forked from lijiext/lammps
95 lines
3.7 KiB
Plaintext
95 lines
3.7 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/partial command :h3
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[Syntax:]
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compute ID group-ID temp/partial xflag yflag zflag :pre
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ID, group-ID are documented in "compute"_compute.html command
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temp/partial = style name of this compute command
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xflag,yflag,zflag = 0/1 for whether to exclude/include this dimension :ul
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[Examples:]
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compute newT flow temp/partial 1 1 0 :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 excluding one or more velocity components. A compute of
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this style can be used by any 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 = dimensionality of the simulation, N = number of atoms in the
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group, k = Boltzmann constant, and T = temperature. The calculation
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of KE excludes the x, y, or z dimensions if xflag, yflag, or zflag =
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0. The dim parameter is adjusted to give the correct number of
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degrees of freedom.
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A kinetic energy tensor, stored as a 6-element vector, is also
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calculated by this compute for use in the calculation of a pressure
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tensor. The formula for the components of the tensor is the same as
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the above formula, except that v^2 is replaced by vx*vy for the xy
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component, etc. The 6 components of the vector are ordered xx, yy,
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zz, xy, xz, yz.
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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 velocity components 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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This compute calculates a global scalar (the temperature) and a global
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vector of length 6 (KE tensor), which can be accessed by indices 1-6.
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These values can be used by any command that uses global scalar or
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vector values from a compute as input. See "this
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section"_Section_howto.html#4_15 for an overview of LAMMPS output
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options.
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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, "compute
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temp/region"_compute_temp_region.html, "compute
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pressure"_compute_pressure.html
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[Default:] none
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