2006-09-22 00:22:34 +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 table command
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</H3>
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2012-01-26 00:00:02 +08:00
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<H3>pair_style table/gpu command
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</H3>
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2011-10-07 01:32:51 +08:00
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<H3>pair_style table/omp command
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</H3>
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2006-09-22 00:22:34 +08:00
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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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2009-08-11 04:13:44 +08:00
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<P>For the <I>linear</I> style, the pair distance is used to find 2
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surrounding table values from which an energy or force is computed by
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linear interpolation.
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2006-09-22 00:22:34 +08:00
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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 at each of the <I>N</I> values in the table. The pair distance is
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used to find the appropriate set of coefficients which are used to
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evaluate a cubic polynomial which computes the energy or force.
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2006-09-22 00:22:34 +08:00
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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.
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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 tabulated 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. The format of this file is described below.
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</P>
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<HR>
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2008-04-03 07:07:12 +08:00
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<P>Here are some guidelines for using the pair_style table command to
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best effect:
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</P>
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<UL><LI>Vary the number of table points; you may need to use more than you think
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to get good resolution.
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<LI>Always use the <A HREF = "pair_write.html">pair_write</A> command to produce a plot
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of what the final interpolated potential looks like. This can show up
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interpolation "features" you may not like.
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<LI>Start with the linear style; it's the style least likely to have problems.
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<LI>Use <I>N</I> in the pair_style command equal to the "N" in the tabulation
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file, and use the "RSQ" or "BITMAP" parameter, so additional interpolation
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is not needed. See discussion below.
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<LI>Make sure that your tabulated forces and tabulated energies are consistent
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(dE/dr = -F) along the entire range of r values.
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2008-04-03 07:05:25 +08:00
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<LI>Use as large an inner cutoff as possible. This avoids fitting splines
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to very steep parts of the potential.
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</UL>
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<HR>
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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 and its value is the number of table
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entries that follow. Note that this may be different than the <I>N</I>
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specified in the <A HREF = "pair_style.html">pair_style table</A> command. Let
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Ntable = <I>N</I> in the pair_style command, and Nfile = "N" in the
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tabulated file. What LAMMPS does is a preliminary interpolation by
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creating splines using the Nfile tabulated values as nodal points. It
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uses these to interpolate as needed to generate energy and force
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values at Ntable different points. The resulting tables of length
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Ntable are then used as described above, when computing energy and
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force for individual pair distances. This means that if you want the
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interpolation tables of length Ntable to match exactly what is in the
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tabulated file (with effectively no preliminary interpolation), you
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should set Ntable = Nfile, and use the "RSQ" or "BITMAP" parameter.
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The internal table abscissa is RSQ (separation distance squared).
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</P>
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<P>All other parameters are optional. If "R" or "RSQ" or "BITMAP" does
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not appear, then the distances in each line of the table are used
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as-is to perform spline interpolation. In this case, the table values
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can be spaced in <I>r</I> uniformly or however you wish to position table
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values in regions of large gradients.
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2006-09-22 00:22:34 +08:00
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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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The distance values in the table file are ignored in this case.
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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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2007-06-26 08:03:39 +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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</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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</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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2006-09-28 03:12:31 +08:00
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</P>
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2007-06-26 08:03:39 +08:00
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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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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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<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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