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367 lines
14 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/cut/gpu command
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</H3>
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<H3>pair_style coul/cut/kk command
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</H3>
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<H3>pair_style coul/cut/omp 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/debye/gpu command
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</H3>
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<H3>pair_style coul/debye/kk command
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</H3>
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<H3>pair_style coul/debye/omp command
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</H3>
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<H3>pair_style coul/dsf command
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</H3>
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<H3>pair_style coul/dsf/gpu command
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</H3>
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<H3>pair_style coul/dsf/kk command
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</H3>
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<H3>pair_style coul/dsf/omp 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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<H3>pair_style coul/long/omp command
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</H3>
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<H3>pair_style coul/long/gpu command
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</H3>
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<H3>pair_style coul/long/kk command
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</H3>
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<H3>pair_style coul/msm command
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</H3>
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<H3>pair_style coul/msm/omp command
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</H3>
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<H3>pair_style coul/streitz command
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</H3>
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<H3>pair_style coul/wolf command
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</H3>
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<H3>pair_style coul/wolf/kk command
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</H3>
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<H3>pair_style coul/wolf/omp command
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</H3>
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<H3>pair_style tip4p/cut command
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</H3>
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<H3>pair_style tip4p/long command
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</H3>
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<H3>pair_style tip4p/cut/omp command
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</H3>
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<H3>pair_style tip4p/long/omp 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/dsf alpha cutoff
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pair_style coul/long cutoff
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pair_style coul/long/gpu cutoff
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pair_style coul/wolf alpha cutoff
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pair_style coul/streitz cutoff keyword alpha
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pair_style tip4p/cut otype htype btype atype qdist cutoff
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pair_style tip4p/long otype htype btype atype qdist 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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<LI>alpha = damping parameter (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/dsf 0.05 10.0
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pair_coeff * *
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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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<PRE>pair_style coul/msm 10.0
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pair_coeff * *
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</PRE>
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<PRE>pair_style coul/wolf 0.2 9.0
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pair_coeff * *
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</PRE>
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<PRE>pair_style coul/streitz 12.0 ewald
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pair_style coul/streitz 12.0 wolf 0.30
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pair_coeff * * AlO.streitz Al O
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</PRE>
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<PRE>pair_style tip4p/cut 1 2 7 8 0.15 12.0
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pair_coeff * *
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</PRE>
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<PRE>pair_style tip4p/long 1 2 7 8 0.15 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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<HR>
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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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<HR>
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<P>Style <I>coul/dsf</I> computes Coulombic interactions via the damped
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shifted force model described in <A HREF = "#Fennell">Fennell</A>, given by:
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</P>
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<CENTER><IMG SRC = "Eqs/pair_coul_dsf.jpg">
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</CENTER>
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<P>where <I>alpha</I> is the damping parameter and erfc() is the
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complementary error-function. The potential corrects issues in the
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Wolf model (described below) to provide consistent forces and energies
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(the Wolf potential is not differentiable at the cutoff) and smooth
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decay to zero.
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</P>
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<HR>
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<P>Style <I>coul/wolf</I> computes Coulombic interactions via the Wolf
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summation method, described in <A HREF = "#Wolf">Wolf</A>, given by:
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</P>
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<CENTER><IMG SRC = "Eqs/pair_coul_wolf.jpg">
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</CENTER>
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<P>where <I>alpha</I> is the damping parameter, and erc() and erfc() are
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error-fuction and complementary error-function terms. This potential
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is essentially a short-range, spherically-truncated,
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charge-neutralized, shifted, pairwise <I>1/r</I> summation. With a
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manipulation of adding and substracting a self term (for i = j) to the
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first and second term on the right-hand-side, respectively, and a
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small enough <I>alpha</I> damping parameter, the second term shrinks and
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the potential becomes a rapidly-converging real-space summation. With
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a long enough cutoff and small enough alpha parameter, the energy and
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forces calcluated by the Wolf summation method approach those of the
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Ewald sum. So it is a means of getting effective long-range
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interactions with a short-range potential.
