forked from lijiext/lammps
83 lines
3.0 KiB
Plaintext
83 lines
3.0 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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compute temp/com command :h3
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[Syntax:]
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compute ID group-ID temp/com :pre
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ID, group-ID are documented in "compute"_compute.html command
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temp/com = style name of this compute command :ul
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[Examples:]
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compute 1 all temp/com
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compute myTemp mobile temp/com :pre
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[Description:]
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Define a computation that calculates the temperature of a group of
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atoms, after subtracting out the center-of-mass velocity of the group.
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This is useful if the group is expected to have a non-zero net
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velocity for some reason. A compute of this style can be used by any
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command that computes a temperature,
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e.g. "thermo_modify"_thermo_modify.html, "fix
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temp/rescale"_fix_temp_rescale.html, "fix npt"_fix_npt.html, etc.
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The temperature is calculated by the formula KE = dim/2 N k T, where
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KE = total kinetic energy of the group of atoms (sum of 1/2 m v^2),
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dim = 2 or 3 = dimensionality of the simulation, N = number of atoms
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in the group, k = Boltzmann constant, and T = temperature.
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A 6-component kinetic energy tensor is also calculated by this compute
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for use in the computation of a pressure tensor. The formula for the
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components of the tensor is the same as the above formula, except that
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v^2 is replaced by vx * vy for the xy component, etc.
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The number of atoms contributing to the temperature is assumed to be
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constant for the duration of the run; use the {dynamic} option of the
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"compute_modify"_compute_modify.html command if this is not the case.
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The removal of the center-of-mass velocity by this fix is essentially
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computing the temperature after a "bias" has been removed from the
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velocity of the atoms. If this compute is used with a fix command
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that performs thermostatting then this bias will be subtracted from
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each atom, thermostatting of the remaining thermal velocity will be
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performed, and the bias will be added back in. Thermostatting fixes
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that work in this way include "fix nvt"_fix_nvt.html, "fix
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temp/rescale"_fix_temp_rescale.html, "fix
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temp/berendsen"_fix_temp_berendsen, and "fix
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langevin"_fix_langevin.html.
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This compute subtracts out degrees-of-freedom due to fixes that
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constrain molecular motion, such as "fix shake"_fix_shake.html and
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"fix rigid"_fix_rigid.html. This means the temperature of groups of
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atoms that include these constraints will be computed correctly. If
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needed, the subtracted degrees-of-freedom can be altered using the
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{extra} option of the "compute_modify"_compute_modify.html command.
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See "this howto section"_Section_howto.html#4_16 of the manual for a
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discussion of different ways to compute temperature and perform
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thermostatting.
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[Output info:]
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The scalar value calculated by this compute is "intensive", meaning it
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is independent of the number of atoms in the simulation. The vector
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values are "extensive", meaning they scale with the number of atoms in
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the simulation.
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[Restrictions:] none
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[Related commands:]
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"compute temp"_compute_temp.html
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[Default:] none
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