forked from lijiext/lammps
98 lines
3.7 KiB
Plaintext
98 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 pressure command :h3
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[Syntax:]
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compute ID group-ID pressure temp-ID keyword ... :pre
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ID, group-ID are documented in "compute"_compute.html command
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pressure = style name of this compute command
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temp-ID = ID of compute that calculates temperature
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zero or more keywords may be appended
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keyword = {ke} or {pair} or {bond} or {angle} or {dihedral} or {improper} or {kspace} or {fix} :ul
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[Examples:]
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compute 1 all pressure myTemp
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compute 1 all pressure thermo_temp pair bond :pre
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[Description:]
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Define a computation that calculates the pressure of the entire system
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of atoms. The specified group must be "all". See the "compute
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stress/atom"_compute_stress_atom.html command if you want per-atom
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pressure (stress). These per-atom values could be summed for a group
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of atoms via the "compute sum"_compute_sum.html command.
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The pressure is computed by the formula
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:c,image(Eqs/pressure.jpg)
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where N is the number of atoms in the system (see discussion of DOF
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below), Kb is the Boltzmann constant, T is the temperature, d is the
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dimensionality of the system (2 or 3 for 2d/3d), V is the system
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volume (or area in 2d), and the second term is the virial, computed
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within LAMMPS for all pairwise as well as 2-body, 3-body, and 4-body,
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and long-range interactions. "Fixes"_fix.html that impose constraints
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(e.g. the "fix shake"_fix_shake.html command) also contribute to the
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virial term.
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A 6-component pressure tensor is also calculated by this compute whose
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componenents can be output by the "thermo_style
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custom"_thermo_style.html command or accessed by other
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"compute"_compute.html and "fix"_fix.html commands. The equation for
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the components of the tensor is the same as in above formula, except
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that the first term uses the components of the kinetic energy tensor
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(vx * vy instead of v^2 for temperature) and the second term uses Rx *
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Fy for the Wxy component of the virial tensor, etc.
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If no extra keywords are listed, the entire equation above is
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calculated which includes a temperature term (or kinetic energy
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tensor) and the virial as the sum of pair, bond, angle, dihedral,
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improper, kspace (long-range), and fix contributions. If any extra
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keywords are listed, then only those components are summed to compute
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ke and/or the virial.
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The temperature and kinetic energy tensor is not calculated by this
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compute, but rather by the temperature compute specified with the
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command. Normally this compute should calculate the temperature of
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all atoms for consistency with the virial term, but any compute style
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that calculates temperature can be used, e.g. one that excludes frozen
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atoms or other degrees of freedom.
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Note that the N is the above formula is really degrees-of-freedom
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where the DOF is specified by the temperature compute. See the
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various "compute temperature"_compute.html styles for details.
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A compute of this style with the ID of "thermo_press" is created when
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LAMMPS starts up, as if this command were in the input script:
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compute thermo_press all pressure thermo_temp :pre
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where "thermo_temp" is the ID of a similarly defined compute of style
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"temp". See the "thermo_style" command for more details.
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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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stress/atom"_compute_stress_atom.html,
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"thermo_style"_themo_style.html,
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[Default:] none
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