forked from lijiext/lammps
107 lines
4.2 KiB
Plaintext
107 lines
4.2 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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compute temp/ramp command :h3
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[Syntax:]
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compute ID group-ID temp/ramp vdim vlo vhi dim clo chi keyword value ... :pre
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ID, group-ID are documented in "compute"_compute.html command
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temp/ramp = style name of this compute command
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vdim = {vx} or {vy} or {vz}
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vlo,vhi = subtract velocities between vlo and vhi (velocity units)
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dim = {x} or {y} or {z}
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clo,chi = lower and upper bound of domain to subtract from (distance units)
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zero or more keyword/value pairs may be appended
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keyword = {units} :ul
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{units} value = {lattice} or {box} :pre
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[Examples:]
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compute 2nd middle temp/ramp vx 0 8 y 2 12 units lattice :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 an ramped velocity profile before
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computing the kinetic energy. A compute of this style can be used by
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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 meaning of the arguments for this command which define the
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velocity ramp are the same as for the "velocity ramp"_velocity.html
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command which was presumably used to impose the velocity.
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After the ramp velocity has been subtracted from the specified
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dimension for each atom, the temperature is calculated by the formula
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KE = dim/2 N k T, where KE = total kinetic energy of the group of
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atoms (sum of 1/2 m v^2), dim = 2 or 3 = dimensionality of the
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simulation, N = number of atoms in the group, k = Boltzmann constant,
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and T = temperature.
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The {units} keyword determines the meaning of the distance units used
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for coordinates (c1,c2) and velocities (vlo,vhi). A {box} value
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selects standard distance units as defined by the "units"_units.html
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command, e.g. Angstroms for units = real or metal. A {lattice} value
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means the distance units are in lattice spacings; e.g. velocity =
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lattice spacings / tau. The "lattice"_lattice.html command must have
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been previously used to define the lattice spacing.
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A 6-component kinetic energy tensor is also calculated by this compute
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for use in the calculation 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 ramped velocity component by this fix is
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essentially computing the temperature after a "bias" has been removed
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from the velocity of the atoms. If this compute is used with a fix
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command that performs thermostatting then this bias will be subtracted
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from each atom, thermostatting of the remaining thermal velocity will
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be performed, and the bias will be added back in. Thermostatting
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fixes 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, "compute
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temp/profie"_compute_temp_profile.html, "compute
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temp/deform"_compute_temp_deform.html, "compute
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pressure"_compute_pressure.html
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[Default:]
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The option default is units = lattice.
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