forked from lijiext/lammps
103 lines
4.1 KiB
HTML
103 lines
4.1 KiB
HTML
<HTML>
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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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</CENTER>
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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 temp-ID keyword ...
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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>temp-ID = ID of compute that calculates temperature
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<LI>zero or more keywords may be appended
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<LI>keyword = <I>ke</I> or <I>pair</I> or <I>bond</I> or <I>angle</I> or <I>dihedral</I> or <I>improper</I> or <I>kspace</I> or <I>fix</I>
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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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compute 1 all pressure thermo_temp pair bond
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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 the entire system
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of atoms. The specified group must be "all". See the <A HREF = "compute_stress_atom.html">compute
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stress/atom</A> command if you want per-atom
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pressure (stress). These per-atom values could be summed for a group
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of atoms via the <A HREF = "compute_sum.html">compute sum</A> command.
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</P>
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<P>The pressure is computed by the 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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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, and 4-body,
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and 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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</P>
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<P>A 6-component pressure tensor is also calculated by this compute whose
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componenents can be output by the <A HREF = "thermo_style.html">thermo_style
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custom</A> command or accessed by other
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<A HREF = "compute.html">compute</A> and <A HREF = "fix.html">fix</A> commands. The equation for
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the components of the tensor is the same as in above formula, except
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that the first term uses the components of the kinetic energy tensor
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(vx * vy instead of v^2 for temperature) and the second term uses Rx *
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Fy for the Wxy component of the virial tensor, etc.
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</P>
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<P>If no extra keywords are listed, the entire equation above is
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calculated which includes a temperature term (or kinetic energy
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tensor) and the virial as the sum of pair, bond, angle, dihedral,
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improper, kspace (long-range), and fix contributions. If any extra
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keywords are listed, then only those components are summed to compute
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ke and/or the virial.
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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 with the
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command. Normally this compute should calculate the temperature of
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all atoms for consistency with the virial term, but any compute style
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that calculates temperature can be used, e.g. one that excludes frozen
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atoms or other degrees of freedom.
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</P>
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<P>Note that the N is the above formula is really degrees-of-freedom
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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>A compute of this style with the ID of "thermo_press" is created when
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LAMMPS starts up, as if this command were in the input script:
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</P>
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<PRE>compute thermo_press all pressure thermo_temp
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</PRE>
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<P>where "thermo_temp" is the ID of a similarly defined compute of style
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"temp". See the "thermo_style" command for more details.
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</P>
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<P><B>Output info:</B>
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</P>
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<P>The scalar value calculated by this compute is "intensive", meaning it
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is independent of the number of atoms in the simulation. The vector
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values are "extensive", meaning they scale with the number of atoms in
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the simulation.
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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 = "compute_stress_atom.html">compute
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stress/atom</A>,
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<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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