<UL><LI>ID, group-ID are documented in <A HREF = "compute.html">compute</A> command
<LI>stress/atom = style name of this compute command
<LI>zero or more keywords may be appended
<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> or <I>virial</I>
</UL>
<P><B>Examples:</B>
</P>
<PRE>compute 1 mobile stress/atom
compute 1 all stress/atom pair bond
</PRE>
<P><B>Description:</B>
</P>
<P>Define a computation that computes the symmetric per-atom stress
tensor for each atom in a group. The tensor for each atom has 6
components and is stored as a 6-element vector in the following order:
xx, yy, zz, xy, xz, yz. See the <A HREF = "compute_pressure.html">compute
pressure</A> command if you want the stress tensor
(pressure) of the entire system.
</P>
<P>The stress tensor for atom <I>I</I> is given by the following formula,
where <I>a</I> and <I>b</I> take on values x,y,z to generate the 6 components of
the symmetric tensor:
</P>
<CENTER><IMG SRC = "Eqs/stress_tensor.jpg">
</CENTER>
<P>The first term is a kinetic energy contribution for atom <I>I</I>. The
second term is a pairwise energy contribution where <I>n</I> loops over the
<I>Np</I> neighbors of atom <I>I</I>, <I>r1</I> and <I>r2</I> are the positions of the 2
atoms in the pairwise interaction, and <I>F1</I> and <I>F2</I> are the forces on
the 2 atoms resulting from the pairwise interaction. The third term
is a bond contribution of similar form for the <I>Nb</I> bonds which atom
<I>I</I> is part of. There are similar terms for the <I>Na</I> angle, <I>Nd</I>
dihedral, and <I>Ni</I> improper interactions atom <I>I</I> is part of. There
is also a term for the KSpace contribution from long-range Coulombic
interactions, if defined. Finally, there is a term for the <I>Nf</I>
<A HREF = "fix.html">fixes</A> that apply internal constraint forces to atom <I>I</I>.
Currently, only the <A HREF = "fix_shake.html">fix shake</A> and <A HREF = "fix_rigid.html">fix
rigid</A> commands contribute to this term.
</P>
<P>As the coefficients in the formula imply, a virial contribution
produced by a small set of atoms (e.g. 4 atoms in a dihedral or 3
atoms in a Tersoff 3-body interaction) is assigned in equal portions
to each atom in the set. E.g. 1/4 of the dihedral virial to each of
the 4 atoms, or 1/3 of the fix virial due to SHAKE constraints applied
to atoms in a a water molecule via the <A HREF = "fix_shake.html">fix shake</A>
command.
</P>
<P>If no extra keywords are listed, all of the terms in this formula are
included in the per-atom stress tensor. If any extra keywords are
listed, only those terms are summed to compute the tensor. The
<I>virial</I> keyword means include all terms except the kinetic energy
<I>ke</I>.
</P>
<P>Note that the stress for each atom is due to its interaction with all
other atoms in the simulation, not just with other atoms in the group.