forked from lijiext/lammps
122 lines
5.1 KiB
Plaintext
122 lines
5.1 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,Section_commands.html#comm)
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:line
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fix langevin command :h3
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[Syntax:]
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fix ID group-ID langevin Tstart Tstop damp seed keyword values ... :pre
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ID, group-ID are documented in "fix"_fix.html command :ulb,l
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langevin = style name of this fix command :l
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Tstart,Tstop = desired temperature at start/end of run (temperature units) :l
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damp = damping parameter (time units) :l
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seed = random # seed to use for white noise (integer > 0 and < 900000000) :l
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zero or more keyword/value pairs may be appended to the args :l
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keyword = {axes} or {scale} or {region}
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{axes} values = xflag yflag zflag
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xflag,yflag,zflag = 0/1 to exclude/include a dimension in the thermostat
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{scale} values = type ratio
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type = atom type (1-N)
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ratio = factor to scale the damping coefficient by
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{region} values = region-ID
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region-ID = ID of region to apply thermostat to :pre
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:ule
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[Examples:]
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fix 3 boundary langevin 1.0 1.0 1000.0 699483
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fix 1 all langevin 1.0 1.1 100.0 48279 axes 0 1 1
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fix 3 boundary langevin 1.0 1.0 1000.0 699483 region boundary :pre
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[Description:]
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Apply a Langevin thermostat to a group of atoms which models an
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interaction with a background implicit solvent. Used with "fix
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nve"_fix_nve.html, this command performs Brownian dynamics (BD), since
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the total force on each atom will have the form:
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F = Fc + Ff + Fr :pre
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Fc is the conservative force computed via the usual inter-particle
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interactions ("pair_style"_pair_style.html,
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"bond_style"_bond_style.html, etc).
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The Ff and Fr terms are added by this fix. Ff = - gamma v and is a
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frictional drag or viscous damping term proportional to the particle's
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velocity. Gamma for each atom is computed as m/damp, where m is the
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mass of the particle and damp is the damping factor specified by the
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user.
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Fr is a force due to solvent atoms at a temperature T randomly bumping
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into the particle. As derived from the fluctuation/dissipation
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theorum, its magnitude is proportional to sqrt(T m / dt damp), where T
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is the desired temperature, m is the mass of the particle, dt is the
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timestep size, and damp is the damping factor. Random numbers are
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used to randomize the direction and magnitude of this force.
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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.
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The {damp} parameter is specified in time units and determines how
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rapidly the temperature is relaxed. For example, a value of 100.0
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means to relax the temperature in a timespan of (roughly) 100 time
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units (tau or fmsec or psec - see the "units"_units.html command).
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The damp factor can be thought of as inversely related to the
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viscosity of the solvent. I.e. a small relaxation time implies a
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hi-viscosity solvent and vice versa. See the discussion about gamma
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and viscosity in the documentation for the "fix
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viscous"_fix_viscous.html command for more details.
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The random # {seed} should be a non-zero integer with 1 to 8 digits.
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A Marsaglia random number generator is used. Each processor uses the
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input seed to generate its own unique seed and its own stream of
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random numbers. Thus the dynamics of the system will not be identical
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on two runs on different numbers of processors. Also, the state of
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the random number generator is not saved in a restart file. This
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means you cannot do exact restarts when a fix {langevin} command is
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used.
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The keyword {axes} can be used to specify which dimensions to add Ff
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and Fr to. A flag of 0 means skip that dimension; a flag of 1 means
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include that dimension. The default is 1 for all 3 dimensions.
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The keyword {scale} allows the damp factor to be scaled up or down by
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the specified factor for atoms of that type. It can be used multiple
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times to adjust damp for several atom types. Note that specifying a
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ratio of 2 increase the relaxation time which is equivalent to the the
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solvent's viscosity acting on particles with 1/2 the diameter. This
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is the opposite effect of scale factors used by the "fix
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viscous"_fix_viscous.html command, since the damp factor in fix
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{langevin} is inversely related to the gamma factor in fix {viscous}.
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Also note that the damping factor in fix {langevin} includes the
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particle mass in Ff, unlike fix {viscous}. Thus the mass and size of
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different atom types should be accounted for in the choice of ratio
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values.
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The keyword {region} applies the fix only to atoms that are in the
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specified geometric region (and in the fix group). Since atoms can
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enter/leave a region, this test is performed each timestep.
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As noted above, fix {langevin} does not update the coordinates or
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velocities of its atoms, only the forces. It is normally used with a
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"fix nve"_fix_nve.html that does the time integration. Fix {langevin}
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should not normally be used on atoms that also have their temperature
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controlled by another fix - e.g. a "nvt"_fix_nvt.html or
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"temp/rescale"_fix_temp_rescale.html fix.
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[Restrictions:] none
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[Related commands:]
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"fix nvt"_fix_nvt.html, "fix temp/rescale"_fix_temp_rescale.html, "fix
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viscous"_fix_viscous.html
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[Default:] none
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