\(\renewcommand{\AA}{\text{Å}}\)

# pair_write command

## Syntax

    pair_write itype jtype N style inner outer file keyword Qi Qj

- itype,jtype = 2 atom types (numeric or type label)

- N = # of values

- style = r or rsq or bitmap

- inner,outer = inner and outer cutoff (distance units)

- file = name of file to write values to

- keyword = section name in file for this set of tabulated values

- Qi,Qj = 2 atom charges (charge units) (optional)

## Examples

    pair_write 1 3 500 r 1.0 10.0 table.txt LJ
    pair_write 1 1 1000 rsq 2.0 8.0 table.txt Yukawa_1_1 -0.5 0.5

    labelmap atom 1 C 2 H
    pair_write C H 500 r 1.0 10.0 table.txt LJ

## Description

Write energy and force values to a file as a function of distance for the currently defined pair potential. This is useful for plotting the potential function or otherwise debugging its values. If the file already exists, the table of values is appended to the end of the file to allow multiple tables of energy and force to be included in one file. In case a new file is created, the first line will be a comment containing a “DATE:” and “UNITS:” tag with the current date and the current units setting as argument. For subsequent invocations of the pair_write command, the current units setting is compared against the entry in the file, if present, and pair_write will refuse to add a table if the units are not the same.

The energy and force values are computed at distances from inner to outer for 2 interacting atoms of type itype and jtype, using the appropriate pair_coeff coefficients. If the style is r, then N distances are used, evenly spaced in r; if the style is rsq, N distances are used, evenly spaced in r^2.

For example, for N = 7, style = r, inner = 1.0, and outer = 4.0, values are computed at r = 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0.

If the style is bitmap, then 2^N values are written to the file in a format and order consistent with how they are read in by the pair_coeff command for pair style table. For reasonable accuracy in a bitmapped table, choose N >= 12, an inner value that is smaller than the distance of closest approach of 2 atoms, and an outer value <= cutoff of the potential.

If the pair potential is computed between charged atoms, the charges of the pair of interacting atoms can optionally be specified. If not specified, values of Qi = Qj = 1.0 are used.

The file is written in the format used as input for the pair_style table option with keyword as the section name. Each line written to the file lists an index number (1-N), a distance (in distance units), an energy (in energy units), and a force (in force units).

## Restrictions

All force field coefficients for pair and other kinds of interactions must be set before this command can be invoked.

Due to how the pairwise force is computed, an inner value > 0.0 must be specified even if the potential has a finite value at r = 0.0.

The pair_write command can only be used for pairwise additive interactions for which a Pair::single() function can be and has been implemented. This excludes for example manybody potentials or TIP4P coulomb styles.

## Related commands

pair_style table, pair_style, pair_coeff

## Default

none