2007-02-10 05:40:32 +08:00
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"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 compute-ID :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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compute-ID = ID of compute that calculates temperature :ul
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[Examples:]
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compute 1 all pressure myTemp :pre
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[Description:]
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Define a computation that calculates the pressure of atoms averaged
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over the entire system. The specified group must be "all". See the
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"dump custom"_dump.html command for how to dump the per-atom stress
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tensor if you want more localized information about pressure (stress)
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in your system.
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The pressure is computed by the standard 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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2007-07-04 03:54:12 +08:00
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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, 4-body, and
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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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2007-02-10 05:40:32 +08:00
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A 6-component pressure tensor is also calculated by this compute which
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can be output by the "thermo_style custom"_thermo_style.html command.
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The formula for the components of the tensor is the same as in above
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formula, except that the first term uses the components of the kinetic
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energy tensor (vx * vy instead of v^2 for temperature) and the second
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term uses Rx * Fy for the Wxy component of the virial tensor, etc.
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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 as the last
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argument of the command. Normally this compute should calculate the
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temperature of all atoms for consistency with the virial term, but any
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compute style that calculates temperature can be used, e.g. one that
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excludes frozen atoms or other degrees of freedom.
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2007-07-04 03:54:12 +08:00
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Note that the N is the above formula is really degrees-of-freedom/d
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2007-02-10 05:40:32 +08:00
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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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[Restrictions:] none
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[Related commands:]
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"compute temp"_compute_temp.html, "thermo_style"_themo_style.html
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[Default:] none
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