forked from lijiext/lammps
174 lines
6.6 KiB
HTML
174 lines
6.6 KiB
HTML
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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/cut command
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</H3>
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<H3>pair_style lj/cut/coul/cut command
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</H3>
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<H3>pair_style lj/cut/coul/debye command
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</H3>
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<H3>pair_style lj/cut/coul/long command
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</H3>
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<H3>pair_style lj/cut/coul/long/tip4p 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/cut</I> or <I>lj/cut/coul/cut</I> or <I>lj/cut/coul/debye</I> or <I>lj/cut/coul/long</I> or <I>lj/cut/coul/long/tip4p</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/cut</I> args = cutoff
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cutoff = global cutoff for Lennard Jones interactions (distance units)
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<I>lj/cut/coul/cut</I> args = cutoff (cutoff2)
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cutoff = global cutoff for LJ (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/cut/coul/debye</I> args = kappa cutoff (cutoff2)
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kappa = Debye length (inverse distance units)
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cutoff = global cutoff for LJ (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/cut/coul/long</I> args = cutoff (cutoff2)
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cutoff = global cutoff for LJ (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/cut/coul/long/tip4p</I> args = otype htype btype atype qdist cutoff (cutoff2)
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otype,htype = atom types for TIP4P O and H
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btype,atype = bond and angle types for TIP4P waters
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qdist = distance from O atom to massless charge (distance units)
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cutoff = global cutoff for LJ (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/cut 2.5
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pair_coeff * * 1 1
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pair_coeff 1 1 1 1.1 2.8
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</PRE>
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<PRE>pair_style lj/cut/coul/cut 10.0
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pair_style lj/cut/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/cut/coul/debye 1.5 3.0
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pair_style lj/cut/coul/debye 1.5 2.5 5.0
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pair_coeff * * 1.0 1.0
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pair_coeff 1 1 1.0 1.5 2.5
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pair_coeff 1 1 1.0 1.5 2.5 5.0
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</PRE>
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<PRE>pair_style lj/cut/coul/long 10.0
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pair_style lj/cut/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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<PRE>pair_style lj/cut/coul/long/tip4p 1 2 7 8 0.3 12.0
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pair_style lj/cut/coul/long/tip4p 1 2 7 8 0.3 12.0 10.0
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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/cut</I> styles compute the standard 6/12 Lennard-Jones potential,
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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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<P>Rc is the cutoff.
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</P>
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<P>Style <I>lj/cut/coul/cut</I> adds a Coulombic pairwise interaction given by
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</P>
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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, and epsilon is the dielectric constant which can be set
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by the <A HREF = "dielectric.html">dielectric</A> command. If one cutoff is
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specified in the pair_style command, it is used for both the LJ and
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Coulombic terms. If two cutoffs are specified, they are used as
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cutoffs for the LJ and Coulombic terms respectively.
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</P>
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<P>Style <I>lj/cut/coul/debye</I> adds an additional exp() damping factor
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to the Coulombic term, given by
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</P>
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<CENTER><IMG SRC = "Eqs/pair_debye.jpg">
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</CENTER>
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<P>where kappa is the Debye length. This potential is another way to
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mimic the screening effect of a polar solvent.
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</P>
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<P>Style <I>lj/cut/coul/long</I> computes the same Coulombic interactions as
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style <I>lj/cut/coul/cut</I> except that an additional damping factor is
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applied to the Coulombic term so it can be used in conjunction with
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the <A HREF = "kspace_style.html">kspace_style</A> command and its <I>ewald</I> or <I>pppm</I>
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option. The Coulombic cutoff specified for this style means that
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pairwise interactions within this distance are computed directly;
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interactions outside that distance are computed in K-space.
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</P>
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<P>Style <I>lj/cut/coul/long/tip4p</I> implements the TIP4P water model of
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<A HREF = "#Jorgensen">(Jorgensen)</A>, which introduces a massless site located a
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short distance away from the oxygen atom along the bisector of the HOH
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angle. The atomic types of the oxygen and hydrogen atoms, the bond
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and angle types for OH and HOH interactions, and the distance to the
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massless charge site are specified as pair_style arguments. See the
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<A HREF = "Section_howto.html#4_8">howto section</A> for more information on how to
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use this pair style.
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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:
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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. You cannot specify 2 cutoffs for style <I>lj/cut</I>, since it
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has no Coulombic terms.
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</P>
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<P>For <I>lj/cut/coul/long</I> and <I>lj/cut/coul/long/tip4p</I> only the LJ cutoff
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can be specified since a Coulombic cutoff cannot be specified for an
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individual I,J type pair. All type pairs use the same global
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Coulombic cutoff specified in the pair_style command.
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</P>
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<P><B>Restrictions:</B>
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</P>
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<P>The <I>lj/cut/coul/long</I> styles is part of the "kspace" package. It is
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only enabled if LAMMPS was built with that package. See the <A HREF = "Section_start.html#2_2">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/cut/coul/long</I> style. See
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the "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 = "Jorgensen"></A>
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<P><B>(Jorgensen)</B> Jorgensen, Chandrasekhar, Madura, Impey, Klein, J Chem
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Phys, 79, 926 (1983).
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</P>
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</HTML>
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