lammps/doc/angle_table.txt

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angle_style table command :h3

[Syntax:]

angle_style table style N :pre

style = {linear} or {spline} = method of interpolation
N = use N values in table :ul

[Examples:]

angle_style table linear 1000
angle_coeff 3 file.table ENTRY1 :pre

[Description:]

Style {table} creates interpolation tables of length {N} from angle
potential and force values listed in a file(s) as a function of angle
The files are read by the "angle_coeff"_angle_coeff.html command.

The interpolation tables are created by fitting cubic splines to the
file values and interpolating energy and force values at each of {N}
distances.  During a simulation, these tables are used to interpolate
energy and force values as needed.  The interpolation is done in one
of 2 styles: {linear} or {spline}.

For the {linear} style, the angle is used to find 2 surrounding table
values from which an energy or force is computed by linear
interpolation.

For the {spline} style, a cubic spline coefficients are computed and
stored at each of the {N} values in the table.  The angle is used to
find the appropriate set of coefficients which are used to evaluate a
cubic polynomial which computes the energy or force.

The following coefficients must be defined for each angle type via the
"angle_coeff"_angle_coeff.html command as in the example above.

filename
keyword :ul

The filename specifies a file containing tabulated energy and force
values.  The keyword specifies a section of the file.  The format of
this file is described below.

:line

The format of a tabulated file is as follows (without the
parenthesized comments):

# Angle potential for harmonic (one or more comment or blank lines) :pre

HAM                           (keyword is the first text on line)
N 181 FP 0 0 EQ 90.0          (N, FP, EQ parameters)
                              (blank line)
N 181 FP 0 0                  (N, FP parameters)
1 0.0 200.5 2.5               (index, angle, energy, force)
2 1.0 198.0 2.5
...
181 180.0 0.0 0.0 :pre

A section begins with a non-blank line whose 1st character is not a
"#"; blank lines or lines starting with "#" can be used as comments
between sections.  The first line begins with a keyword which
identifies the section.  The line can contain additional text, but the
initial text must match the argument specified in the
"angle_coeff"_angle_coeff.html 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.

The parameter "N" is required and its value is the number of table
entries that follow.  Note that this may be different than the {N}
specified in the "angle_style table"_angle_style.html command.  Let
Ntable = {N} in the angle_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 angles.  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.

The "FP" parameter is optional.  If used, it is followed by two values
fplo and fphi, which are the derivatives of the force at the innermost
and outermost angle settings.  These values are needed by the spline
construction routines.  If not specified by the "FP" parameter, they
are estimated (less accurately) by the first two and last two force
values in the table.

The "EQ" parameter is also optional.  If used, it is followed by a the
equilibrium angle value, which is used, for example, by the "fix
shake"_fix_shake.html command.  If not used, the equilibrium angle is
set to 180.0.

Following a blank line, the next N lines list the tabulated values.
On each line, the 1st value is the index from 1 to N, the 2nd value is
the angle value (in degrees), the 3rd value is the energy (in energy
units), and the 4th is the force (in force units).  The angle values
must increase from one line to the next.

Note that one file can contain many sections, each with a tabulated
potential.  LAMMPS reads the file section by section until it finds
one that matches the specified keyword.

[Restrictions:]

This angle style can only be used if LAMMPS was built with the
"molecular" package (which it is by default).  See the "Making
LAMMPS"_Section_start.html#2_3 section for more info on packages.

[Related commands:]

"angle_coeff"_angle_coeff.html

[Default:] none