# angle_write command

## Syntax

``` LAMMPS
angle_write atype N file keyword
```

-   atype = angle 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
angle_write 1  500 table.txt Harmonic_1
angle_write 3 1000 table.txt Harmonic_3
```

## Description

::: versionadded
8Feb2023
:::

Write energy and force values to a file as a function of angle for the
currently defined angle potential. Force in this context means the force
with respect to the 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 [angle
style table](angle_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 angles ranging from 0
degrees to 180 degrees for 3 interacting atoms forming an angle type
atype, using the appropriate [angle_coeff](angle_coeff) coefficients. N
evenly spaced angles are used.

For example, for N = 6, values are computed at $\theta = 0, 36,
72, 108, 144, 180$.

The file is written in the format used as input for the [angle_style
table](angle_table) option with *keyword* as the section name. Each line
written to the file lists an index number (1-N), an 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 *angle_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
*angle_write* will refuse to add a table to an existing file if the
units are not the same.

## Restrictions

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

The table of the angle energy and force data data is created by using a
separate, internally created, new LAMMPS instance with a dummy system of
3 atoms for which the angle potential energy is computed after
transferring the angle style and coefficients and arranging the 3 atoms
into the corresponding geometries. The angle force is then determined
from the potential energies through numerical differentiation. As a
consequence of this approach, not all angle styles are compatible. The
following conditions must be met:

-   The angle style must be able to write its coefficients to a data
    file. This condition excludes for example [angle style
    hybrid](angle_hybrid) and [angle style table](angle_table).
-   The potential function must not have any terms that depend on
    geometry properties other than the angle. This condition excludes
    for example [angle style class2](angle_class2) all angle types for
    [angle style charmm](angle_charmm) that have non-zero Urey-Bradley
    terms. Please note that the *write_angle* command has no way of
    checking for this condition, so the resulting tables may be bogus if
    the requirement is not met. It is thus recommended to make careful
    tests for any created tables.

## Related commands

[angle_style table](angle_table), [bond_write](bond_write),
[dihedral_write](dihedral_write), [angle_style](angle_style),
[angle_coeff](angle_coeff)

## Default

none