\(\renewcommand{\AA}{\text{Å}}\)

# compute ackland/atom command

## Syntax

    compute ID group-ID ackland/atom keyword/value

- ID, group-ID are documented in 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

    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). 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 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 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 page for more info.

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

## Related commands

compute centro/atom

## Default

The keyword legacy defaults to no.

(Ackland) Ackland, Jones, Phys Rev B, 73, 054104 (2006).