2006-09-22 00:22:34 +08:00
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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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<HR>
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<H3>pair_style lj/class2 command
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</H3>
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<H3>pair_style lj/class2/coul/cut command
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</H3>
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<H3>pair_style lj/class2/coul/long command
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</H3>
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<P><B>Syntax:</B>
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</P>
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<PRE>pair_style style args
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</PRE>
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<UL><LI>style = <I>lj/class2</I> or <I>lj/class2/coul/cut</I> or <I>lj/class2/coul/long</I>
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<LI>args = list of arguments for a particular style
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</UL>
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<PRE> <I>lj/class2</I> args = cutoff
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cutoff = global cutoff for class 2 interactions (distance units)
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<I>lj/class2/coul/cut</I> args = cutoff (cutoff2)
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cutoff = global cutoff for class 2 (and Coulombic if only 1 arg) (distance units)
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cutoff2 = global cutoff for Coulombic (optional) (distance units)
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<I>lj/class2/coul/long</I> args = cutoff (cutoff2)
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cutoff = global cutoff for class 2 (and Coulombic if only 1 arg) (distance units)
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cutoff2 = global cutoff for Coulombic (optional) (distance units)
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</PRE>
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<P><B>Examples:</B>
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</P>
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<PRE>pair_style lj/class2 10.0
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pair_coeff * * 100.0 2.5
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pair_coeff 1 2* 100.0 2.5 9.0
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</PRE>
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<PRE>pair_style lj/class2/coul/cut 10.0
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pair_style lj/class2/coul/cut 10.0 8.0
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pair_coeff * * 100.0 3.0
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pair_coeff 1 1 100.0 3.5 9.0
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pair_coeff 1 1 100.0 3.5 9.0 9.0
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</PRE>
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<PRE>pair_style lj/class2/coul/long 10.0
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pair_style lj/class2/coul/long 10.0 8.0
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pair_coeff * * 100.0 3.0
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pair_coeff 1 1 100.0 3.5 9.0
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</PRE>
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<P><B>Description:</B>
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</P>
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<P>The <I>lj/class2</I> styles compute a 6/9 Lennard-Jones potential given by
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</P>
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<CENTER><IMG SRC = "Eqs/pair_class2.jpg">
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</CENTER>
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<P>Rc is the cutoff.
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</P>
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<P>The <I>lj/class2/coul/cut</I> and <I>lj/class2/coul/long</I> styles add a
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2007-02-10 05:40:32 +08:00
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Coulombic term as described for the <A HREF = "pair_lj.html">lj/cut</A> pair
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2006-09-22 00:22:34 +08:00
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styles.
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</P>
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2006-09-28 03:12:31 +08:00
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<P>See <A HREF = "#Sun">(Sun)</A> for a description of the COMPASS class2 force field.
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</P>
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2006-09-22 00:22:34 +08:00
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<P>The following coefficients must be defined for each pair of atoms
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types via the <A HREF = "pair_coeff.html">pair_coeff</A> command as in the examples
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above, or in the data file or restart files read by the
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<A HREF = "read_data.html">read_data</A> or <A HREF = "read_restart.html">read_restart</A>
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2007-06-25 22:36:36 +08:00
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commands, or by mixing as described below:
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2006-09-22 00:22:34 +08:00
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</P>
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<UL><LI>epsilon (energy units)
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<LI>sigma (distance units)
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<LI>cutoff1 (distance units)
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<LI>cutoff2 (distance units)
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</UL>
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<P>The latter 2 coefficients are optional. If not specified, the global
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class 2 and Coulombic cutoffs are used. If only one cutoff is
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specified, it is used as the cutoff for both class 2 and Coulombic
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interactions for this type pair. If both coefficients are specified,
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they are used as the class 2 and Coulombic cutoffs for this type pair.
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You cannot specify 2 cutoffs for style <I>lj/class2</I>, since it has no
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Coulombic terms.
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</P>
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<P>For <I>lj/class2/coul/long</I> only the class 2 cutoff can be specified
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since a Coulombic cutoff cannot be specified for an individual I,J
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type pair. All type pairs use the same global Coulombic cutoff
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specified in the pair_style command.
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</P>
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2007-06-25 22:36:36 +08:00
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<P>: line
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</P>
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2007-04-20 07:25:27 +08:00
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<P>If the pair_coeff command is not used to define coefficients for a
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particular I != J type pair, the mixing rule for epsilon and sigma for
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all class2 potentials is to use the <I>sixthpower</I> formulas documented
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by the <A HREF = "pair_modify.html">pair_modify</A> command. The <A HREF = "pair_modify.html">pair_modify
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mix</A> setting is thus ignored for class2 potentials
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for epsilon and sigma. However it is still followed for mixing the
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cutoff distance.
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</P>
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2007-06-25 22:36:36 +08:00
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<P><B>Mixing, shift, table, tail correction, per-atom energy/stress, and
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restart info</B>:
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</P>
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<P>For atom type pairs I,J and I != J, the epsilon and sigma coefficients
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and cutoff distance for all of the lj/class2 pair styles can be mixed.
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Epsilon and sigma are always mixed with the value <I>sixthpower</I>. The
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cutoff distance is mixed by whatever option is set by the pair_modify
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command (default = geometric). See the "pair_modify" command for
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details.
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</P>
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<P>All of the lj/class2 pair styles support the
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<A HREF = "pair_modify.html">pair_modify</A> shift option for the energy of the
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Lennard-Jones portion of the pair interaction.
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</P>
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<P>The <I>lj/class2/coul/long</I> pair style does not support the
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<A HREF = "pair_modify.html">pair_modify</A> table option since a tabulation
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capability has not yet been added to this potential.
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</P>
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<P>All of the lj/class2 pair styles support the
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<A HREF = "pair_modify.html">pair_modify</A> tail option for adding a long-range
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tail correction to the energy and pressure of the Lennard-Jones
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portion of the pair interaction.
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</P>
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<P>All of the lj/class2 pair styles can calculate per-atom energy and
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stress, as used by the <A HREF = "compute_epair_atom.html">compute epair/atom</A>,
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<A HREF = "compute_stress_atom.html">compute stress/atom</A>, and <A HREF = "dump.html">dump
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custom</A> commands.
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</P>
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<P>All of the lj/class2 pair styles write their information to <A HREF = "restart.html">binary
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restart files</A>, so pair_style and pair_coeff commands do
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not need to be specified in an input script that reads a restart file.
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</P>
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2006-09-22 00:22:34 +08:00
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<P><B>Restrictions:</B>
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</P>
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2007-06-25 22:36:36 +08:00
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<P>All of these pair styles are part of the "class2" package. They are
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only enabled if LAMMPS was built with that package. See the <A HREF = "Section_start.html#2_3">Making
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LAMMPS</A> section for more info.
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</P>
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<P><B>Related commands:</B>
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</P>
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<P><A HREF = "pair_coeff.html">pair_coeff</A>
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</P>
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<P><B>Default:</B> none
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</P>
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2006-09-28 03:12:31 +08:00
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<HR>
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<A NAME = "Sun"></A>
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<P><B>(Sun)</B> Sun, J Phys Chem B 102, 7338-7364 (1998).
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</P>
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2006-09-22 00:22:34 +08:00
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</HTML>
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