2007-06-20 21:15:18 +08:00
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<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 dipole/cut command
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</H3>
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2011-08-09 05:51:11 +08:00
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<H3>pair_style dipole/sf command
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</H3>
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2007-06-20 21:15:18 +08:00
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<P><B>Syntax:</B>
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</P>
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<PRE>pair_style dipole/cut cutoff (cutoff2)
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</PRE>
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2011-08-09 05:51:11 +08:00
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<PRE>pair_style dipole/sf cutoff (cutoff2)
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</PRE>
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2007-06-20 21:15:18 +08:00
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<UL><LI>cutoff = global cutoff LJ (and Coulombic if only 1 arg) (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 dipole/cut 10.0
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pair_coeff * * 1.0 1.0
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pair_coeff 2 3 1.0 1.0 2.5 4.0
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</PRE>
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2011-08-09 05:51:11 +08:00
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<PRE>pair_style dipole/sf 9.0
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pair_coeff * * 1.0 1.0
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pair_coeff 2 3 1.0 1.0 2.5 4.0
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</PRE>
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2007-06-20 21:15:18 +08:00
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<P><B>Description:</B>
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</P>
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2008-02-20 05:47:37 +08:00
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<P>Style <I>dipole/cut</I> computes interactions between pairs of particles
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2007-06-20 21:15:18 +08:00
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that each have a charge and/or a point dipole moment. In addition to
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the usual Lennard-Jones interaction between the particles (Elj) the
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charge-charge (Eqq), charge-dipole (Eqp), and dipole-dipole (Epp)
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interactions are computed by these formulas for the energy (E), force
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(F), and torque (T) between particles I and J.
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</P>
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<CENTER><IMG SRC = "Eqs/pair_dipole.jpg">
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</CENTER>
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<P>where qi and qj are the charges on the two particles, pi and pj are
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the dipole moment vectors of the two particles, r is their separation
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distance, and the vector r = Ri - Rj is the separation vector between
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the two particles. Note that Eqq and Fqq are simply Coulombic energy
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and force, Fij = -Fji as symmetric forces, and Tij != -Tji since the
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torques do not act symmetrically. These formulas are discussed in
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<A HREF = "#Allen">(Allen)</A> and in <A HREF = "#Toukmaji">(Toukmaji)</A>.
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</P>
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2011-08-09 05:51:11 +08:00
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<P>Style <I>dipole/sf</I> computes "shifted-force" interactions between pairs
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of particles that each have a charge and/or a point dipole moment. In
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general, a shifted-force potential is a (sligthly) modified potential
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containing extra terms that make both the energy and its derivative go
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to zero at the cutoff distance; this removes (cutoff-related) problems
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in energy conservation and any numerical instability in the equations
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of motion <A HREF = "#Allen">(Allen)</A>. Shifted-force interactions for the
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Lennard-Jones (E_LJ), charge-charge (Eqq), charge-dipole (Eqp),
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dipole-charge (Epq) and dipole-dipole (Epp) potentials are computed by
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these formulas for the energy (E), force (F), and torque (T) between
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particles I and J:
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</P>
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<CENTER><IMG SRC = "Eqs/pair_dipole_sf.jpg">
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</CENTER>
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2011-08-17 07:39:57 +08:00
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<CENTER><IMG SRC = "Eqs/pair_dipole_sf2.jpg">
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</CENTER>
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<P>where epsilon and sigma are the standard LJ parameters, r_c is the
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cutoff, qi and qj are the charges on the two particles, pi and pj are
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2011-08-09 05:51:11 +08:00
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the dipole moment vectors of the two particles, r is their separation
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distance, and the vector r = Ri - Rj is the separation vector between
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the two particles. Note that Eqq and Fqq are simply Coulombic energy
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and force, Fij = -Fji as symmetric forces, and Tij != -Tji since the
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torques do not act symmetrically. The shifted-force formula for the
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Lennard-Jones potential is reported in <A HREF = "#Stoddard">(Stoddard)</A>. The
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original (unshifted) formulas for the electrostatic potentials, forces
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and torques can be found in <A HREF = "#Price">(Price)</A>. The shifted-force
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electrostatic potentials have been obtained by applying equation 5.13
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of <A HREF = "#Allen">(Allen)</A>. The formulas for the corresponding forces and
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torques have been obtained by applying the 'chain rule' as in appendix
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C.3 of <A HREF = "#Allen">(Allen)</A>.
