# fix_modify command

## Syntax

fix_modify fix-ID keyword value ...

• fix-ID = ID of the fix to modify

• one or more keyword/value pairs may be appended

• keyword = temp or press or energy or virial or respa or dynamic/dof or bodyforces

temp value = compute ID that calculates a temperature
press value = compute ID that calculates a pressure
energy value = yes or no
virial value = yes or no
respa value = 1 to max respa level or 0 (for outermost level)
dynamic/dof value = yes or no
yes/no = do or do not re-compute the number of degrees of freedom (DOF) contributing to the temperature
bodyforces value = early or late
early/late = compute rigid-body forces/torques early or late in the timestep

## Examples

fix_modify 3 temp myTemp press myPress
fix_modify 1 energy yes
fix_modify tether respa 2


## Description

Modify one or more parameters of a previously defined fix. Only specific fix styles support specific parameters. See the doc pages for individual fix commands for info on which ones support which fix_modify parameters.

The temp keyword is used to determine how a fix computes temperature. The specified compute ID must have been previously defined by the user via the compute command and it must be a style of compute that calculates a temperature. All fixes that compute temperatures define their own compute by default, as described in their documentation. Thus this option allows the user to override the default method for computing T.

The press keyword is used to determine how a fix computes pressure. The specified compute ID must have been previously defined by the user via the compute command and it must be a style of compute that calculates a pressure. All fixes that compute pressures define their own compute by default, as described in their documentation. Thus this option allows the user to override the default method for computing P.

The energy keyword can be used with fixes that support it. energy yes adds a contribution to the potential energy of the system. The fix’s global and per-atom energy is included in the calculation performed by the compute pe or compute pe/atom commands. See the thermo_style command for info on how potential energy is output. For fixes that tally a global energy, it can be printed by using the keyword f_ID in the thermo_style custom command, where ID is the fix-ID of the appropriate fix.

Note

You must also specify the energy yes setting for a fix if you are using it when performing an energy minimization and if you want the energy and forces it produces to be part of the optimization criteria.

The virial keyword can be used with fixes that support it. virial yes adds a contribution to the virial of the system. The fix’s global and per-atom virial is included in the calculation performed by the compute pressure or compute stress/atom commands. See the thermo_style command for info on how pressure is output.

Note

You must specify the virial yes setting for a fix if you are doing box relaxation and if you want virial contribution of the fix to be part of the relaxation criteria, although this seems unlikely.

Note

This option is only supported by fixes that explicitly say so. For some of these (e.g. the fix shake command) the default setting is virial yes, for others it is virial no.

For fixes that set or modify forces, it may be possible to select at which r-RESPA level the fix operates via the respa keyword. The RESPA level at which the fix is active can be selected. This is a number ranging from 1 to the number of levels. If the RESPA level is larger than the current maximum, the outermost level will be used, which is also the default setting. This default can be restored using a value of 0 for the RESPA level. The affected fix has to be enabled to support this feature; if not, fix_modify will report an error. Active fixes with a custom RESPA level setting are reported with their specified level at the beginning of a r-RESPA run.

The dynamic/dof keyword determines whether the number of atoms N in the fix group and their associated degrees of freedom are re-computed each time a temperature is computed. Only fix styles that calculate their own internal temperature use this option. Currently this is only the fix rigid/nvt/small and fix rigid/npt/small commands for the purpose of thermostatting rigid body translation and rotation. By default, N and their DOF are assumed to be constant. If you are adding atoms or molecules to the system (see the fix pour, fix deposit, and fix gcmc commands) or expect atoms or molecules to be lost (e.g. due to exiting the simulation box or via fix evaporate), then this option should be used to insure the temperature is correctly normalized.

Note

Other thermostatting fixes, such as fix nvt, do not use the dynamic/dof keyword because they use a temperature compute to calculate temperature. See the compute_modify dynamic/dof command for a similar way to insure correct temperature normalization for those thermostats.

The bodyforces keyword determines whether the forces and torques acting on rigid bodies are computed early at the post-force stage of each timestep (right after per-atom forces have been computed and communicated among processors), or late at the final-integrate stage of each timestep (after any other fixes have finished their post-force tasks). Only the rigid-body integration fixes use this option, which includes fix rigid and fix rigid/small, and their variants, and also fix poems.

The default is late. If there are other fixes that add forces to individual atoms, then the rigid-body constraints will include these forces when time-integrating the rigid bodies. If early is specified, then new fixes can be written that use or modify the per-body force and torque, before time-integration of the rigid bodies occurs. Note however this has the side effect, that fixes such as fix addforce, fix setforce, fix spring, which add forces to individual atoms will have no effect on the motion of the rigid bodies if they are specified in the input script after the fix rigid command. LAMMPS will give a warning if that is the case.

none

## Default

The option defaults are temp = ID defined by fix, press = ID defined by fix, energy = no, virial = different for each fix style, respa = 0, bodyforce = late.