2007-10-04 08:21:28 +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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2012-10-03 23:40:18 +08:00
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<H3>pair_style lj/long/coul/long command
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</H3>
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<H3>pair_style lj/long/coul/long/omp command
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2011-10-07 01:32:51 +08:00
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</H3>
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2012-10-09 22:51:20 +08:00
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<H3>pair_style lj/long/tip4p/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 style args
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</PRE>
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<UL><LI>style = <I>lj/long/coul/long</I> or <I>lj/long/tip4p/long</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/long/coul/long</I> args = flag_lj flag_coul cutoff (cutoff2)
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flag_lj = <I>long</I> or <I>cut</I>
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<I>long</I> = use Kspace long-range summation for dispersion 1/r^6 term
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<I>cut</I> = use a cutoff
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flag_coul = <I>long</I> or <I>off</I>
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<I>long</I> = use Kspace long-range summation for Coulombic 1/r term
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<I>off</I> = omit Coulombic term
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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/tip4p/long</I> args = flag_lj flag_coul otype htype btype atype qdist cutoff (cutoff2)
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flag_lj = <I>long</I> or <I>cut</I>
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<I>long</I> = use Kspace long-range summation for dispersion 1/r^6 term
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<I>cut</I> = use a cutoff
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flag_coul = <I>long</I> or <I>off</I>
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<I>long</I> = use Kspace long-range summation for Coulombic 1/r term
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<I>off</I> = omit Coulombic term
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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/long/coul/long cut off 2.5
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pair_style lj/long/coul/long cut long 2.5 4.0
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pair_style lj/long/coul/long long long 2.5 4.0
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pair_coeff * * 1 1
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pair_coeff 1 1 1 3 4
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</PRE>
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<PRE>pair_style lj/long/tip4p/long long long 1 2 7 8 0.15 12.0
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pair_style lj/long/tip4p/long long long 1 2 7 8 0.15 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>Style <I>lj/long/coul/long</I> computes the standard 12/6 Lennard-Jones and
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Coulombic potentials, 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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<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, epsilon is the dielectric constant which can be set by
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the <A HREF = "dielectric.html">dielectric</A> command, and Rc is the cutoff. If
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one cutoff is specified in the pair_style command, it is used for both
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the LJ and Coulombic terms. If two cutoffs are specified, they are
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used as cutoffs for the LJ and Coulombic terms respectively.
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</P>
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<P>The purpose of this pair style is to capture long-range interactions
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resulting from both attractive 1/r^6 Lennard-Jones and Coulombic 1/r
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interactions. This is done by use of the <I>flag_lj</I> and <I>flag_coul</I>
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settings. The <A HREF = "#Veld">In 't Veld</A> paper has more details on when it is
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appropriate to include long-range 1/r^6 interactions, using this
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potential.
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</P>
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<P>Style <I>lj/cut/tip4p/long</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.
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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 style. Note that the
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neighobr list cutoff for Coulomb interactions is effectively extended
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by a distance 2*qdist when using the TIP4P pair style, to account for
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the offset distance of the fictitious charges on O atoms in water
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molecules. Thus it is typically best in an efficiency sense to use a
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LJ cutoff >= Coulomb cutoff + 2*qdist, to shrink the size of the
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neighbor list. This leads to slightly larger cost for the long-range
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calculation, so you can test the trade-off for your model.
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</P>
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<P>If <I>flag_lj</I> is set to <I>long</I>, no cutoff is used on the LJ 1/r^6
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dispersion term. The long-range portion is calculated by using the
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<A HREF = "kspace_style.html">kspace_style ewald/n</A> command. The specified LJ
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cutoff then determines which portion of the LJ interactions are
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computed directly by the pair potential versus which part is computed
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in reciprocal space via the Kspace style. If <I>flag_lj</I> is set to
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<I>cut</I>, the LJ interactions are simply cutoff, as with <A HREF = "pair_lj.html">pair_style
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lj/cut</A>.
