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git-svn-id: svn://svn.icms.temple.edu/lammps-ro/trunk@3037 f3b2605a-c512-4ea7-a41b-209d697bcdaa

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doc/pair_table.html
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@ -42,25 +42,23 @@ of 4 styles: <I>lookup</I>, <I>linear</I>, <I>spline</I>, or <I>bitmap</I>.
<P>For the <I>lookup</I> style, the distance between 2 atoms is used to find
the nearest table entry, which is the energy or force.
</P>
<P>For the <I>linear</I> style, the distance is used to find 2 surrounding
table values from which an energy or force is computed by linear
interpolation.
<P>For the <I>linear</I> style, the pair distance is used to find 2
surrounding table values from which an energy or force is computed by
linear interpolation.
</P>
<P>For the <I>spline</I> style, a cubic spline coefficients are computed and
stored each of the <I>N</I> values in the table. The pair distance is used
to find the appropriate set of coefficients which are used to evaluate
a cubic polynomial which computes the energy or force.
stored at each of the <I>N</I> values in the table. The pair distance is
used to find the appropriate set of coefficients which are used to
evaluate a cubic polynomial which computes the energy or force.
</P>
<P>For the <I>bitmap</I> style, the N means to create interpolation tables
that are 2^N in length. The pair distance is used to index into the
that are 2^N in length. <The pair distance is used to index into the
table via a fast bit-mapping technique <A HREF = "#Wolff">(Wolff)</A> and a linear
interpolation is performed between adjacent table values.
</P>
<P>The following coefficients must be defined for each pair of atoms
types via the <A HREF = "pair_coeff.html">pair_coeff</A> command as in the examples
above, or in the data file or restart files read by the
<A HREF = "read_data.html">read_data</A> or <A HREF = "read_restart.html">read_restart</A>
commands:
above.
</P>
<UL><LI>filename
<LI>keyword
@ -88,11 +86,12 @@ interpolation "features" you may not like.
<LI>Start with the linear style; it's the style least likely to have problems.
<LI>Use <I>N</I> in the pair_style command equal to the "N" in the tabulation
file, so additional interpolation is not needed. See discussion
below.
<LI>Use as large an inner cutoff as possible. This avoids fitting splines
to very steep parts of the potential.
<LI>Make sure the tabulated potential you are feeding to LAMMPS is not
pathological.
</UL>
<HR>
@ -118,12 +117,25 @@ command. The next line lists (in any order) one or more parameters
for the table. Each parameter is a keyword followed by one or more
numeric values.
</P>
<P>The parameter "N" is required; its value is the number of table
entries that follow. All other parameters are optional. If "R" or
"RSQ" or "BITMAP" does not appear, then the distances in each line of
the table are used as-is to perform spline interpolation. In this
case, the table values can be spaced in <I>r</I> uniformly or however you
wish to position table values in regions of large gradients.
<P>The parameter "N" is required and its value is the number of table
entries that follow. Note that this may be different than the <I>N</I>
specified in the <A HREF = "pair_style.html">pair_style table</A> command. Let
Ntable = <I>N</I> in the pair_style command, and Nfile = "N" in the
tabulated file. What LAMMPS does is a preliminary interpolation by
creating splines using the Nfile tabulated values as nodal points. It
uses these to interpolate as needed to generate energy and force
values at Ntable different points. The resulting tables of length
Ntable are then used as described above, when computing energy and
force for individual pair distances. This means that if you want the
interpolation tables of length Ntable to match exactly what is in the
tabulated file (with effectively no preliminary interpolation), you
should set Ntable = Nfile.
</P>
<P>All other parameters are optional. If "R" or "RSQ" or "BITMAP" does
not appear, then the distances in each line of the table are used
as-is to perform spline interpolation. In this case, the table values
can be spaced in <I>r</I> uniformly or however you wish to position table
values in regions of large gradients.
</P>
<P>If used, the parameters "R" or "RSQ" are followed by 2 values <I>rlo</I>
and <I>rhi</I>. If specified, the distance associated with each energy and