Syntax:
compute ID group-ID pressure compute-ID
Examples:
compute 1 all pressure myTemp
Description:
Define a computation that calculates the pressure of atoms averaged over the entire system. The specified group must be "all". See the dump custom command for how to dump the per-atom stress tensor if you want more localized information about pressure (stress) in your system.
The pressure is computed by the standard formula
where N is the number of atoms in the system (see discussion of DOF below), Kb is the Boltzmann constant, T is the temperature, d is the dimensionality of the system (2 or 3 for 2d/3d), V is the system volume (or area in 2d), and the second term is the virial, computed within LAMMPS for all pairwise as well as 2-body, 3-body, 4-body, and long-range interactions. Fixes that impose constraints (e.g. the fix shake command) also contribute to the virial term.
A 6-component pressure tensor is also calculated by this compute which can be output by the thermo_style custom command. The formula for the components of the tensor is the same as in above formula, except that the first term uses the components of the kinetic energy tensor (vx * vy instead of v^2 for temperature) and the second term uses Rx * Fy for the Wxy component of the virial tensor, etc.
The temperature and kinetic energy tensor is not calculated by this compute, but rather by the temperature compute specified as the last argument of the command. Normally this compute should calculate the temperature of all atoms for consistency with the virial term, but any compute style that calculates temperature can be used, e.g. one that excludes frozen atoms or other degrees of freedom.
Note that the N is the above formula is really degrees-of-freedom/d where the DOF is specified by the temperature compute. See the various compute temperature styles for details.
Restrictions: none
Related commands:
Default: none