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<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>pair_style nb3b/harmonic command
</H3>
<P><B>Syntax:</B>
</P>
<PRE>pair_style nb3b/harmonic
</PRE>
<P><B>Examples:</B>
</P>
<PRE>pair_style nb3b/harmonic
pair_coeff * * MgOH.nb3bharmonic Mg O H
</PRE>
<P><B>Description:</B>
</P>
<P>This pair style computes a nonbonded 3-body harmonic potential for the
energy E of a system of atoms as
</P>
<CENTER><IMG SRC = "Eqs/pair_3bnbharm.jpg">
</CENTER>
<P>where <I>theta_0</I> is the equilibrium value of the angle and <I>K</I> is a
prefactor. Note that the usual 1/2 factor is included in <I>K</I>. The form
of the potential is identical to that used in angle_style <I>harmonic</I>,
but in this case, the atoms do not need to be explicitly bonded.
</P>
<P>Only a single pair_coeff command is used with this style which
specifies a potential file with parameters for specified elements.
These are mapped to LAMMPS atom types by specifying N additional
arguments after the filename in the pair_coeff command, where N is the
number of LAMMPS atom types:
</P>
<UL><LI>filename
<LI>N element names = mapping of elements to atom types
</UL>
<P>See the <A HREF = "pair_coeff.html">pair_coeff</A> doc page for alternate ways
to specify the path for the potential file.
</P>
<P>As an example, imagine a file SiC.nb3bharmonic has potential values
for Si and C. If your LAMMPS simulation has 4 atoms types and you
want the 1st 3 to be Si, and the 4th to be C, you would use the
following pair_coeff command:
</P>
<PRE>pair_coeff * * SiC.nb3bharmonic Si Si Si C
</PRE>
<P>The 1st 2 arguments must be * * so as to span all LAMMPS atom types.
The first three Si arguments map LAMMPS atom types 1,2,3 to the Si
element in the potential file. The final C argument maps LAMMPS atom
type 4 to the C element in the potential file. If a mapping value is
specified as NULL, the mapping is not performed. This can be used
when the potential is used as part of the <I>hybrid</I> pair style. The
NULL values are placeholders for atom types that will be used with
other potentials. An example of a pair_coeff command for use with the
<I>hybrid</I> pair style is:
</P>
<P>pair_coeff * * 3bnbharm MgOH.3bnbharm Mg O H
</P>
<P>Three-body nonbonded harmonic files in the <I>potentials</I> directory of
the LAMMPS distribution have a ".nb3bharmonic" suffix. Lines that are
not blank or comments (starting with #) define parameters for a
triplet of elements.
</P>
<P>Each entry has six arguments. The first three are atom types as
referenced in the LAMMPS input file. The first argument specifies the
central atom. The fourth argument indicates the <I>K</I> parameter. The
fifth argument indicates <I>theta_0</I>. The sixth argument indicates a
separation cutoff in Angstroms.
</P>
<P>For a given entry, if the second and third arguments are identical,
then the entry is for a cutoff for the distance between types 1 and 2
(values for <I>K</I> and <I>theta_0</I> are irrelevant in this case).
</P>
<P>For a given entry, if the first three arguments are all different,
then the entry is for the <I>K</I> and <I>theta_0</I> parameters (the cutoff in
this case is irrelevant).
</P>
<P>It is <I>not</I> required that the potential file contain entries for all
of the elements listed in the pair_coeff command. It can also contain
entries for additional elements not being used in a particular
simulation; LAMMPS ignores those entries.
</P>
<HR>
<P><B>Restrictions:</B>
</P>
<P>This pair style can only be used if LAMMPS was built with the
USER_MISC package. See the <A HREF = "Section_start.html#start_3">Making LAMMPS</A>
section for more info on packages.
</P>
<P><B>Related commands:</B>
</P>
<P><A HREF = "pair_coeff.html">pair_coeff</A>
</P>
<P><B>Default:</B> none
</P>
</HTML>