2007-02-10 05:40:32 +08:00
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<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>
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<HR>
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<H3>compute pressure command
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</H3>
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<P><B>Syntax:</B>
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</P>
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<PRE>compute ID group-ID pressure compute-ID
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</PRE>
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<UL><LI>ID, group-ID are documented in <A HREF = "compute.html">compute</A> command
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<LI>pressure = style name of this compute command
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<LI>compute-ID = ID of compute that calculates temperature
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</UL>
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<P><B>Examples:</B>
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</P>
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<PRE>compute 1 all pressure myTemp
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</PRE>
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<P><B>Description:</B>
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</P>
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<P>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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<A HREF = "dump.html">dump custom</A> 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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</P>
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<P>The pressure is computed by the standard formula
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</P>
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<CENTER><IMG SRC = "Eqs/pressure.jpg">
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</CENTER>
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<P>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. <A HREF = "fix.html">Fixes</A> that impose constraints
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(e.g. the <A HREF = "fix_shake.html">fix shake</A> 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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</P>
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<P>A 6-component pressure tensor is also calculated by this compute which
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can be output by the <A HREF = "thermo_style.html">thermo_style custom</A> 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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</P>
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<P>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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</P>
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2007-07-04 03:54:12 +08:00
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<P>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 <A HREF = "compute.html">compute temperature</A> styles for details.
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</P>
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<P><B>Restrictions:</B> none
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</P>
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<P><B>Related commands:</B>
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</P>
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<P><A HREF = "compute_temp.html">compute temp</A>, <A HREF = "themo_style.html">thermo_style</A>
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</P>
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<P><B>Default:</B> none
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</P>
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</HTML>
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