forked from lijiext/lammps
158 lines
7.1 KiB
Plaintext
158 lines
7.1 KiB
Plaintext
"LAMMPS WWW Site"_lws - "LAMMPS Documentation"_ld - "LAMMPS Commands"_lc :c
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:link(lws,http://lammps.sandia.gov)
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:link(ld,Manual.html)
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:link(lc,Commands_all.html)
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:line
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fix_modify command :h3
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[Syntax:]
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fix_modify fix-ID keyword value ... :pre
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fix-ID = ID of the fix to modify :ulb,l
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one or more keyword/value pairs may be appended :l
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keyword = {temp} or {press} or {energy} or {virial} or {respa} or {dynamic/dof} or {bodyforces} :l
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{temp} value = compute ID that calculates a temperature
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{press} value = compute ID that calculates a pressure
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{energy} value = {yes} or {no}
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{virial} value = {yes} or {no}
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{respa} value = {1} to {max respa level} or {0} (for outermost level)
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{dynamic/dof} value = {yes} or {no}
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yes/no = do or do not re-compute the number of degrees of freedom (DOF) contributing to the temperature
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{bodyforces} value = {early} or {late}
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early/late = compute rigid-body forces/torques early or late in the timestep :pre
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:ule
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[Examples:]
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fix_modify 3 temp myTemp press myPress
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fix_modify 1 energy yes
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fix_modify tether respa 2 :pre
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[Description:]
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Modify one or more parameters of a previously defined fix. Only
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specific fix styles support specific parameters. See the doc pages
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for individual fix commands for info on which ones support which
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fix_modify parameters.
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The {temp} keyword is used to determine how a fix computes
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temperature. The specified compute ID must have been previously
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defined by the user via the "compute"_compute.html command and it must
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be a style of compute that calculates a temperature. All fixes that
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compute temperatures define their own compute by default, as described
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in their documentation. Thus this option allows the user to override
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the default method for computing T.
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The {press} keyword is used to determine how a fix computes pressure.
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The specified compute ID must have been previously defined by the user
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via the "compute"_compute.html command and it must be a style of
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compute that calculates a pressure. All fixes that compute pressures
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define their own compute by default, as described in their
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documentation. Thus this option allows the user to override the
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default method for computing P.
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The {energy} keyword can be used with fixes that support it.
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{energy yes} adds a contribution to the potential energy of the
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system. The fix's global and per-atom
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energy is included in the calculation performed by the "compute
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pe"_compute_pe.html or "compute pe/atom"_compute_pe_atom.html
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commands. See the "thermo_style"_thermo_style.html command for info
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on how potential energy is output. For fixes that tally a global
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energy, it can be printed by using the keyword f_ID in the
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thermo_style custom command, where ID is the fix-ID of the appropriate
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fix.
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NOTE: You must also specify the {energy yes} setting for a fix if you
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are using it when performing an "energy minimization"_minimize.html
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and if you want the energy and forces it produces to be part of the
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optimization criteria.
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The {virial} keyword can be used with fixes that support it.
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{virial yes} adds a contribution to the virial of the
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system. The fix's global and per-atom
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virial is included in the calculation performed by the "compute
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pressure"_compute_pressure.html or
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"compute stress/atom"_compute_stress_atom.html
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commands. See the "thermo_style"_thermo_style.html command for info
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on how pressure is output.
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NOTE: You must specify the {virial yes} setting for a fix if you
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are doing "box relaxation"_fix_box_relax.html and
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if you want virial contribution of the fix to be part of the
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relaxation criteria, although this seems unlikely.
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NOTE: This option is only supported by fixes that explicitly say
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so. For some of these (e.g. the "fix shake"_fix_shake.html command)
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the default setting is {virial yes}, for others it is {virial no}.
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For fixes that set or modify forces, it may be possible to select at
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which "r-RESPA"_run_style.html level the fix operates via the {respa}
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keyword. The RESPA level at which the fix is active can be selected.
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This is a number ranging from 1 to the number of levels. If the RESPA
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level is larger than the current maximum, the outermost level will be
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used, which is also the default setting. This default can be restored
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using a value of {0} for the RESPA level. The affected fix has to be
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enabled to support this feature; if not, {fix_modify} will report an
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error. Active fixes with a custom RESPA level setting are reported
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with their specified level at the beginning of a r-RESPA run.
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The {dynamic/dof} keyword determines whether the number of atoms N in
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the fix group and their associated degrees of freedom are re-computed
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each time a temperature is computed. Only fix styles that calculate
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their own internal temperature use this option. Currently this is
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only the "fix rigid/nvt/small"_fix_rigid.html and "fix
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rigid/npt/small"_fix_rigid.html commands for the purpose of
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thermostatting rigid body translation and rotation. By default, N and
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their DOF are assumed to be constant. If you are adding atoms or
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molecules to the system (see the "fix pour"_fix_pour.html, "fix
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deposit"_fix_deposit.html, and "fix gcmc"_fix_gcmc.html commands) or
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expect atoms or molecules to be lost (e.g. due to exiting the
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simulation box or via "fix evaporate"_fix_evaporate.html), then
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this option should be used to insure the temperature is correctly
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normalized.
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NOTE: Other thermostatting fixes, such as "fix nvt"_fix_nh.html, do
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not use the {dynamic/dof} keyword because they use a temperature
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compute to calculate temperature. See the "compute_modify
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dynamic/dof"_compute_modify.html command for a similar way to insure
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correct temperature normalization for those thermostats.
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The {bodyforces} keyword determines whether the forces and torques
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acting on rigid bodies are computed {early} at the post-force stage of
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each timestep (right after per-atom forces have been computed and
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communicated among processors), or {late} at the final-integrate stage
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of each timestep (after any other fixes have finished their post-force
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tasks). Only the rigid-body integration fixes use this option, which
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includes "fix rigid"_fix_rigid.html and "fix
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rigid/small"_fix_rigid.html, and their variants, and also "fix
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poems"_fix_poems.html.
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The default is {late}. If there are other fixes that add forces to
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individual atoms, then the rigid-body constraints will include these
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forces when time-integrating the rigid bodies. If {early} is
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specified, then new fixes can be written that use or modify the
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per-body force and torque, before time-integration of the rigid bodies
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occurs. Note however this has the side effect, that fixes such as
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"fix addforce"_fix_addforce.html, "fix setforce"_fix_setforce.html,
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"fix spring"_fix_spring.html, which add forces to individual atoms
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will have no effect on the motion of the rigid bodies if they are
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specified in the input script after the fix rigid command. LAMMPS
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will give a warning if that is the case.
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[Restrictions:] none
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[Related commands:]
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"fix"_fix.html, "compute temp"_compute_temp.html, "compute
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pressure"_compute_pressure.html, "thermo_style"_thermo_style.html
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[Default:]
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The option defaults are temp = ID defined by fix, press = ID defined
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by fix, energy = no, virial = different for each fix style, respa = 0,
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bodyforce = late.
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