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187 lines
7.9 KiB
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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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<H3>pair_style table 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 table style N
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</PRE>
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<UL><LI>style = <I>lookup</I> or <I>linear</I> or <I>spline</I> or <I>bitmap</I> = method of interpolation
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<LI>N = use N values in <I>lookup</I>, <I>linear</I>, <I>spline</I> tables
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<LI>N = use 2^N values in <I>bitmap</I> tables
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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 table linear 1000
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pair_style table bitmap 12
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pair_coeff * 3 morse.table ENTRY1
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pair_coeff * 3 morse.table ENTRY1 7.0
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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>table</I> creates interpolation tables of length <I>N</I> from pair
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potential and force values listed in a file(s) as a function of
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distance. The files are read by the <A HREF = "pair_coeff.html">pair_coeff</A>
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command.
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</P>
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<P>The interpolation tables are created by fitting cubic splines to the
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file values and interpolating energy and force values at each of <I>N</I>
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distances. During a simulation, these tables are used to interpolate
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energy and force values as needed. The interpolation is done in one
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of 4 styles: <I>lookup</I>, <I>linear</I>, <I>spline</I>, or <I>bitmap</I>.
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</P>
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<P>For the <I>lookup</I> style, the distance between 2 atoms is used to find
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the nearest table entry, which is the energy or force.
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</P>
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<P>For the <I>linear</I> style, the distance is used to find 2 surrounding
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table values from which an energy or force is computed by linear
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interpolation.
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</P>
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<P>For the <I>spline</I> style, a cubic spline coefficients are computed and
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stored each of the <I>N</I> values in the table. The pair distance is used
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to find the appropriate set of coefficients which are used to evaluate
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a cubic polynomial which computes the energy or force.
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</P>
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<P>For the <I>bitmap</I> style, the N means to create interpolation tables
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that are 2^N in length. The pair distance is used to index into the
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table via a fast bit-mapping technique <A HREF = "#Wolff">(Wolff)</A> and a linear
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interpolation is performed between adjacent table values.
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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>filename
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<LI>keyword
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<LI>cutoff (distance units)
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</UL>
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<P>The filename specifies a file containing tabulated energy and force
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values. The keyword specifies a section of the file. The cutoff is
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an optional coefficient. If not specified, the outer cutoff in the
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table itself (see below) will be used to build an interpolation table
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that extend to the largest tablulated distance. If specified, only
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file values up to the cutoff are used to create the interpolation
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table.
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</P>
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<P>The format of a tabulated file is as follows (without the
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parenthesized comments):
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</P>
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<PRE># Morse potential for Fe (one or more comment or blank lines)
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</PRE>
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<PRE>MORSE_FE (keyword is first text on line)
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N 500 R 1.0 10.0 (N, R, RSQ, BITMAP, FPRIME parameters)
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(blank)
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1 1.0 25.5 102.34 (index, r, energy, force)
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2 1.02 23.4 98.5
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...
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500 10.0 0.001 0.003
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</PRE>
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<P>A section begins with a non-blank line whose 1st character is not a
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"#"; blank lines or lines starting with "#" can be used as comments
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between sections. The first line begins with a keyword which
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identifies the section. The line can contain additional text, but the
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initial text must match the argument specified in the pair_coeff
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command. The next line lists (in any order) one or more parameters
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for the table. Each parameter is a keyword followed by one or more
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numeric values.
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</P>
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<P>The parameter "N" is required; its value is the number of table
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entries that follow. All other parameters are optional. If "R" or
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"RSQ" or "BITMAP" does not appear, then the distances in each line of
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the table are used as-is to perform spline interpolation. In this
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case, the table values can be spaced in <I>r</I> uniformly or however you
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wish to position table values in regions of large gradients.
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</P>
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<P>If used, the parameters "R" or "RSQ" are followed by 2 values <I>rlo</I>
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and <I>rhi</I>. If specified, the distance associated with each energy and
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force value is computed from these 2 values (at high accuracy), rather
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than using the (low-accuracy) value listed in each line of the table.
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For "R", distances uniformly spaced between <I>rlo</I> and <I>rhi</I> are
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computed; for "RSQ", squared distances uniformly spaced between
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<I>rlo*rlo</I> and <I>rhi*rhi</I> are computed.
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</P>
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<P>If used, the parameter "BITMAP" is also followed by 2 values <I>rlo</I> and
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<I>rhi</I>. These values, along with the "N" value determine the ordering
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of the N lines that follow and what distance is associated with each.
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This ordering is complex, so it is not documented here, since this
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file is typically produced by the <A HREF = "pair_write.html">pair_write</A> command
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with its <I>bitmap</I> option. When the table is in BITMAP format, the "N"
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parameter in the file must be equal to 2^M where M is the value
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specified in the pair_style command. Also, a cutoff parameter cannot
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be used as an optional 3rd argument in the pair_coeff command; the
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entire table extent as specified in the file must be used.
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</P>
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<P>If used, the parameter "FPRIME" is followed by 2 values <I>fplo</I> and
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<I>fphi</I> which are the derivative of the force at the innermost and
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outermost distances listed in the table. These values are needed by
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the spline construction routines. If not specified by the "FPRIME"
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parameter, they are estimated (less accurately) by the first 2 and
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last 2 force values in the table. This parameter is not used by
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BITMAP tables.
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</P>
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<P>Following a blank line, the next N lines list the tabulated values.
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On each line, the 1st value is the index from 1 to N, the 2nd value is
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r (in distance units), the 3rd value is the energy (in energy units),
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and the 4th is the force (in force units). The r values must increase
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from one line to the next (unless the BITMAP parameter is specified).
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</P>
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<P>Note that one file can contain many sections, each with a tabulated
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potential. LAMMPS reads the file section by section until it finds
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one that matches the specified keyword.
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</P>
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<HR>
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<P><B>Mixing, shift, table, tail correction, per-atom energy/stress,
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restart, rRESPA info</B>:
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</P>
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<P>This pair style 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>The <A HREF = "pair_modify.html">pair_modify</A> shift, table, and tail options are
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not relevant for this pair style.
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</P>
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<P>This pair style can calculate per-atom energy and stress, as used by
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the <A HREF = "compute_epair_atom.html">compute epair/atom</A>, <A HREF = "compute_stress_atom.html">compute
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stress/atom</A>, and <A HREF = "dump.html">dump custom</A>
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commands.
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</P>
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<P>This pair style writes the settings for the "pair_style table" command
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to <A HREF = "restart.html">binary restart files</A>, so a pair_style command does
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not need to specified in an input script that reads a restart file.
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However, the coefficient information is not stored in the restart
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file, since it is tabulated in the potential files. Thus, pair_coeff
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commands do need to be specified in the restart input script.
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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> none
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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 = "Wolff"></A>
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<P><B>(Wolff)</B> Wolff and Rudd, Comp Phys Comm, 120, 200-32 (1999).
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</P>
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</HTML>
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