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</P>
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2007-06-20 21:15:18 +08:00
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<P>If one cutoff is specified in the pair_style command, it is used for
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both the LJ and Coulombic (q,p) terms. If two cutoffs are specified,
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they are used as cutoffs for the LJ and Coulombic (q,p) terms
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respectively.
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</P>
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2009-03-16 22:46:18 +08:00
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<P>Atoms with dipole moments should be integrated using the <A HREF = "fix_nve_sphere.html">fix
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nve/sphere update dipole</A> command to rotate the
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dipole moments. The <A HREF = "compute_temp_sphere.html">compute temp/sphere</A>
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command can be used to monitor the temperature, since it includes
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rotational degrees of freedom. The <A HREF = "atom_style.html">atom_style
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dipole</A> command should be used since it defines the
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point dipoles and their rotational state. The magnitude of the dipole
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moment for each type of particle can be defined by the
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<A HREF = "dipole.html">dipole</A> command or in the "Dipoles" section of the data
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file read in by the <A HREF = "read_data.html">read_data</A> command. Their initial
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orientation can be defined by the <A HREF = "set.html">set dipole</A> command or in
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the "Atoms" section of the data file.
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2007-06-20 21:15:18 +08:00
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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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2007-06-25 22:36:36 +08:00
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commands, or by mixing as described below:
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2007-06-20 21:15:18 +08:00
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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>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.
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</P>
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2007-06-25 22:36:36 +08:00
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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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2007-06-25 22:36:36 +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 distances for this pair style can be mixed. The default
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mix value is <I>geometric</I>. See the "pair_modify" command for details.
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</P>
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<P>For atom type pairs I,J and I != J, the A, sigma, d1, and d2
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coefficients and cutoff distance for this pair style can be mixed. A
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is an energy value mixed like a LJ epsilon. D1 and d2 are distance
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values and are mixed like sigma. The default mix value is
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<I>geometric</I>. See the "pair_modify" command for details.
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</P>
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2011-08-09 05:51:11 +08:00
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<P>This pair style does not support the <A HREF = "pair_modify.html">pair_modify</A>
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shift option for the energy of the Lennard-Jones portion of the pair
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interaction; such energy goes to zero at the cutoff by construction.
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2007-06-20 21:15:18 +08:00
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</P>
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2007-06-25 22:36:36 +08:00
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<P>The <A HREF = "pair_modify.html">pair_modify</A> table option is not relevant
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for this pair style.
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</P>
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<P>This pair style does 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>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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2007-10-04 08:21:14 +08:00
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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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2007-10-12 07:09:49 +08:00
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<I>inner</I>, <I>middle</I>, <I>outer</I> keywords.
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2007-10-04 08:21:14 +08:00
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</P>
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2007-06-25 22:36:36 +08:00
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<P><B>Restrictions:</B>
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2007-06-20 21:15:18 +08:00
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</P>
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2011-08-09 05:51:11 +08:00
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<P>The <I>dipole/cut</I> style is part of the "dipole" package. It is only
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2011-08-26 00:46:23 +08:00
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enabled if LAMMPS was built with that package. See the <A HREF = "Section_start.html#start_3">Making
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2011-08-09 05:51:11 +08:00
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LAMMPS</A> section for more info.
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</P>
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<P>The <I>dipole/sf</I> style is part of the "user-misc" package. It is only
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2011-08-26 00:46:23 +08:00
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enabled if LAMMPS was built with that package. See the <A HREF = "Section_start.html#start_3">Making
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2007-06-25 22:36:36 +08:00
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LAMMPS</A> section for more info.
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2007-06-20 21:15:18 +08:00
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</P>
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<P><B>Related commands:</B>
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</P>
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2010-05-07 23:11:21 +08:00
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<P><A HREF = "pair_coeff.html">pair_coeff</A>
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2007-06-20 21:15:18 +08:00
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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 = "Allen"></A>
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<P><B>(Allen)</B> Allen and Tildesley, Computer Simulation of Liquids,
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Clarendon Press, Oxford, 1987.
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</P>
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<A NAME = "Toukmaji"></A>
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<P><B>(Toukmaji)</B> Toukmaji, Sagui, Board, and Darden, J Chem Phys, 113,
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10913 (2000).
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</P>
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2011-08-09 05:51:11 +08:00
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<A NAME = "Stoddard"></A>
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<P><B>(Stoddard)</B> Stoddard and Ford, Phys Rev A, 8, 1504 (1973).
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</P>
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<A NAME = "Price"></A>
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<P><B>(Price)</B> Price, Stone and Alderton, Mol Phys, 52, 987 (1984).
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</P>
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2007-06-20 21:15:18 +08:00
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</HTML>
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