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</P>
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<P>If <I>flag_coul</I> is set to <I>long</I>, no cutoff is used on the Coulombic
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interactions. The long-range portion is calculated by using any
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style, including <I>ewald/n</I> of the <A HREF = "kspace_style.html">kspace_style</A>
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command. Note that if <I>flag_lj</I> is also set to long, then only the
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<I>ewald/n</I> Kspace style can perform the long-range calculations for
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both the LJ and Coulombic interactions. If <I>flag_coul</I> is set to
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<I>off</I>, Coulombic interactions are not computed.
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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>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.
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</P>
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<P>Note that if you are using <I>flag_lj</I> set to <I>long</I>, you
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cannot specify a LJ cutoff for an atom type pair, since only one
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global LJ cutoff is allowed. Similarly, if you are using <I>flag_coul</I>
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set to <I>long</I>, you cannot specify a Coulombic cutoff for an atom type
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pair, since only one global Coulombic cutoff is allowed.
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</P>
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2012-10-09 22:51:20 +08:00
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<P>For <I>lj/long/tip4p/long</I> only the LJ cutoff can be specified
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since a Coulombic cutoff cannot be specified for an individual I,J
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type pair. All type pairs use the same global Coulombic cutoff
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specified in the pair_style command.
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</P>
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2007-10-04 08:21:28 +08:00
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<HR>
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2011-12-14 04:35:35 +08:00
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<P>Styles with a <I>cuda</I>, <I>gpu</I>, <I>omp</I>, or <I>opt</I> suffix are functionally
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the same as the corresponding style without the suffix. They have
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been optimized to run faster, depending on your available hardware, as
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discussed in <A HREF = "Section_accelerate.html">Section_accelerate</A> of the
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manual. The accelerated styles take the same arguments and should
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produce the same results, except for round-off and precision issues.
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2011-10-07 01:32:51 +08:00
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</P>
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<P>These accelerated styles are part of the USER-CUDA, GPU, USER-OMP and OPT
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packages, respectively. They are only enabled if LAMMPS was built with
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those packages. See the <A HREF = "Section_start.html#start_3">Making LAMMPS</A>
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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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2011-12-14 04:35:35 +08:00
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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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2011-10-07 01:32:51 +08:00
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</P>
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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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2007-10-04 08:21:28 +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 distance for all of the lj/cut pair styles can be mixed.
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The default mix value is <I>geometric</I>. See the "pair_modify" command
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for details.
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</P>
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<P>This pair style supports the <A HREF = "pair_modify.html">pair_modify</A> shift
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option for the energy of the Lennard-Jones portion of the pair
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interaction, assuming <I>flag_lj</I> is <I>cut</I>.
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</P>
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<P>This pair style supports the <A HREF = "pair_modify.html">pair_modify</A> table
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option since it can tabulate the short-range portion of the long-range
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Coulombic interaction.
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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 a long-range tail correction to the
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Lennard-Jones portion of the energy and 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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<P>This pair style supports the use of the <I>inner</I>, <I>middle</I>, and <I>outer</I>
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keywords of the <A HREF = "run_style.html">run_style respa</A> command, meaning the
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pairwise forces can be partitioned by distance at different levels of
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the rRESPA hierarchy. See the <A HREF = "run_style.html">run_style</A> command for
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details.
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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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2012-10-03 07:18:29 +08:00
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<P>These styles are part of the KSPACE package. They are only enabled if
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LAMMPS was built with that package. See the <A HREF = "Section_start.html#start_3">Making
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2012-10-03 06:54:29 +08:00
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LAMMPS</A> section for more info. Note that
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the KSPACE package is installed by default.
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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 = "Veld"></A>
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<P><B>(In 't Veld)</B> In 't Veld, Ismail, Grest, J Chem Phys (accepted) (2007).
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</P>
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</HTML>
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