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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>pair_style gayberne command
</H3>
<P><B>Syntax:</B>
</P>
<PRE>pair_style gayberne gamma upsilon mu cutoff
</PRE>
<UL><LI>gamma = shift for potential minimum (typically 1)
<LI>upsilon = exponent for eta orientation-dependent energy function
<LI>mu = exponent for chi orientation-dependent energy function
<LI>cutoff = global cutoff for interactions (distance units)
</UL>
<P><B>Examples:</B>
</P>
<PRE>pair_style gayberne 1.0 1.0 1.0 10.0
pair_coeff * * 1.0 1.7 1.7 3.4 3.4 1.0 1.0 1.0
</PRE>
<P><B>Description:</B>
</P>
<P>Style <I>gayberne</I> computes a Gay-Berne anisotropic LJ interaction
<A HREF = "#Beradi">(Beradi)</A> between pairs of ellipsoidal particles via the
formulas
</P>
<CENTER><IMG SRC = "Eqs/pair_gayberne.jpg">
</CENTER>
<P>where A1 and A2 are the transformation matrices from the simulation
box frame to the body frame and r12 is the center to center vector
between the particles. Ur controls the shifted distance dependent
interaction based on the distance of closest approach of the two
particles (h12) and the user-specified shift parameter gamma.
</P>
<P>For large uniform molecules it has been shown that the energy
parameters are approximately representable in terms of local contact
curvatures <A HREF = "#Everaers">(Everaers)</A>:
</P>
<CENTER><IMG SRC = "Eqs/pair_gayberne2.jpg">
</CENTER>
<P>The variable names utilized as potential parameters are for the most
part taken from <A HREF = "#Everaers">(Everaers)</A> in order to be consistent with
its RE-squared potential fix. Details on the upsilon and mu
parameters are given <A HREF = "Eqs/pair_gayberne_extra.pdf">here</A>.
Use of this pair style requires the
<A HREF = "fix_nve_asphere.html">fix nve/asphere</A> in order to integrate particle
rotation. Additionally, <A HREF = "atom_style.html">atom_style ellipsoid</A> should
be used since it defines the rotation state of the ellipsoidal
particles.
</P>
<P>More details of the Gay-Berne formulation are given in the references
listed below and in <A HREF = "Eqs/pair_gayberne_extra.pdf">this document</A>.
</P>
<P>The following coefficients must be defined for each pair of atoms
types via the <A HREF = "pair_coeff.html">pair_coeff</A> command as in the examples
above, or in the data file or restart files read by the
<A HREF = "read_data.html">read_data</A> or <A HREF = "read_restart.html">read_restart</A>
commands:
</P>
<UL><LI>epsilon = well depth (energy units)
<LI>sigma = minimum effective particle radii (distance units)
<LI>a = ellipsoid radius in x dimension (distance units)
<LI>b = ellipsoid radius in y dimension (distance units)
<LI>c = ellipsoid radius in z dimension (distance units)
<LI>epsilon_a = relative well depth for side-to-side interactions
<LI>epsilon_b = relative well depth for face-to-face interactions
<LI>epsilon_c = relative well depth for end-to-end interactions
<LI>cutoff (distance units)
</UL>
<P>The last coefficient is optional. If not specified, the global
cutoff specified in the pair_style command is used.
</P>
<P>The epsilon and sigma parameters are mixed for I != J atom pairings
the same as Lennard-Jones parameters; see the <A HREF = "pair_modify.html">pair_modify
mix</A> documentation for details. The other parameters
(except cutoff) are really specific to a single atom type, and not a
pair of atoms. Thus they are applied to atom type I only.
</P>
<P><B>Restrictions:</B>
</P>
<P>Can only be used if LAMMPS was built with the "asphere" package. Can
only be used with <A HREF = "atom_style.html">atom_style ellipsoid</A>.
</P>
<P>The use of this potential requires additional fixes as described
above. The "shift yes" option currently cannot be used with this
potential to shift energies to 0 at the cutoff due to the anisotropic
dependence of the interaction. Angular velocities are all set to zero
initially. The Gay-Berne potential does not become isotropic as r
increases <A HREF = "#Everaers">(Everaers)</A>. The distance of closest approach
approximation becomes less accurate as the shape of ellipsoids becomes
more dissimilar (high aspect ratio particles).
</P>
<P><B>Related commands:</B>
</P>
<P><A HREF = "pair_coeff.html">pair_coeff</A>, <A HREF = "fix_nve_asphere.html">fix nve/asphere</A>,
<A HREF = "compute_temp_asphere.html">compute temp/asphere</A>
</P>
<P><B>Default:</B> none
</P>
<HR>
<A NAME = "Everaers"></A>
<P><B>(Everaers)</B> Everaers and Ejtehadi, Phys Rev E, 67, 041710 (2003).
</P>
<A NAME = "Berardi"></A>
<P><B>(Berardi)</B> Berardi, Fava, Zannoni, Chem Phys Lett, 297, 8-14 (1998).
</P>
<A NAME = "Perram"></A>
<P><B>(Perram)</B> Perram and Rasmussen, Phys Rev E, 54, 6565-6572 (1996).
</P>
<A NAME = "Allen"></A>
<P><B>(Allen)</B> Allen and Germano, Mol Phys 104, 3225-3235 (2006).
</P>
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