2006-09-22 00:22:34 +08:00
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"LAMMPS WWW Site"_lws - "LAMMPS Documentation"_ld - "LAMMPS Commands"_lc :c
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:link(lws,http://lammps.sandia.gov)
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:link(ld,Manual.html)
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:link(lc,Section_commands.html#comm)
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:line
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pair_style table command :h3
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[Syntax:]
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pair_style table style N :pre
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style = {lookup} or {linear} or {spline} or {bitmap} = method of interpolation
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N = use N values in {lookup}, {linear}, {spline} tables
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N = use 2^N values in {bitmap} tables :ul
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[Examples:]
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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 :pre
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[Description:]
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Style {table} creates interpolation tables of length {N} 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 "pair_coeff"_pair_coeff.html
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command.
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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 {N}
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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: {lookup}, {linear}, {spline}, or {bitmap}.
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For the {lookup} 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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For the {linear} 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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For the {spline} style, a cubic spline coefficients are computed and
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stored each of the {N} 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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For the {bitmap} 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 "(Wolff)"_#Wolff and a linear
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interpolation is performed between adjacent table values.
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The following coefficients must be defined for each pair of atoms
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types via the "pair_coeff"_pair_coeff.html 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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"read_data"_read_data.html or "read_restart"_read_restart.html
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commands:
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filename
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keyword
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cutoff (distance units) :ul
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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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2008-03-01 09:13:20 +08:00
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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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2008-04-03 07:05:25 +08:00
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table. The format of this file is described below.
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:line
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Here are some guidelines for using the pair_style table command to
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best effect:
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Vary the number of table points; you may need to use more than you think
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to get good resolution. :ulb,l
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Always use the "pair_write"_pair_write.html 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. :l
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Start with the linear style; it's the style least likely to have problems. :l
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Use as large an inner cutoff as possible. This avoids fitting splines
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to very steep parts of the potential. :l
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2008-04-03 07:07:12 +08:00
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Make sure the tabulated potential you are feeding to LAMMPS is not
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pathological. :l,ule
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:line
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The format of a tabulated file is as follows (without the
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parenthesized comments):
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# Morse potential for Fe (one or more comment or blank lines) :pre
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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 :pre
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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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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 {r} uniformly or however you
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wish to position table values in regions of large gradients.
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If used, the parameters "R" or "RSQ" are followed by 2 values {rlo}
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and {rhi}. 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 {rlo} and {rhi} are
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computed; for "RSQ", squared distances uniformly spaced between
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{rlo*rlo} and {rhi*rhi} are computed.
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If used, the parameter "BITMAP" is also followed by 2 values {rlo} and
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{rhi}. 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 "pair_write"_pair_write.html command
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with its {bitmap} 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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If used, the parameter "FPRIME" is followed by 2 values {fplo} and
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{fphi} 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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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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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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2007-06-26 08:03:39 +08:00
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:line
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2008-01-22 02:12:34 +08:00
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[Mixing, shift, table, tail correction, restart, rRESPA info]:
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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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The "pair_modify"_pair_modify.html shift, table, and tail options are
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not relevant for this pair style.
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This pair style writes the settings for the "pair_style table" command
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to "binary restart files"_restart.html, 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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2007-10-04 08:21:14 +08:00
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This pair style can only be used via the {pair} keyword of the
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"run_style respa"_run_style.html command. It does not support the
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{inner}, {middle}, {outer} keywords.
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2007-06-26 08:03:39 +08:00
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:line
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2006-09-28 03:12:31 +08:00
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2007-06-26 08:03:39 +08:00
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[Restrictions:] none
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[Related commands:]
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"pair_coeff"_pair_coeff.html
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[Default:] none
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:line
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:link(Wolff)
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[(Wolff)] Wolff and Rudd, Comp Phys Comm, 120, 200-32 (1999).
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