forked from lijiext/lammps
139 lines
4.9 KiB
HTML
139 lines
4.9 KiB
HTML
<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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</CENTER>
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<HR>
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<H3>pair_style coul/cut command
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</H3>
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<H3>pair_style coul/debye command
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</H3>
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<H3>pair_style 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 coul/cut cutoff
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pair_style coul/debye kappa cutoff
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pair_style coul/long cutoff
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</PRE>
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<UL><LI>cutoff = global cutoff for Coulombic interactions
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<LI>kappa = Debye length (inverse 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 coul/cut 2.5
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pair_coeff * *
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pair_coeff 2 2 3.5
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</PRE>
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<PRE>pair_style coul/debye 1.4 3.0
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pair_coeff * *
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pair_coeff 2 2 3.5
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</PRE>
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<PRE>pair_style coul/long 10.0
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pair_coeff * *
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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>coul/cut</I> style computes the standard Coulombic interaction
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potential 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. The cutoff Rc truncates
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the interaction distance.
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</P>
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<P>Style <I>coul/debye</I> adds an additional exp() damping factor to the
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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>coul/long</I> computes the same Coulombic interactions as style
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<I>coul/cut</I> except that an additional damping factor is applied so it
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can be used in conjunction with the <A HREF = "kspace_style.html">kspace_style</A>
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command and its <I>ewald</I> or <I>pppm</I> option. The Coulombic cutoff
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specified for this style means that pairwise interactions within this
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distance are computed directly; interactions outside that distance are
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computed in reciprocal space.
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</P>
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<P>These potentials are designed to be combined with other pair
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potentials via the <A HREF = "pair_hybrid.html">pair_style hybrid/overlay</A>
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command. This is because they have no repulsive core. Hence if they
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are used by themselves, there will be no repulsion to keep two
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oppositely charged particles from overlapping each other.
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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>cutoff (distance units)
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</UL>
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<P>For <I>coul/cut</I> and <I>coul/debye</I>, the cutoff coefficient is optional.
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If it is not used (as in some of the examples above), the default
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global value specified in the pair_style command is used.
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</P>
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<P>For <I>coul/long</I> no cutoff can be specified for an individual I,J type
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pair via the pair_coeff command. 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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<HR>
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<P><B>Mixing, shift, table, tail correction, restart, rRESPA info</B>:
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</P>
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<P>For atom type pairs I,J and I != J, the cutoff distance for the
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<I>coul/cut</I> style can be mixed. The default mix value is <I>geometric</I>.
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See the "pair_modify" command for details.
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</P>
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<P>The <A HREF = "pair_modify.html">pair_modify</A> shift option is not relevant
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for these pair styles.
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</P>
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<P>The <I>coul/long</I> style supports the <A HREF = "pair_modify.html">pair_modify</A>
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table option for tabulation of the short-range portion of the
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long-range Coulombic interaction.
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</P>
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<P>These pair styles do not support the <A HREF = "pair_modify.html">pair_modify</A>
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tail option for adding long-range tail corrections to energy and
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pressure.
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</P>
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<P>These pair styles write their 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 can only be used via the <I>pair</I> keyword of the
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<A HREF = "run_style.html">run_style respa</A> command. It does not support the
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<I>inner</I>, <I>middle</I>, <I>outer</I> keywords.
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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>The <I>coul/long</I> style is part of the "kspace" package. It is only
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enabled if LAMMPS was built with that package (which it is by
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default). See the <A HREF = "Section_start.html#2_3">Making LAMMPS</A> section for
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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>coul/long</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>, <A HREF = "pair_hybrid.html">pair_style
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hybrid/overlay</A>
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</P>
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<P><B>Default:</B> none
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</P>
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</HTML>
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