forked from lijiext/lammps
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180 lines
7.3 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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<HR>
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<H3>pair_style buck/long/coul/long command
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</H3>
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<H3>pair_style buck/long/coul/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 buck/long/coul/long flag_buck flag_coul cutoff (cutoff2)
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</PRE>
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<UL><LI>flag_buck = <I>long</I> or <I>cut</I>
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<PRE> <I>long</I> = use Kspace long-range summation for the dispersion term 1/r^6
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<I>cut</I> = use a cutoff
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</PRE>
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<LI>flag_coul = <I>long</I> or <I>off</I>
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<PRE> <I>long</I> = use Kspace long-range summation for the Coulombic term 1/r
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<I>off</I> = omit the Coulombic term
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</PRE>
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<LI>cutoff = global cutoff for Buckingham (and Coulombic if only 1 cutoff) (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 buck/long/coul/long cut off 2.5
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pair_style buck/long/coul/long cut long 2.5 4.0
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pair_style buck/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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<P><B>Description:</B>
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</P>
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<P>The <I>buck/long/coul/long</I> style computes a Buckingham potential (exp/6
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instead of Lennard-Jones 12/6) and Coulombic potential, given by
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</P>
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<CENTER><IMG SRC = "Eqs/pair_buck.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>Rc is the cutoff. If one cutoff is specified in the pair_style
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command, it is used for both the Buckingham and Coulombic terms. If
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two cutoffs are specified, they are used as cutoffs for the Buckingham
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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 Buckingham and Coulombic 1/r
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interactions. This is done by use of the <I>flag_buck</I> and <I>flag_coul</I>
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settings. The "<A HREF = "#Ismail">Ismail</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>If <I>flag_buck</I> is set to <I>long</I>, no cutoff is used on the Buckingham
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1/r^6 dispersion term. The long-range portion is calculated by using
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the <A HREF = "kspace_style.html">kspace_style ewald/n</A> command. The specified
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Buckingham cutoff then determines which portion of the Buckingham
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interactions are computed directly by the pair potential versus which
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part is computed in reciprocal space via the Kspace style. If
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<I>flag_buck</I> is set to <I>cut</I>, the Buckingham interactions are simply
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cutoff, as with <A HREF = "pair_buck.html">pair_style buck</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_buck</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 Buckingham and Coulombic interactions. If <I>flag_coul</I> is set
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to <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:
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</P>
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<UL><LI>A (energy units)
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<LI>rho (distance units)
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<LI>C (energy-distance^6 units)
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<LI>cutoff (distance units)
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<LI>cutoff2 (distance units)
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</UL>
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<P>The second coefficient, rho, must be greater than zero.
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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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Buckingham and Coulombic cutoffs specified in the pair_style command
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are used. If only one cutoff is specified, it is used as the cutoff
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for both Buckingham and Coulombic interactions for this type pair. If
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both coefficients are specified, they are used as the Buckingham and
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Coulombic cutoffs for this type pair. Note that if you are using
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<I>flag_buck</I> set to <I>long</I>, you cannot specify a Buckingham cutoff for
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an atom type pair, since only one global Buckingham cutoff is allowed.
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Similarly, if you are using <I>flag_coul</I> set to <I>long</I>, you cannot
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specify a Coulombic cutoff for an atom type pair, since only one
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global Coulombic cutoff is allowed.
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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>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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</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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<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>This pair styles does not support mixing. Thus, coefficients for all
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I,J pairs must be specified explicitly.
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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 exp() and 1/r^6 portion of the pair
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interaction, assuming <I>flag_buck</I> is <I>cut</I>.
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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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shift option for the energy of the Buckingham portion of the pair
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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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table option since a tabulation capability has not yet been added to
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this potential.
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</P>
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<P>This pair style write 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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<P>This style is part of the KSPACE package. It is 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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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 = "Ismail"></A>
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<P><B>(Ismail)</B> Ismail, Tsige, In 't Veld, Grest, Molecular Physics
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(accepted) (2007).
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</P>
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</HTML>
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