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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>
</CENTER>
<HR>
<H3>compute stress/atom command
</H3>
<P><B>Syntax:</B>
</P>
<PRE>compute ID group-ID stress/atom
</PRE>
<UL><LI>ID, group-ID are documented in <A HREF = "compute.html">compute</A> command
<LI>stress/atom = style name of this compute command
</UL>
<P><B>Examples:</B>
</P>
<PRE>compute 1 mobile stress/atom
</PRE>
<P><B>Description:</B>
</P>
<P>Define a computation that computes the per-atom stress tensor for each
atom in a group. The 6 components can be output via the <A HREF = "dump.html">dump
custom</A> command.
</P>
<P>The stress tensor is computed for only pairwise forces where the <I>ab</I>
component of stress on atom <I>i</I> is given by
</P>
<CENTER><IMG SRC = "Eqs/stress_tensor.jpg">
</CENTER>
<P>where the first term is a kinetic energy component for atom <I>i</I>, <I>j</I>
loops over the <I>N</I> neighbors of atom <I>i</I>, and <I>Fb</I> is one of 3
components of force on atom <I>i</I> due to atom <I>j</I>. Both <I>a</I> and <I>b</I>
take on values x,y,z to generate the 6 components of the symmetric
tensor.
</P>
<P>Note that this formula for stress does not include virial
contributions from intra-molecular interactions (e.g. bonds, angles,
torsions, etc). Also note that this quantity is the negative of the
per-atom pressure tensor. It is also really a stress-volume
formulation. It would need to be divided by a per-atom volume to have
units of stress, but an individual atom's volume is not easy to
compute in a deformed solid. Thus, if you sum the diagonal components
of the per-atom stress tensor for all atoms in the system and divide
the sum by 3V, where V is the volume of the system, you should get -P,
where P is the total pressure of the system.
</P>
<P>Computation of per-atom stress tensor components requires a loop thru
the neighbor list and inter-processor communication, so it can be
inefficient to compute/dump this quantity too frequently or to have
multiple compute/dump commands, each of a <I>stress/atom</I> style.
</P>
<P><B>Restrictions:</B> none
</P>
<P><B>Related commands:</B> none
</P>
<P><B>Default:</B> none
</P>
</HTML>
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