<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 coul/cut command </H3> <H3>pair_style coul/long command </H3> <P><B>Syntax:</B> </P> <PRE>pair_style coul/cut cutoff pair_style coul/long cutoff </PRE> <UL><LI>cutoff = global cutoff for Coulombic interactions </UL> <P><B>Examples:</B> </P> <PRE>pair_style coul/cut 2.5 pair_coeff * * pair_coeff 2 2 3.5 </PRE> <PRE>pair_style coul/long 10.0 pair_coeff * * </PRE> <P><B>Description:</B> </P> <P>The <I>coul/cut</I> style computes the standard Coulombic interaction potential given by </P> <CENTER><IMG SRC = "Eqs/pair_coulomb.jpg"> </CENTER> <P>where C is an energy-conversion constant, Qi and Qj are the charges on the 2 atoms, and epsilon is the dielectric constant which can be set by the <A HREF = "dielectric.html">dielectric</A> command. The cutoff Rc truncates the interaction distance. </P> <P>Style <I>coul/long</I> computes the same Coulombic interactions as style <I>coul/cut</I> except that an additional damping factor is applied so it can be used in conjunction with the <A HREF = "kspace_style.html">kspace_style</A> command and its <I>ewald</I> or <I>pppm</I> option. The Coulombic cutoff specified for this style means that pairwise interactions within this distance are computed directly; interactions outside that distance are computed in reciprocal space. </P> <P>These potentials are designed to be combined with other pair potentials via the <A HREF = "pair_hybrid.html">pair_style hybrid/overlay</A> command. This is because they have no repulsive core. Hence if they are used by themselves, there will be no repulsion to keep two oppositely charged particles from overlapping each other. </P> <P>The following coefficients must be defined for each pair of atoms types via the <A HREF = "pair_coeff.html">pair_coeff</A> command as in the examples 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>cutoff (distance units) </UL> <P>For <I>coul/cut</I>, the cutoff coefficient is optional. If it is not used (as in som of the examples above), the default global value specified in the pair_style command is used. </P> <P>For <I>coul/long</I> no cutoff can be specified for an individual I,J type pair via the pair_coeff command. All type pairs use the same global Coulombic cutoff specified in the pair_style command. </P> <HR> <P><B>Mixing, shift, table, tail correction, per-atom energy/stress, and restart info</B>: </P> <P>For atom type pairs I,J and I != J, the cutoff distance for the <I>coul/cut</I> style can be mixed. The default mix value is <I>geometric</I>. See the "pair_modify" command for details. </P> <P>The <A HREF = "pair_modify.html">pair_modify</A> shift option is not relevant for these pair styles. </P> <P>The <I>coul/long</I> style supports the <A HREF = "pair_modify.html">pair_modify</A> table option for tabulation of the short-range portion of the long-range Coulombic interaction. </P> <P>These pair styles do not support the <A HREF = "pair_modify.html">pair_modify</A> tail option for adding long-range tail corrections to energy and pressure. </P> <P>These pair styles can calculate per-atom energy and stress, as used by the <A HREF = "compute_epair_atom.html">compute epair/atom</A>, <A HREF = "compute_stress_atom.html">compute stress/atom</A>, and <A HREF = "dump.html">dump custom</A> commands. </P> <P>These pair styles write their 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> <HR> <P><B>Restrictions:</B> </P> <P>The <I>coul/long</I> style is part of the "kspace" package. It is only enabled if LAMMPS was built with that package (which it is by default). See the <A HREF = "Section_start.html#2_3">Making LAMMPS</A> section for more info. </P> <P>On some 64-bit machines, compiling with -O3 appears to break the Coulombic tabling option used by the <I>coul/long</I> style. See the "Additional build tips" section of the Making LAMMPS documentation pages for workarounds on this issue. </P> <P><B>Related commands:</B> </P> <P><A HREF = "pair_coeff.html">pair_coeff</A>, <A HREF = "pair_hybrid.html">pair_style hybrid/overlay</A> </P> <P><B>Default:</B> none </P> </HTML>