forked from lijiext/lammps
101 lines
4.0 KiB
Plaintext
101 lines
4.0 KiB
Plaintext
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"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 nvt/asphere command :h3
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[Syntax:]
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fix ID group-ID nvt/asphere Tstart Tstop Tdamp keyword value ... :pre
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ID, group-ID are documented in "fix"_fix.html command :ulb,l
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nvt/asphere = style name of this fix command :l
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Tstart,Tstop = desired temperature at start/end of run :l
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Tdamp = temperature damping parameter (time units) :l
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zero or more keyword/value pairs may be appended to the args :l
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keyword = {drag} :l
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{drag} value = drag factor added to thermostat (0.0 = no drag) :pre
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:ule
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[Examples:]
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fix 1 all nvt/asphere 300.0 300.0 100.0
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fix 1 all nvt/asphere 300.0 300.0 100.0 drag 0.2 :pre
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[Description:]
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Perform constant NVT integration to update positions, velocities,
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and angular velocities each timestep for aspherical or ellipsoidal
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particles in the group using a Nose/Hoover temperature thermostat.
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V is volume; T is temperature. This creates a system trajectory
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consistent with the canonical ensemble.
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The desired temperature at each timestep is a ramped value during the
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run from {Tstart} to {Tstop}. The "run"_run.html command documents
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how to make the ramping take place across multiple runs. The {Tdamp}
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parameter is specified in time units and determines how rapidly the
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temperature is relaxed. For example, a value of 100.0 means to relax
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the temperature in a timespan of (roughly) 100 time units (tau or
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fmsec or psec - see the "units"_units.html command).
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In some cases (e.g. for solids) the temperature of the system can
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oscillate undesirably when a Nose/Hoover thermostat is applied. The
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optional {drag} keyword will damp these oscillations, although it
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alters the Nose/Hoover equations. A value of 0.0 (no drag) leaves the
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Nose/Hoover formalism unchanged. A non-zero value adds a drag term;
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the larger the value specified, the greater the damping effect.
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Performing a short run and monitoring the temperature is the best way
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to determine if the drag term is working. Typically a value between
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0.2 to 2.0 is sufficient to damp oscillations after a few periods.
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This fix computes a temperature each timestep. To do this, the fix
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creates its own compute of style "temp/asphere", as if this command
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had been issued:
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compute fix-ID_temp group-ID temp/asphere :pre
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See the "compute temp/asphere"_compute_temp_asphere.html command for
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details. Note that the ID of the new compute is the fix-ID with
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underscore + "temp" appended and the group for the new compute is
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the same as the fix group.
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Note that this is NOT the compute used by thermodynamic output (see
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the "thermo_style"_thermo_style.html command) with ID = {thermo_temp}.
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This means you can change the attributes of this fix's temperature
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(e.g. its degrees-of-freedom) via the
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"compute_modify"_compute_modify.html command or print this temperature
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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} will have no
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effect on this fix. Alternatively, you can directly assign a new
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compute (for calculating temeperature) that you have defined to this
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fix via the "fix_modify"_fix_modify.html command.
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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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Can only be used if LAMMPS was built with the "asphere" package.
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The final Tstop cannot be 0.0 since it would make the target T = 0.0
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at some timestep during the simulation which is not allowed in
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the Nose/Hoover formulation.
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[Related commands:]
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"fix nvt"_fix_nvt.html, "fix nve_asphere"_fix_nve_asphere.html, "fix
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npt_asphere"_fix_npt_asphere.html, "fix_modify"_fix_modify.html
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[Default:]
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The keyword defaults are drag = 0.0.
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