lammps/doc/fix.txt

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fix command :h3

[Syntax:]

fix ID group-ID style args :pre

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 :ul

[Examples:]

fix 1 all nve
fix 3 all nvt 300.0 300.0 0.01
fix mine top setforce 0.0 NULL 0.0 :pre

[Description:]

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 dozens of fixes
defined in LAMMPS and new ones can be added - see "this
section"_Section_modify.html for a discussion.

Each fix style has its own documentation page which describes its
arguments and what it does.  For example, see the "fix
setforce"_fix_setforce.html page for information on style {setforce}.

Fixes perform their operations at different stages of the timestep.
If 2 or more fixes both operate at the same stage of the timestep,
they are invoked in the order they were specified in the input script.

Specifying a new fix with the same ID as an existing fix effectively
replaces the old fix (and its parameters) with the new fix.  This can
only be done if the new fix has the same style as the existing fix.

Fixes can be deleted with the "unfix"_unfix.html command.  Note that
this 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.  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!

Here is an alphabetic list of fix styles defined in LAMMPS:

"fix addforce"_fix_addforce.html - add a force to each atom
"fix aveforce"_fix_aveforce.html - add an averaged force to each atom
"fix com"_fix_com.html - compute a center-of-mass
"fix drag"_fix_drag.html - drag atoms towards a defined coordinate
"fix efield"_fix_efield.html - impose electric field on system
"fix enforce2d"_fix_enforce2d.html - zero out z-dimension velocity and force
"fix freeze"_fix_freeze.html - freeze atoms in a granular simulation
"fix gran/diag"_fix_gran_diag.html - compute granular diagnostics
"fix gravity"_fix_gravity.html - add gravity to atoms in a granular simulation
"fix gyration"_fix_gyration.html - compute radius of gyration
"fix indent"_fix_indent.html - impose force due to an indenter
"fix insert"_fix_insert.html - add new atoms to a granular simulation
"fix langevin"_fix_langevin.html - Langevin temperature control
"fix lineforce"_fix_lineforce.html - constrain atoms to move in a line
"fix msd"_fix_msd.html - compute mean-squared displacement \
     (i.e. diffusion coefficient)
"fix momentum"_fix_momentum.html - zero the linear and/or angular momentum \
  of a group of atoms
"fix nph"_fix_nph.html - constant NPH time integration via Nose/Hoover
"fix npt"_fix_npt.html - constant NPT time integration via Nose/Hoover
"fix nve"_fix_nve.html - constant NVE time integration
"fix nve/gran"_fix_nve_gran.html - NVE time integration for \
     granular systems
"fix nvt"_fix_nvt.html - constant NVT time integration via Nose/Hoover
"fix orient/fcc"_fix_orient_fcc.html - add grain boundary migration force
"fix planeforce"_fix_planeforce.html - constrain atoms to move in a plane
"fix poems"_fix_poems.html - constrain clusters of atoms to move \
  as coupled rigid bodies
"fix print"_fix_print.html - print text and variables during a simulation
"fix rdf"_fix_rdf.html - compute radial distribution functions
"fix recenter"_fix_recenter.html - constrain the center-of-mass position \
  of a group of atoms
"fix rigid"_fix_rigid.html - constrain one or more clusters of atoms to \
     move as a rigid body
"fix setforce"_fix_setforce.html - set the force on each atom
"fix shake"_fix_shake.html - SHAKE constraints on bonds and/or angles
"fix spring"_fix_spring.html - apply harmonic spring force to group of atoms
"fix spring/rg"_fix_spring_rg.html - spring on radius of gyration of \
     group of atoms
"fix spring/self"_fix_spring_self.html - spring from each atom to its origin
"fix temp/rescale"_fix_temp_rescale.html - temperature control by \
     velocity rescaling
"fix tmd"_fix_tmd.html - guide a group of atoms to a new configuration
"fix uniaxial"_fix_uniaxial.html - uniaxial straining of system while \
     preserving total volume
"fix viscous"_fix_viscous.html - viscous damping for granular simulations
"fix volume/rescale"_fix_volume_rescale.html - density control by \
     volume rescaling
"fix wall/gran"_fix_wall_gran.html - frictional wall(s) for \
     granular simulations
"fix wall/lj93"_fix_wall_lj93.html - Lennard-Jones 9/3 wall
"fix wall/reflect"_fix_wall_reflect.html - reflecting wall(s)
"fix wiggle"_fix_wiggle.html - oscillate walls and frozen atoms :ul

[Restrictions:]

Some fix styles are part of specific packages.  They are only enabled
if LAMMPS was built with that package.  See the "Making
LAMMPS"_Section_start.html#2_2 section for more info.

The {freeze}, {gran/diag}, {gravity}, {insert}, {nve/gran}, and
{wall/gran} styles are part of the "granular" package.

The {poems} style is part of the "poems" package.

[Related commands:]

"unfix"_unfix.html, "fix_modify"_fix_modify.html

[Default:] none