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</P>
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<HR>
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<P>Style <I>coul/streitz</I> is the Coulomb pair interaction defined as part
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of the Streitz-Mintmire potential, as described in <A HREF = "#Streitz">this
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paper</A>, in which charge distribution about an atom is modeled
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as a Slater 1<I>s</I> orbital. More details can be found in the referenced
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paper. To fully reproduce the published Streitz-Mintmire potential,
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which is a variable charge potential, style <I>coul/streitz</I> must be
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used with <A HREF = "pair_eam.html">pair_style eam/alloy</A> (or some other
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short-range potential that has been parameterized appropriately) via
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the <A HREF = "pair_hybrid.html">pair_style hybrid/overlay</A> command. Likewise,
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charge equilibration must be performed via the <A HREF = "fix_qeq.html">fix
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qeq/slater</A> command. For example:
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</P>
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<PRE>pair_style hybrid/overlay coul/streitz 12.0 wolf 0.31 eam/alloy
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pair_coeff * * coul/streitz AlO.streitz Al O
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pair_coeff * * eam/alloy AlO.eam.alloy Al O
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fix 1 all qeq/slater 1 12.0 1.0e-6 100 coul/streitz
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</PRE>
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<P>The keyword <I>wolf</I> in the coul/streitz command denotes computing
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Coulombic interactions via Wolf summation. An additional damping
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parameter is required for the Wolf summation, as described for the
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coul/wolf potential above. Alternatively, Coulombic interactions can
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be computed via an Ewald summation. For example:
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</P>
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<PRE>pair_style hybrid/overlay coul/streitz 12.0 ewald eam/alloy
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kspace_style ewald 1e-6
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</PRE>
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<P>Keyword <I>ewald</I> does not need a damping parameter, but a
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<A HREF = "kspace_style.html">kspace_style</A> must be defined, which can be style
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<I>ewald</I> or <I>pppm</I>. The Ewald method was used in Streitz and
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Mintmire's original paper, but a Wolf summation offers a speed-up in
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some cases.
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</P>
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<P>For the fix qeq/slater command, the <I>qfile</I> can be a filename that
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contains QEq parameters as discussed on the <A HREF = "fix_qeq.html">fix qeq</A>
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command doc page. Alternatively <I>qfile</I> can be replaced by
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"coul/streitz", in which case the fix will extract QEq parameters from
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the coul/streitz pair style itself.
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</P>
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<P>See the examples/strietz directory for an example input script that
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uses the Streitz-Mintmire potential. The potentials directory has the
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AlO.eam.alloy and AlO.streitz potential files used by the example.
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</P>
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<P>Note that the Streiz-Mintmire potential is generally used for oxides,
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but there is no conceptual problem with extending it to nitrides and
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carbides (such as SiC, TiN). Pair coul/strietz used by itself or with
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any other pair style such as EAM, MEAM, Tersoff, or LJ in
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hybrid/overlay mode. To do this, you would need to provide a
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Streitz-Mintmire parameterizaion for the material being modeled.
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</P>
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<HR>
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<P>Styles <I>coul/long</I> and <I>coul/msm</I> compute the same Coulombic
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interactions as style <I>coul/cut</I> except that an additional damping
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factor is applied so it can be used in conjunction with the
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<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 reciprocal space.
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</P>
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<P>Styles <I>tip4p/cut</I> and <I>tip4p/long</I> implement the coulomb part of
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the TIP4P water model of <A HREF = "#Jorgensen">(Jorgensen)</A>, which introduces
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a massless site located a short distance away from the oxygen atom
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along the bisector of the HOH angle. The atomic types of the oxygen and
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hydrogen atoms, the bond and angle types for OH and HOH interactions,
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and the distance to the massless charge site are specified as
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pair_style arguments. Style <I>tip4p/cut</I> uses a global cutoff for
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Coulomb interactions; style <I>tip4p/long</I> is for use with a long-range
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Coulombic solver (Ewald or PPPM).
