# dihedral_write command

## Syntax

``` LAMMPS
dihedral_write dtype N file keyword
```

-   dtype = dihedral type
-   N = \# of values
-   file = name of file to write values to
-   keyword = section name in file for this set of tabulated values

## Examples

``` LAMMPS
dihedral_write 1  500 table.txt Harmonic_1
dihedral_write 3 1000 table.txt Harmonic_3
```

## Description

::: versionadded
8Feb2023
:::

Write energy and force values to a file as a function of the dihedral
angle for the currently defined dihedral potential. Force in this
context means the force with respect to the dihedral angle, not the
force on individual atoms. This is useful for plotting the potential
function or otherwise debugging its values. The resulting file can also
be used as input for use with [dihedral style table](dihedral_table).

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. The individual sections may be identified by the *keyword*.

The energy and force values are computed for dihedrals ranging from 0
degrees to 360 degrees for 4 interacting atoms forming an dihedral type
dtype, using the appropriate [dihedral_coeff](dihedral_coeff)
coefficients. N evenly spaced dihedrals are used. Since 0 and 360
degrees are the same dihedral angle, the latter entry is skipped.

For example, for N = 6, values would be computed at
$\phi = 0, 60, 120, 180, 240, 300$.

The file is written in the format used as input for the [dihedral_style
table](dihedral_table) option with *keyword* as the section name. Each
line written to the file lists an index number (1-N), an dihedral angle
(in degrees), an energy (in energy units), and a force (in force units
per radians\^2). In case a new file is created, the first line will be a
comment with a \"DATE:\" and \"UNITS:\" tag with the current date and
[units](units) settings. For subsequent invocations of the
*dihedral_write* command for the same file, data will be appended and
the current units settings will be compared to the data from the header,
if present. The *dihedral_write* will refuse to add a table to an
existing file if the units are not the same.

## Restrictions

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

The table of the dihedral energy and force data data is created by using
a separate, internally created, new LAMMPS instance with a dummy system
of 4 atoms for which the dihedral potential energy is computed after
transferring the dihedral style and coefficients and arranging the 4
atoms into the corresponding geometries. The dihedral force is then
determined from the potential energies through numerical
differentiation. As a consequence of this approach, not all dihedral
styles are compatible. The following conditions must be met:

-   The dihedral style must be able to write its coefficients to a data
    file. This condition excludes for example [dihedral style
    hybrid](dihedral_hybrid) and [dihedral style table](dihedral_table).
-   The potential function must not have any terms that depend on
    geometry properties other than the dihedral. This condition excludes
    for example [dihedral style class2](dihedral_class2). Please note
    that the *write_dihedral* command has no way of checking for this
    condition. It will check the style name against an internal list of
    known to be incompatible styles. The resulting tables may be bogus
    for unlisted dihedral styles if the requirement is not met. It is
    thus recommended to make careful tests for any created tables.

## Related commands

[dihedral_style table](dihedral_table), [bond_write](bond_write),
[angle_write](angle_write), [dihedral_style](dihedral_style),
[dihedral_coeff](dihedral_coeff)

## Default

none