2007-02-10 05:40:32 +08:00
|
|
|
<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>
|
2007-11-02 00:46:50 +08:00
|
|
|
<P>Define a computation that calculates the pressure of the entire system
|
|
|
|
|
of atoms. The specified group must be "all". See the <A HREF = "compute_stress_atom.html">compute
|
|
|
|
|
stress/atom</A> command if you want per-atom
|
|
|
|
|
pressure (stress). These per-atom values could be summed for a group
|
|
|
|
|
of atoms via the <A HREF = "compute_sum.html">compute sum</A> command.
|
2007-02-10 05:40:32 +08:00
|
|
|
</P>
|
2007-11-02 00:46:50 +08:00
|
|
|
<P>The pressure is computed by the formula
|
2007-02-10 05:40:32 +08:00
|
|
|
</P>
|
|
|
|
|
<CENTER><IMG SRC = "Eqs/pressure.jpg">
|
|
|
|
|
</CENTER>
|
|
|
|
|
<P>where N is the number of atoms in the system (see discussion of DOF
|
2007-07-04 03:54:12 +08:00
|
|
|
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. <A HREF = "fix.html">Fixes</A> that impose constraints
|
|
|
|
|
(e.g. the <A HREF = "fix_shake.html">fix shake</A> command) also contribute to the
|
|
|
|
|
virial term.
|
2007-02-10 05:40:32 +08:00
|
|
|
</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>
|
2007-07-04 03:54:12 +08:00
|
|
|
<P>Note that the N is the above formula is really degrees-of-freedom/d
|
2007-02-10 05:40:32 +08:00
|
|
|
where the DOF is specified by the temperature compute. See the
|
|
|
|
|
various <A HREF = "compute.html">compute temperature</A> styles for details.
|
|
|
|
|
</P>
|
2007-11-02 00:46:50 +08:00
|
|
|
<P>A compute of this style with the ID of "thermo_press" is created when
|
|
|
|
|
LAMMPS starts up, as if this command were in the input script:
|
|
|
|
|
</P>
|
|
|
|
|
<PRE>compute thermo_press all pressure thermo_temp
|
|
|
|
|
</PRE>
|
|
|
|
|
<P>where "thermo_temp" is the ID of a similarly defined compute of style
|
|
|
|
|
"temp". See the "thermo_style" command for more details.
|
|
|
|
|
</P>
|
2007-02-10 05:40:32 +08:00
|
|
|
<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>
|
