2011-08-30 02:44:24 +08:00
|
|
|
<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 lj/cubic command
|
|
|
|
|
</H3>
|
|
|
|
|
<P><B>Syntax:</B>
|
|
|
|
|
</P>
|
|
|
|
|
<PRE>pair_style lj/cubic
|
|
|
|
|
</PRE>
|
|
|
|
|
<P><B>Examples:</B>
|
|
|
|
|
</P>
|
|
|
|
|
<PRE>pair_style lj/cubic
|
|
|
|
|
pair_coeff * * 1.0 0.8908987
|
|
|
|
|
</PRE>
|
|
|
|
|
<P><B>Description:</B>
|
|
|
|
|
</P>
|
|
|
|
|
<P>The <I>lj/cubic</I> style computes a truncated LJ interaction potential whose
|
2011-08-31 02:01:33 +08:00
|
|
|
energy and force are continuous everywhere.
|
|
|
|
|
Inside the inflection point the interaction is identical to the
|
|
|
|
|
standard 12/6 <A HREF = "pair_lj.html">Lennard-Jones</A> potential.
|
|
|
|
|
The LJ function outside the inflection point is replaced
|
|
|
|
|
with a cubic function of distance. The energy, force and second
|
|
|
|
|
derivative are continuous at the inflection point.
|
|
|
|
|
The cubic coefficient A3 is chosen so
|
|
|
|
|
that both energy and force go to zero at the cutoff distance.
|
|
|
|
|
Outside the cutoff distance the energy and force are zero.
|
|
|
|
|
</P>
|
|
|
|
|
<CENTER><IMG SRC = "Eqs/pair_lj_cubic.jpg">
|
|
|
|
|
</CENTER>
|
|
|
|
|
<P>The location of the inflection point rs is defined
|
2011-08-30 02:44:24 +08:00
|
|
|
by the LJ diameter, rs/sigma = (26/7)^1/6. The cutoff distance
|
2011-08-31 02:01:33 +08:00
|
|
|
is defined by rc/rs = 67/48. The analytic expression for the
|
|
|
|
|
the cubic coefficient
|
|
|
|
|
A3*rmin^3/epsilon = 27.93357 is given in the paper
|
|
|
|
|
Holian and Ravelo <A HREF = "#Holian">(Holian)</A>.
|
2011-08-30 02:44:24 +08:00
|
|
|
</P>
|
2011-08-31 02:01:33 +08:00
|
|
|
<P>This potential is commonly used to study the mechanical behavior
|
2011-08-30 02:44:24 +08:00
|
|
|
of FCC solids, as in the paper by Holian and Ravelo <A HREF = "#Holian">(Holian)</A>.
|
|
|
|
|
</P>
|
2011-08-31 02:01:33 +08:00
|
|
|
<P>The following coefficients must be defined for each pair of atom
|
2011-08-30 02:44:24 +08:00
|
|
|
types via the <A HREF = "pair_coeff.html">pair_coeff</A> command as in the example
|
|
|
|
|
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, or by mixing as described below:
|
|
|
|
|
</P>
|
|
|
|
|
<UL><LI>epsilon (energy units)
|
|
|
|
|
<LI>sigma (distance units)
|
|
|
|
|
</UL>
|
|
|
|
|
<P>Note that sigma is defined in the LJ formula as the zero-crossing
|
|
|
|
|
distance for the potential, not as the energy minimum, which
|
2011-08-31 02:01:33 +08:00
|
|
|
is located at rmin = 2^(1/6)*sigma. In the above example, sigma = 0.8908987,
|
|
|
|
|
so rmin = 1.
|
2011-08-30 02:44:24 +08:00
|
|
|
</P>
|
|
|
|
|
<HR>
|
|
|
|
|
|
|
|
|
|
<P><B>Mixing, shift, table, tail correction, restart, rRESPA info</B>:
|
|
|
|
|
</P>
|
|
|
|
|
<P>For atom type pairs I,J and I != J, the epsilon and sigma coefficients
|
|
|
|
|
and cutoff distance for all of the lj/cut pair styles can be mixed.
|
|
|
|
|
The default mix value is <I>geometric</I>. See the "pair_modify" command
|
|
|
|
|
for details.
|
|
|
|
|
</P>
|
|
|
|
|
<P>The lj/cubic pair style does not support the
|
|
|
|
|
<A HREF = "pair_modify.html">pair_modify</A> shift option,
|
|
|
|
|
since pair interaction is already smoothed to 0.0 at the
|
|
|
|
|
cutoff.
|
|
|
|
|
</P>
|
|
|
|
|
<P>The <A HREF = "pair_modify.html">pair_modify</A> table option is not relevant
|
|
|
|
|
for this pair style.
|
|
|
|
|
</P>
|
|
|
|
|
<P>The lj/cubic pair style does not support the
|
|
|
|
|
<A HREF = "pair_modify.html">pair_modify</A> tail option for adding long-range tail
|
|
|
|
|
corrections to energy and pressure, since there are no corrections for
|
|
|
|
|
a potential that goes to 0.0 at the cutoff.
|
|
|
|
|
</P>
|
|
|
|
|
<P>The lj/cubic pair style writes its information to <A HREF = "restart.html">binary
|
|
|
|
|
restart files</A>, so pair_style and pair_coeff commands do
|
|
|
|
|
not need to be specified in an input script that reads a restart file.
|
|
|
|
|
</P>
|
|
|
|
|
<P>The lj/cubic pair style can only be used via the <I>pair</I>
|
|
|
|
|
keyword of the <A HREF = "run_style.html">run_style respa</A> command. It does not
|
|
|
|
|
support the <I>inner</I>, <I>middle</I>, <I>outer</I> keywords.
|
|
|
|
|
</P>
|
|
|
|
|
<HR>
|
|
|
|
|
|
|
|
|
|
<P><B>Restrictions:</B> none
|
|
|
|
|
</P>
|
|
|
|
|
<P><B>Related commands:</B>
|
|
|
|
|
</P>
|
|
|
|
|
<P><A HREF = "pair_coeff.html">pair_coeff</A>
|
|
|
|
|
</P>
|
|
|
|
|
<P><B>Default:</B> none
|
|
|
|
|
</P>
|
|
|
|
|
<HR>
|
|
|
|
|
|
|
|
|
|
<A NAME = "Holian"></A>
|
|
|
|
|
|
|
|
|
|
<P><B>(Holian)</B> Holian and Ravelo, Phys Rev B, 51, 11275 (1995).
|
|
|
|
|
</P>
|
|
|
|
|
</HTML>
|