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</P>
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<P>IMPORTANT NOTE: For each TIP4P water molecule in your system, the atom
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IDs for the O and 2 H atoms must be consecutive, with the O atom
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first. This is to enable LAMMPS to "find" the 2 H atoms associated
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with each O atom. For example, if the atom ID of an O atom in a TIP4P
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water molecule is 500, then its 2 H atoms must have IDs 501 and 502.
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</P>
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<P>See the <A HREF = "Section_howto.html#howto_8">howto section</A> for more
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information on how to use the TIP4P pair styles and lists of
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parameters to set. Note that the neighobr list cutoff for Coulomb
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interactions is effectively extended by a distance 2*qdist when using
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the TIP4P pair style, to account for the offset distance of the
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fictitious charges on O atoms in water molecules. Thus it is
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typically best in an efficiency sense to use a LJ cutoff >= Coulomb
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cutoff + 2*qdist, to shrink the size of the neighbor list. This leads
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to slightly larger cost for the long-range calculation, so you can
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test the trade-off for your model.
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</P>
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<HR>
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<P>Note that 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 moving arbitrarily close to each
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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> and <I>coul/msm</I> no cutoff can be specified for an
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individual I,J type pair via the pair_coeff command. All type pairs
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use the same global Coulombic cutoff specified in the pair_style
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command.
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</P>
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<HR>
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<P>Styles with a <I>cuda</I>, <I>gpu</I>, <I>intel</I>, <I>kk</I>, <I>omp</I>, or <I>opt</I> suffix are
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functionally the same as the corresponding style without the suffix.
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They have been optimized to run faster, depending on your available
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hardware, as discussed in <A HREF = "Section_accelerate.html">Section_accelerate</A>
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of the manual. The accelerated styles take the same arguments and
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should produce the same results, except for round-off and precision
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issues.
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</P>
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<P>These accelerated styles are part of the USER-CUDA, GPU, USER-INTEL,
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KOKKOS, USER-OMP and OPT packages, respectively. They are only
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enabled if LAMMPS was built with those packages. See the <A HREF = "Section_start.html#start_3">Making
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LAMMPS</A> section for more info.
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</P>
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<P>You can specify the accelerated styles explicitly in your input script
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by including their suffix, or you can use the <A HREF = "Section_start.html#start_7">-suffix command-line
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switch</A> when you invoke LAMMPS, or you can
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use the <A HREF = "suffix.html">suffix</A> command in your input script.
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</P>
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<P>See <A HREF = "Section_accelerate.html">Section_accelerate</A> of the manual for
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more instructions on how to use the accelerated styles effectively.
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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>, <I>coul/msm</I> and <I>tip4p/long</I> styles are part of the
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KSPACE package. They are only enabled if LAMMPS was built with that
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package (which it is by default).
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See the <A HREF = "Section_start.html#start_3">Making LAMMPS</A> section
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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>, <A HREF = "pair_hybrid.html">pair_style
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hybrid/overlay</A>
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<A HREF = "kspace_style.html">kspace_style</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 = "Wolf"></A>
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<P><B>(Wolf)</B> D. Wolf, P. Keblinski, S. R. Phillpot, J. Eggebrecht, J Chem
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Phys, 110, 8254 (1999).
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</P>
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<A NAME = "Fennell"></A>
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<P><B>(Fennell)</B> C. J. Fennell, J. D. Gezelter, J Chem Phys, 124,
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234104 (2006).
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</P>
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<A NAME = "Streitz"></A>
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<P><B>(Streitz)</B> F. H. Streitz, J. W. Mintmire, Phys Rev B, 50, 11996-12003
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(1994).
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</P>
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</HTML>
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