# compute ackland/atom command

## Syntax

``` LAMMPS
compute ID group-ID ackland/atom keyword/value
```

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

-   ackland/atom = style name of this compute command

-   zero or more keyword/value pairs may be appended

-   keyword = *legacy*

        *legacy* args = *yes* or *no* = use (\ *yes*\ ) or do not use (\ *no*\ ) legacy Ackland algorithm implementation

## Examples

``` LAMMPS
compute 1 all ackland/atom
compute 1 all ackland/atom legacy yes
```

## Description

Defines a computation that calculates the local lattice structure
according to the formulation given in [(Ackland)](Ackland).
Historically, LAMMPS had two, slightly different implementations of the
algorithm from the paper. With the *legacy* keyword, it is possible to
switch between the pre-2015 (\*legacy yes\*) and post-2015
implementation (\*legacy no\*). The post-2015 variant is the default.

In contrast to the [centro-symmetry parameter](compute_centro_atom) this
method is stable against temperature boost, because it is based not on
the distance between particles but the angles. Therefore statistical
fluctuations are averaged out a little more. A comparison with the
Common Neighbor Analysis metric is made in the paper.

The result is a number which is mapped to the following different
lattice structures:

-   0 = UNKNOWN
-   1 = BCC
-   2 = FCC
-   3 = HCP
-   4 = ICO

The neighbor list needed to compute this quantity is constructed each
time the calculation is performed (i.e. each time a snapshot of atoms is
dumped). Thus it can be inefficient to compute/dump this quantity too
frequently or to have multiple compute/dump commands, each of which
computes this quantity.-

## 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.

## Restrictions

This compute is part of the EXTRA-COMPUTE package. It is only enabled if
LAMMPS was built with that package. See the [Build
package](Build_package) page for more info.

The per-atom vector values will be unitless since they are the integers
defined above.

## Related commands

[compute centro/atom](compute_centro_atom)

## Default

The keyword *legacy* defaults to *no*.

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

::: {#Ackland}
**(Ackland)** Ackland, Jones, Phys Rev B, 73, 054104 (2006).
:::