forked from lijiext/lammps
167 lines
6.9 KiB
Plaintext
167 lines
6.9 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,Section_commands.html#comm)
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:line
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fix nph command :h3
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[Syntax:]
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fix ID group-ID nph p-style args keyword value ... :pre
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ID, group-ID are documented in "fix"_fix.html command :ulb,l
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nph = style name of this fix command :l
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p-style = {xyz} or {xy} or {yz} or {xz} or {aniso} :l
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{xyz} args = Pstart Pstop Pdamp
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Pstart,Pstop = desired pressure at start/end of run (pressure units)
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Pdamp = pressure damping parameter (time units)
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{xy} or {yz} or {xz} args = Px0 Px1 Py0 Py1 Pz0 Pz1 Pdamp
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Px0,Px1,Py0,Py1,Pz0,Pz1 = desired pressure in x,y,z at
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start/end (0/1) of run (pressure units)
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Pdamp = pressure damping parameter (time units)
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{aniso} args = Px0 Px1 Py0 Py1 Pz0 Pz1 Pdamp
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Px0,Px1,Py0,Py1,Pz0,Pz1 = desired pressure in x,y,z at
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start/end (0/1) of run (pressure units)
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Pdamp = pressure damping parameter (time units) :pre
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zero or more keyword/value pairs may be appended to the args :l
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keyword = {drag} or {dilate} :l
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{drag} value = drag factor added to barostat (0.0 = no drag)
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{dilate} value = {all} or {partial} :pre
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:ule
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[Examples:]
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fix 1 all nph xyz 0.0 0.0 1000.0
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fix 2 all nph xz 5.0 5.0 NULL NULL 5.0 5.0 1000.0 drag 1.0
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fix 2 all nph aniso 0.0 0.0 0.0 0.0 NULL NULL 1000.0 :pre
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[Description:]
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Perform constant NPH integration to update positions and velocities
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each timestep for atoms in the group using a Nose/Hoover pressure
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barostat. P is pressure. This creates a system trajectory consistent
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with the isobaric ensemble. Unlike "fix npt"_fix_npt.html,
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temperature will not be controlled if no other fix is used.
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Temperature can be controlled independently by using ""fix
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langevin"_fix_langevin.html or "fix
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temp/rescale"_fix_temp_rescale.html.
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The atoms in the fix group are the only ones whose velocities and
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positions are updated by the velocity/position update portion of the
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NPT integration.
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Regardless of what atoms are in the fix group, a global pressure is
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computed for all atoms. Similarly, when the size of the simulation
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box is changed, all atoms are re-scaled to new positions, unless the
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keyword {dilate} is specified with a value of {partial}, in which case
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only the atoms in the fix group are re-scaled. The latter can be
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useful for leaving the coordinates of atoms in a solid substrate
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unchanged and controlling the pressure of a surrounding fluid.
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:line
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The pressure can be controlled in one of several styles, as specified
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by the {p-style} argument. In each case, the desired pressure at each
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timestep is a ramped value during the run from the starting value to
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the end value. The "run"_run.html command documents how to make the
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ramping take place across multiple runs.
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Style {xyz} means couple all 3 dimensions together when pressure is
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computed (isotropic pressure), and dilate/contract the 3 dimensions
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together.
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Styles {xy} or {yz} or {xz} means that the 2 specified dimensions are
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coupled together, both for pressure computation and for
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dilation/contraction. The 3rd dimension dilates/contracts
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independently, using its pressure component as the driving force.
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For style {aniso}, all 3 dimensions dilate/contract independently
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using their individual pressure components as the 3 driving forces.
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For any of the styles except {xyz}, any of the independent pressure
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components (e.g. z in {xy}, or any dimension in {aniso}) can have
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their target pressures (both start and stop values) specified as NULL.
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This means that no pressure control is applied to that dimension so
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that the box dimension remains unchanged.
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In some cases (e.g. for solids) the pressure (volume) and/or
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temperature of the system can oscillate undesirably when a Nose/Hoover
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barostat is applied. The optional {drag} keyword will damp these
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oscillations, although it alters the Nose/Hoover equations. A value
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of 0.0 (no drag) leaves the Nose/Hoover formalism unchanged. A
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non-zero value adds a drag term; the larger the value specified, the
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greater the damping effect. Performing a short run and monitoring the
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pressure is the best way to determine if the drag term is working.
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Typically a value between 0.2 to 2.0 is sufficient to damp
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oscillations after a few periods.
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For all pressure styles, the simulation box stays rectangular in
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shape. Parinello-Rahman boundary conditions (tilted box) are not
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implemented in LAMMPS.
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For all styles, the {Pdamp} parameter operates like the {Tdamp}
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parameter, determining the time scale on which pressure is relaxed.
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For example, a value of 1000.0 means to relax the pressure in a
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timespan of (roughly) 1000 time units (tau or fmsec or psec - see the
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"units"_units.html command).
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:line
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This fix computes a temperature and pressure each timestep. To do
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this, the fix creates its own computes of style "temp" and "pressure",
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as if these commands had been issued:
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compute fix-ID_temp group-ID temp :pre
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compute fix-ID_press group-ID pressure fix-ID_temp :pre
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See the "compute temp"_compute_temp.html and "compute
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pressure"_compute_pressure.html commands for details. Note that the
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IDs of the new computes are the fix-ID with underscore + "temp" or
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"press" appended and the group for the new computes is the same as the
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fix group.
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Note that these are NOT the computes used by thermodynamic output (see
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the "thermo_style"_thermo_style.html command) with ID = {thermo_temp}
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and {thermo_press}. This means you can change the attributes of this
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fix's temperature or pressure via the
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"compute_modify"_compute_modify.html command or print this temperature
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or pressure during thermodyanmic output via the "thermo_style
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custom"_thermo_style.html command using the appropriate compute-ID.
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It also means that changing attributes of {thermo_temp} or
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{thermo_press} will have no effect on this fix. Alternatively, you
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can directly assign a new compute (for calculating temeperature or
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pressure) that you have defined to this fix via the
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"fix_modify"_fix_modify.html command. If you do this, note that the
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kinetic energy derived from T should be consistent with the virial
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term computed using all atoms. LAMMPS will warn you if you choose to
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compute temperature on a subset of atoms.
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This fix makes a contribution to the potential energy of the system
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that can be included in thermodynamic output of potential energy using
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the "fix_modify energy"_fix_modify.html option. The contribution can
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also be printed by itself via the keyword {f_fix-ID} in the
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"thermo_style custom"_thermo_style.html command.
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[Restrictions:]
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Any dimension being adjusted by this fix must be periodic. A
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dimension whose target pressures are specified as NULL can be
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non-periodic or periodic.
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You should not use "fix nvt"_fix_nvt.html with this fix. Instead, use
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"fix npt"_fix_npt.html if you want to control both temperature and
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pressure via Nose/Hoover.
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[Related commands:]
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"fix nve"_fix_nve.html, "fix npt"_fix_npt.html,
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"fix_modify"_fix_modify.html
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[Default:]
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The keyword defaults are drag = 0.0 and dilate = all.
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