar and a global vector.
When a fix quantity is accessed, as in many of the output commands discussed below, it can be referenced via the following bracket notation, where ID is the ID of the fix:
entire scalar, vector, or array
one element of vector, one column of array
one element of array
In other words, using one bracket reduces the dimension of the quantity once (vector -> scalar, array -> vector). Using two brackets reduces the dimension twice (array -> scalar). Thus a command that uses scalar fix values as input can also process elements of a vector or array.
Note that commands and variables which use fix quantities typically do not allow for all kinds, e.g. a command may require a vector of values, not a scalar. This means there is no ambiguity about referring to a fix quantity as f_ID even if it produces, for example, both a scalar and vector. The doc pages for various commands explain the details.
In LAMMPS, the values generated by a fix can be used in several ways:
Per-atom values can be output via the dump custom command. Or they can be time-averaged via the fix ave/atom command or reduced by the compute reduce command. Or the per-atom values can be referenced in an atom-style variable.
See the Howto output doc page for a summary of various LAMMPS output options, many of which involve fixes.
The results of fixes that calculate global quantities can be either “intensive” or “extensive” values. Intensive means the value is independent of the number of atoms in the simulation, e.g. temperature. Extensive means the value scales with the number of atoms in the simulation, e.g. total rotational kinetic energy. Thermodynamic output will normalize extensive values by the number of atoms in the system, depending on the “thermo_modify norm” setting. It will not normalize intensive values. If a fix value is accessed in another way, e.g. by a variable, you may want to know whether it is an intensive or extensive value. See the doc page for individual fixes for further info.
Each fix style has its own doc page which describes its arguments and what it does, as listed below. Here is an alphabetic list of fix styles available in LAMMPS. They are also listed in more compact form on the Commands fix doc page.
There are also additional accelerated fix styles included in the LAMMPS distribution for faster performance on CPUs, GPUs, and KNLs. The individual style names on the Commands fix doc page are followed by one or more of (g,i,k,o,t) to indicate which accelerated styles exist.
adapt - change a simulation parameter over time
addforce - add a force to each atom
append/atoms - append atoms to a running simulation
atom/swap - Monte Carlo atom type swapping
ave/atom - compute per-atom time-averaged quantities
ave/chunk - compute per-chunk time-averaged quantities
ave/correlate - compute/output time correlations
ave/histo - compute/output time-averaged histograms
ave/time - compute/output global time-averaged quantities
aveforce - add an averaged force to each atom
balance - perform dynamic load-balancing
bond/break - break bonds on the fly
bond/create - create bonds on the fly
bond/swap - Monte Carlo bond swapping
box/relax - relax box size during energy minimization
deform - change the simulation box size/shape
deposit - add new atoms above a surface
drag - drag atoms towards a defined coordinate
dt/reset - reset the timestep based on velocity, forces
efield - impose electric field on system
ehex - enhanced heat exchange algorithm
electron/stopping - electronic stopping power as a friction force
enforce2d - zero out z-dimension velocity and force
evaporate - remove atoms from simulation periodically
external - callback to an external driver program
freeze - freeze atoms in a granular simulation
gcmc - grand canonical insertions/deletions
gld - generalized Langevin dynamics integrator
gravity - add gravity to atoms in a granular simulation
halt - terminate a dynamics run or minimization
heat - add/subtract momentum-conserving heat
hyper/global - global hyperdynamics
hyper/local - local hyperdynamics
indent - impose force due to an indenter
langevin - Langevin temperature control
latte - wrapper on LATTE density-functional tight-binding code
lineforce - constrain atoms to move in a line
meso - move mesoscopic SPH/SDPD particles in a prescribed fashion
momentum - zero the linear and/or angular momentum of a group of atoms
move - move atoms in a prescribed fashion
msst - multi-scale shock technique (MSST) integration
neb - nudged elastic band (NEB) spring forces
nph - constant NPH time integration via Nose/Hoover
nph/asphere - NPH for aspherical particles
nph/body - NPH for body particles
nph/sphere - NPH for spherical particles
nphug - constant-stress Hugoniostat integration
npt - constant NPT time integration via Nose/Hoover
