2007-10-04 08:21:28 +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/coul 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 lj/coul flag_lj flag_coul cutoff (cutoff2)
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</PRE>
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<UL><LI>flag_lj = <I>long</I> or <I>cut</I>
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<PRE> <I>long</I> = use Kspace long-range summation for the dispersion term 1/r^6
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<I>cut</I> = use a cutoff
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</PRE>
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<LI>flag_coul = <I>long</I> or <I>off</I>
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<PRE> <I>long</I> = use Kspace long-range summation for the Coulombic term 1/r
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<I>off</I> = omit the Coulombic term
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</PRE>
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<LI>cutoff = global cutoff for LJ (and Coulombic if only 1 cutoff) (distance units)
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<LI>cutoff2 = global cutoff for Coulombic (optional) (distance units)
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</UL>
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<P><B>Examples:</B>
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</P>
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<PRE>pair_style lj/coul cut off 2.5
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pair_style lj/coul cut long 2.5 4.0
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pair_style lj/coul long long 2.5 4.0
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pair_coeff * * 1 1
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pair_coeff 1 1 1 3 4
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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/coul</I> style computes the standard 12/6 Lennard-Jones and
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Coulombic potentials, given by
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</P>
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<CENTER><IMG SRC = "Eqs/pair_lj.jpg">
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</CENTER>
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<CENTER><IMG SRC = "Eqs/pair_coulomb.jpg">
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</CENTER>
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<P>where C is an energy-conversion constant, Qi and Qj are the charges on
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the 2 atoms, epsilon is the dielectric constant which can be set by
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the <A HREF = "dielectric.html">dielectric</A> command, and Rc is the cutoff. If
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one cutoff is specified in the pair_style command, it is used for both
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the LJ and Coulombic terms. If two cutoffs are specified, they are
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used as cutoffs for the LJ and Coulombic terms respectively.
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</P>
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<P>The purpose of this pair style is to capture long-range interactions
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resulting from both attractive 1/r^6 Lennard-Jones and Coulombic 1/r
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interactions. This is done by use of the <I>flag_lj</I> and <I>flag_coul</I>
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settings. The <A HREF = "#Veld">In 't Veld</A> paper has more details on when it is
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appropriate to include long-range 1/r^6 interactions, using this
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potential.
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</P>
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<P>If <I>flag_lj</I> is set to <I>long</I>, no cutoff is used on the LJ 1/r^6
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dispersion term. The long-range portion is calculated by using the
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<A HREF = "kspace_style.html">kspace_style ewald/n</A> command. The specified LJ
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cutoff then determines which portion of the LJ interactions are
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computed directly by the pair potential versus which part is computed
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in reciprocal space via the Kspace style. If <I>flag_lj</I> is set to
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<I>cut</I>, the LJ interactions are simply cutoff, as with <A HREF = "pair_lj.html">pair_style
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lj/cut</A>.
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</P>
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<P>If <I>flag_coul</I> is set to <I>long</I>, no cutoff is used on the Coulombic
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interactions. The long-range portion is calculated by using any
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style, including <I>ewald/n</I> of the <A HREF = "kspace_style.html">kspace_style</A>
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command. Note that if <I>flag_lj</I> is also set to long, then only the
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<I>ewald/n</I> Kspace style can perform the long-range calculations for
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both the LJ and Coulombic interactions. If <I>flag_coul</I> is set to
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<I>off</I>, Coulombic interactions are not computed.
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</P>
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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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commands, or by mixing as described below:
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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>Note that sigma is defined in the LJ formula as the zero-crossing
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distance for the potential, not as the energy minimum at 2^(1/6)
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sigma.
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</P>
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<P>The latter 2 coefficients are optional. If not specified, the global
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LJ and Coulombic cutoffs specified in the pair_style command are used.
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If only one cutoff is specified, it is used as the cutoff for both LJ
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and Coulombic interactions for this type pair. If both coefficients
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are specified, they are used as the LJ and Coulombic cutoffs for this
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type pair. Note that if you are using <I>flag_lj</I> set to <I>long</I>, you
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cannot specify a LJ cutoff for an atom type pair, since only one
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global LJ cutoff is allowed. Similarly, if you are using <I>flag_coul</I>
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set to <I>long</I>, you cannot specify a Coulombic cutoff for an atom type
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pair, since only one global Coulombic cutoff is allowed.
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</P>
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<HR>
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2008-01-22 02:12:34 +08:00
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<P><B>Mixing, shift, table, tail correction, restart, rRESPA info</B>:
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2007-10-04 08:21:28 +08:00
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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/cut pair styles can be mixed.
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The default mix value is <I>geometric</I>. See the "pair_modify" command
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for details.
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</P>
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<P>This pair style supports the <A HREF = "pair_modify.html">pair_modify</A> shift
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option for the energy of the Lennard-Jones portion of the pair
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interaction, assuming <I>flag_lj</I> is <I>cut</I>.
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</P>
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<P>This pair style supports the <A HREF = "pair_modify.html">pair_modify</A> table
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option since it can tabulate the short-range portion of the long-range
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Coulombic interaction.
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</P>
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<P>This pair styles supports the <A HREF = "pair_modify.html">pair_modify</A> tail
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option for adding a long-range tail correction to the Lennard-Jones
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portion of the energy and pressure of the pair interaction, assuming
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<I>flag_lj</I> is <I>cut</I>.
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</P>
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<P>This pair style writes its information to <A HREF = "restart.html">binary restart
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files</A>, so pair_style and pair_coeff commands do not need
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to be specified in an input script that reads a restart file.
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</P>
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<P>This pair style supports the use of the <I>inner</I>, <I>middle</I>, and <I>outer</I>
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keywords of the <A HREF = "run_style.html">run_style respa</A> command, meaning the
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pairwise forces can be partitioned by distance at different levels of
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the rRESPA hierarchy. See the <A HREF = "run_style.html">run_style</A> command for
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details.
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</P>
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<HR>
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<P><B>Restrictions:</B>
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</P>
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<P>This style is part of the "user-ewaldn" package. It is only enabled
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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>On some 64-bit machines, compiling with -O3 appears to break the
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Coulombic tabling option used by the <I>lj/coul</I> style. See the
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"Additional build tips" section of the Making LAMMPS documentation
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pages for workarounds on this issue.
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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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<HR>
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<A NAME = "Veld"></A>
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<P><B>(In 't Veld)</B> In 't Veld, Ismail, Grest, J Chem Phys (accepted) (2007).
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</P>
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</HTML>
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