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<HTML>
<CENTER><A HREF = "http://lammps.sandia.gov">LAMMPS WWW Site</A> - <A HREF = "Manual.html">LAMMPS Documentation</A> - <A HREF = "Section_commands.html#comm">LAMMPS Commands</A>
</CENTER>
<HR>
<H3>compute pressure command
</H3>
<P><B>Syntax:</B>
</P>
<PRE>compute ID group-ID pressure compute-ID
</PRE>
<UL><LI>ID, group-ID are documented in <A HREF = "compute.html">compute</A> command
<LI>pressure = style name of this compute command
<LI>compute-ID = ID of compute that calculates temperature
</UL>
<P><B>Examples:</B>
</P>
<PRE>compute 1 all pressure myTemp
</PRE>
<P><B>Description:</B>
</P>
<P>Define a computation that calculates the pressure of atoms averaged
over the entire system. The specified group must be "all". See the
<A HREF = "dump.html">dump custom</A> command for how to dump the per-atom stress
tensor if you want more localized information about pressure (stress)
in your system.
</P>
<P>The pressure is computed by the standard formula
</P>
<CENTER><IMG SRC = "Eqs/pressure.jpg">
</CENTER>
<P>where N is the number of atoms in the system (see discussion of DOF
below), Kb is the Boltzmann constant, T is the temperature, V is the
system volume, and the second term is the virial, computed within
LAMMPS for all pairwise as well as 2-body, 3-body, 4-body bonded
interactions.
</P>
<P>A 6-component pressure tensor is also calculated by this compute which
can be output by the <A HREF = "thermo_style.html">thermo_style custom</A> 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.
</P>
<P>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.
</P>
<P>Note that the N is the above formula is really degrees-of-freedom/3
where the DOF is specified by the temperature compute. See the
various <A HREF = "compute.html">compute temperature</A> styles for details.
</P>
<P><B>Restrictions:</B> none
</P>
<P><B>Related commands:</B>
</P>
<P><A HREF = "compute_temp.html">compute temp</A>, <A HREF = "themo_style.html">thermo_style</A>
</P>
<P><B>Default:</B> none
</P>
</HTML>
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