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