npt/asphere - NPT for aspherical particles
npt/body - NPT for body particles
npt/sphere - NPT for spherical particles
nve - constant NVE time integration
nve/asphere - NVE for aspherical particles
nve/asphere/noforce - NVE for aspherical particles without forces”
nve/body - NVE for body particles
nve/limit - NVE with limited step length
nve/line - NVE for line segments
nve/noforce - NVE without forces (v only)
nve/sphere - NVE for spherical particles
nve/tri - NVE for triangles
nvt - constant NVT time integration via Nose/Hoover
nvt/asphere - NVT for aspherical particles
nvt/body - NVT for body particles
nvt/sllod - NVT for NEMD with SLLOD equations
nvt/sphere - NVT for spherical particles
oneway - constrain particles on move in one direction
orient/bcc - add grain boundary migration force for BCC
orient/fcc - add grain boundary migration force for FCC
planeforce - constrain atoms to move in a plane
plumed - wrapper on PLUMED free energy library
poems - constrain clusters of atoms to move as coupled rigid bodies
pour - pour new atoms/molecules into a granular simulation domain
press/berendsen - pressure control by Berendsen barostat
print - print text and variables during a simulation
property/atom - add customized per-atom values
qeq/comb - charge equilibration for COMB potential
qeq/dynamic - charge equilibration via dynamic method
qeq/fire - charge equilibration via FIRE minimizer
qeq/point - charge equilibration via point method
qeq/shielded - charge equilibration via shielded method
qeq/slater - charge equilibration via Slater method
rattle - RATTLE constraints on bonds and/or angles
reax/c/bonds - write out ReaxFF bond information
reax/c/species - write out ReaxFF molecule information
recenter - constrain the center-of-mass position of a group of atoms
restrain - constrain a bond, angle, dihedral
rigid - constrain one or more clusters of atoms to move as a rigid body with NVE integration
rigid/meso - constrain clusters of mesoscopic SPH/SDPD particles to move as a rigid body
rigid/nph - constrain one or more clusters of atoms to move as a rigid body with NPH integration
rigid/nph/small - constrain many small clusters of atoms to move as a rigid body with NPH integration
rigid/npt - constrain one or more clusters of atoms to move as a rigid body with NPT integration
rigid/npt/small - constrain many small clusters of atoms to move as a rigid body with NPT integration
rigid/nve - constrain one or more clusters of atoms to move as a rigid body with alternate NVE integration
rigid/nve/small - constrain many small clusters of atoms to move as a rigid body with alternate NVE integration
rigid/nvt - constrain one or more clusters of atoms to move as a rigid body with NVT integration
rigid/nvt/small - constrain many small clusters of atoms to move as a rigid body with NVT integration
rigid/small - constrain many small clusters of atoms to move as a rigid body with NVE integration
setforce - set the force on each atom
shake - SHAKE constraints on bonds and/or angles
spring - apply harmonic spring force to group of atoms
spring/chunk - apply harmonic spring force to each chunk of atoms
spring/rg - spring on radius of gyration of group of atoms
spring/self - spring from each atom to its origin
srd - stochastic rotation dynamics (SRD)
store/force - store force on each atom
store/state - store attributes for each atom
temp/berendsen - temperature control by Berendsen thermostat
temp/csld - canonical sampling thermostat with Langevin dynamics
temp/csvr - canonical sampling thermostat with Hamiltonian dynamics
temp/rescale - temperature control by velocity rescaling
tfmc - perform force-bias Monte Carlo with time-stamped method
thermal/conductivity - Muller-Plathe kinetic energy exchange for thermal conductivity calculation
tmd - guide a group of atoms to a new configuration
ttm - two-temperature model for electronic/atomic coupling
tune/kspace - auto-tune KSpace parameters
vector - accumulate a global vector every N timesteps
viscosity - Muller-Plathe momentum exchange for viscosity calculation
viscous - viscous damping for granular simulations
wall/colloid - Lennard-Jones wall interacting with finite-size particles
wall/gran - frictional wall(s) for granular simulations
wall/harmonic - harmonic spring wall
wall/lj1043 - Lennard-Jones 10-4-3 wall
wall/lj126 - Lennard-Jones 12-6 wall
wall/lj93 - Lennard-Jones 9-3 wall
wall/piston - moving reflective piston wall
wall/reflect - reflecting wall(s)
wall/region - use region surface as wall
wall/srd - slip/no-slip wall for SRD particles
Some fix styles are part of specific packages. They are only enabled if LAMMPS was built with that package. See the Build package doc page for more info. The doc pages for individual fixes tell if it is part of a package.