forked from lijiext/lammps
106 lines
4.0 KiB
Plaintext
106 lines
4.0 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 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} or {virial} :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 reduce"_compute_reduce.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 symmetric pressure tensor, stored as a 6-element vector, is also
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calculated by this compute. The 6 components of the vector are
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ordered xx, yy, zz, xy, xz, yz. The equation for the I,J components
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(where I and J = x,y,z) is similar to the above formula, except that
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the first term uses components of the kinetic energy tensor and the
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second term uses components of the virial tensor:
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:c,image(Eqs/pressure_tensor.jpg)
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If no extra keywords are listed, the entire equations above are
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calculated which include a kinetic energy (temperature) term and the
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virial as the sum of pair, bond, angle, dihedral, improper, kspace
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(long-range), and fix contributions to the force on each atom. If any
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extra keywords are listed, then only those components are summed to
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compute temperature or ke and/or the virial. The {virial} keyword
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means include all terms except the kinetic energy {ke}.
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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 in the first formula above is really
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degrees-of-freedom divided by d = dimensionality, where the DOF value
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is calcluated by the temperature compute. See the various "compute
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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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This compute calculates a global scalar (the pressure) and a global
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vector of length 6 (pressure tensor), which can be accessed by indices
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1-6. These values can be used by any command that uses global scalar
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or 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 and vector values calculated by this compute are
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"intensive", meaning they are independent of 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"_thermo_style.html,
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[Default:] none
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