# compute efield/wolf/atom command

## Syntax

``` LAMMPS
compute ID group-ID efield/wolf/atom alpha keyword val
```

-   ID, group-ID are documented in [compute](compute) command

-   efield/atom/wolf = style name of this compute command

-   alpha = damping parameter (inverse distance units)

-   zero or more keyword/value pairs may be appended

-   keyword = *limit* or *cutoff*

        *limit* group2-ID = limit computing the electric field contributions to a group (default: all)
        *cutoff* value = set cutoff for computing contributions to this value (default: maximum cutoff of pair style)

## Examples

``` LAMMPS
compute 1 all efield/wolf/atom 0.2
compute 1 mols efield/wolf/atom 0.25 limit water cutoff 10.0
```

## Description

::: versionadded
8Feb2023
:::

Define a computation that approximates the electric field at each atom
in a group.

$$\vec{E}_i = \frac{\vec{F}coul_i}{q_i} = \sum_{j \neq i} \frac{q_j}{r_{ij}^2} \qquad r < r_c$$

The electric field at the position of the atom *i* is the coulomb force
on a unit charge at that point, which is equivalent to dividing the
Coulomb force by the charge of the individual atom.

In this compute the electric field is approximated as the derivative of
the potential energy using the Wolf summation method, described in
[Wolf](Wolf4), given by:

$$E_i = \frac{1}{2} \sum_{j \neq i}
\frac{q_i q_j {\rm erfc}(\alpha r_{ij})}{r_{ij}} +
\frac{1}{2} \sum_{j \neq i}
\frac{q_i q_j {\rm erf}(\alpha r_{ij})}{r_{ij}} \qquad r < r_c$$

where $\alpha$ is the damping parameter, and *erf()* and *erfc()* are
error-function and complementary error-function terms. This potential is
essentially a short-range, spherically-truncated, charge-neutralized,
force-shifted, pairwise *1/r* summation. With a manipulation of adding
and subtracting a self term (for i = j) to the first and second term on
the right-hand-side, respectively, and a small enough $\alpha$ damping
parameter, the second term shrinks and the potential becomes a
rapidly-converging real-space summation. With a long enough cutoff and
small enough $\alpha$ parameter, the electric field calculated by the
Wolf summation method approaches that computed using the Ewald sum.

The value of the electric field components will be 0.0 for atoms not in
the specified compute group.

When the *limit* keyword is used, only contributions from atoms in the
selected group will be considered, otherwise contributions from all
atoms within the cutoff are included.

When the *cutoff* keyword is used, the cutoff used for the electric
field approximation can be set explicitly. By default it is the largest
cutoff of any pair style force computation.

::: {.admonition .note}
Computational Efficiency

This compute will loop over a full neighbor list just like a pair style
does when computing forces, thus it can be quite time-consuming and slow
down a calculation significantly when its data is used in every time
step. The [compute efield/atom](compute_efield_atom) command of the
DIELECTRIC package is more efficient in comparison, since the electric
field data is collected and stored as part of the force computation at
next to no extra computational cost.
:::

## Output info

This compute calculates a per-atom vector, which can be accessed by any
command that uses per-atom values from a compute as input. See the
[Howto output](Howto_output) page for an overview of LAMMPS output
options.

The vector contains 3 values per atom which are the x-, y-, and
z-direction electric field components in force units.

## Restrictions

This compute is part of the EXTRA-COMPUTE package. It is only enabled if
LAMMPS was built with that package.

## Related commands

[pair_style coul/wolf](pair_coul), [compute
efield/atom](compute_efield_atom)

## Default

The option defaults are *limit* = all and *cutoff* = largest cutoff for
pair styles.

------------------------------------------------------------------------

::: {#Wolf4}
**(Wolf)** D. Wolf, P. Keblinski, S. R. Phillpot, J. Eggebrecht, J Chem
Phys, 110, 8254 (1999).
:::