forked from lijiext/lammps
168 lines
7.2 KiB
Plaintext
168 lines
7.2 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 temp/csvr command :h3
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fix temp/csld command :h3
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[Syntax:]
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fix ID group-ID temp/csvr Tstart Tstop Tdamp seed :pre
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fix ID group-ID temp/csld Tstart Tstop Tdamp seed :pre
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ID, group-ID are documented in "fix"_fix.html command :ulb,l
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temp/csvr or temp/csld = 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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Tstart can be a variable (see below) :pre
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Tdamp = temperature damping parameter (time units) :l
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seed = random number seed to use for white noise (positive integer) :l,ule
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[Examples:]
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fix 1 all temp/csvr 300.0 300.0 100.0 54324 :pre
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fix 1 all temp/csld 100.0 300.0 10.0 123321 :pre
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[Description:]
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Adjust the temperature with a canonical sampling thermostat that uses
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global velocity rescaling with Hamiltonian dynamics ({temp/csvr})
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"(Bussi1)"_#Bussi1, or Langevin dynamics ({temp/csld})
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"(Bussi2)"_#Bussi2. In the case of {temp/csvr} the thermostat is
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similar to the empirical Berendsen thermostat in
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"temp/berendsen"_fix_temp_berendsen.html, but chooses the actual
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scaling factor from a suitably chosen (gaussian) distribution rather
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than having it determined from the time constant directly. In the case
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of {temp/csld} the velocities are updated to a linear combination of
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the current velocities with a gaussian distribution of velocities at
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the desired temperature. Both termostats are applied every timestep.
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The thermostat is applied to only the translational degrees of freedom
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for the particles, which is an important consideration for finite-size
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particles which have rotational degrees of freedom are being
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thermostatted with these fixes. The translational degrees of freedom
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can also have a bias velocity removed from them before thermostatting
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takes place; see the description below.
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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 {Tdamp} parameter is specified in
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time units and determines how rapidly the temperature is relaxed. For
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example, a value of 100.0 means to relax the temperature in a timespan
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of (roughly) 100 time units (tau or fmsec or psec - see the
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"units"_units.html command).
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{Tstart} can be specified as an equal-style "variable"_variable.html.
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In this case, the {Tstop} setting is ignored. If the value is a
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variable, it should be specified as v_name, where name is the variable
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name. In this case, the variable will be evaluated each timestep, and
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its value used to determine the target temperature.
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Equal-style variables can specify formulas with various mathematical
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functions, and include "thermo_style"_thermo_style.html command
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keywords for the simulation box parameters and timestep and elapsed
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time. Thus it is easy to specify a time-dependent temperature.
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IMPORTANT NOTE: Unlike the "fix nvt"_fix_nh.html command which
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performs Nose/Hoover thermostatting AND time integration, these fixes
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do NOT perform time integration. They only modify velocities to effect
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thermostatting. Thus you must use a separate time integration fix,
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like "fix nve"_fix_nve.html to actually update the positions of atoms
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using the modified velocities. Likewise, these fixes should not
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normally be used on atoms that also have their temperature controlled
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by another fix - e.g. by "fix nvt"_fix_nh.html or "fix
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langevin"_fix_langevin.html commands.
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See "this howto section"_Section_howto.html#howto_16 of the manual for
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a discussion of different ways to compute temperature and perform
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thermostatting.
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These fixes compute a temperature each timestep. To do this, the fix
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creates its own compute of style "temp", as if this command had been
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issued:
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compute fix-ID_temp group-ID temp :pre
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See the "compute temp"_compute_temp.html command for details. Note
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that the ID of the new compute is the fix-ID + underscore + "temp",
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and the group for the new compute is 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 thermodynamic 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.
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Like other fixes that perform thermostatting, these fixes can be used
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with "compute commands"_compute.html that calculate a temperature
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after removing a "bias" from the atom velocities. E.g. removing the
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center-of-mass velocity from a group of atoms or only calculating
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temperature on the x-component of velocity or only calculating
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temperature for atoms in a geometric region. This is not done by
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default, but only if the "fix_modify"_fix_modify.html command is used
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to assign a temperature compute to this fix that includes such a bias
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term. See the doc pages for individual "compute
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commands"_compute.html to determine which ones include a bias. In
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this case, the thermostat works in the following manner: the current
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temperature is calculated taking the bias into account, bias is
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removed from each atom, thermostatting is performed on the remaining
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thermal degrees of freedom, and the bias is added back in.
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:line
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[Restart, fix_modify, output, run start/stop, minimize info:]
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No information about these fixes are written to "binary restart
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files"_restart.html.
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The "fix_modify"_fix_modify.html {temp} option is supported by these
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fixes. You can use it to assign a temperature "compute"_compute.html
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you have defined to these fixes which will be used in its thermostatting
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procedure, as described above. For consistency, the group used by
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these fixes and by the compute should be the same.
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These fixes can ramp its target temperature over multiple runs, using
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the {start} and {stop} keywords of the "run"_run.html command. See the
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"run"_run.html command for details of how to do this.
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These fixes are not invoked during "energy minimization"_minimize.html.
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These fixes compute a global scalar which can be accessed by various
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"output commands"_Section_howto.html#howto_15. The scalar is the
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cummulative energy change due to the fix. The scalar value
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calculated by this fix is "extensive".
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[Restrictions:]
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These fixes are not compatible with "fix shake"_fix_shake.html.
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The fix can be used with dynamic groups as defined by the
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"group"_group.html command. Likewise it can be used with groups to
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which atoms are added or deleted over time, e.g. a deposition
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simulation. However, the conservation properties of the thermostat
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and barostat are defined for systems with a static set of atoms. You
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may observe odd behavior if the atoms in a group vary dramatically
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over time or the atom count becomes very small.
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[Related commands:]
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"fix nve"_fix_nve.html, "fix nvt"_fix_nh.html, "fix
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temp/rescale"_fix_temp_rescale.html, "fix langevin"_fix_langevin.html,
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"fix_modify"_fix_modify.html, "compute temp"_compute_temp.html,
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"fix temp/berendsen"_fix_temp_berendsen.html
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[Default:] none
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:line
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:link(Bussi1)
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[(Bussi1)] Bussi, Donadio and Parrinello, J. Chem. Phys. 126, 014101(2007)
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:link(Bussi2)
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[(Bussi2)] Bussi and Parrinello, Phys. Rev. E 75, 056707 (2007)
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