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 respa or dynamic/dof
temp value = compute ID that calculates a temperature press value = compute ID that calculates a pressure energy 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 recompute the number of degrees of freedom (DOF) contributing to the temperature
fix_modify 3 temp myTemp press myPress fix_modify 1 energy yes fix_modify tether respa 2
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.
For fixes that calculate a contribution to the potential energy of the system, the energy keyword will include that contribution in thermodynamic output of potential energy. This is because the energy yes setting must be specified to include the fix’s global or per-atom energy 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.
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.
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 evaporation), then
this option should be used to insure the temperature is correctly
The option defaults are temp = ID defined by fix, press = ID defined by fix, energy = no, respa = 0.