# compute cna/atom command

## Syntax

``` LAMMPS
compute ID group-ID cna/atom cutoff
```

-   ID, group-ID are documented in [compute](compute) command
-   cna/atom = style name of this compute command
-   cutoff = cutoff distance for nearest neighbors (distance units)

## Examples

``` LAMMPS
compute 1 all cna/atom 3.08
```

## Description

Define a computation that calculates the CNA (Common Neighbor Analysis)
pattern for each atom in the group. In solid-state systems the CNA
pattern is a useful measure of the local crystal structure around an
atom. The CNA methodology is described in [(Faken)](Faken) and
[(Tsuzuki)](Tsuzuki1).

Currently, there are five kinds of CNA patterns LAMMPS recognizes:

-   fcc = 1
-   hcp = 2
-   bcc = 3
-   icosahedral = 4
-   unknown = 5

The value of the CNA pattern will be 0 for atoms not in the specified
compute group. Note that normally a CNA calculation should only be
performed on mono-component systems.

The CNA calculation can be sensitive to the specified cutoff value. You
should ensure the appropriate nearest neighbors of an atom are found
within the cutoff distance for the presumed crystal structure (e.g., 12
nearest neighbor for perfect FCC and HCP crystals, 14 nearest neighbors
for perfect BCC crystals). These formulas can be used to obtain a good
cutoff distance:

$$\begin{aligned}
r_{c}^{\mathrm{fcc}} = & \frac{1}{2} \left(\frac{\sqrt{2}}{2} + 1\right) a
  \approx 0.8536 a \\
r_{c}^{\mathrm{bcc}} = & \frac{1}{2}(\sqrt{2} + 1) a
  \approx 1.207 a \\
r_{c}^{\mathrm{hcp}} = & \frac{1}{2}\left(1+\sqrt{\frac{4+2x^{2}}{3}}\right) a
\end{aligned}$$

where $a$ is the lattice constant for the crystal structure concerned
and in the HCP case, $x = (c/a) / 1.633$, where 1.633 is the ideal $c/a$
for HCP crystals.

Also note that since the CNA calculation in LAMMPS uses the neighbors of
an owned atom to find the nearest neighbors of a ghost atom, the
following relation should also be satisfied:

$$r_c + r_s > 2*{\rm cutoff}$$

where $r_c$ is the cutoff distance of the potential, $r_s$ is the skin
distance as specified by the [neighbor](neighbor) command, and cutoff is
the argument used with the compute cna/atom command. LAMMPS will issue a
warning if this is not the case.

The neighbor list needed to compute this quantity is constructed each
time the calculation is performed (e.g. 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 with a
*cna/atom* style.

## 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 per-atom vector values will be a number from 0 to 5, as explained
above.

## Restrictions

> none

## Related commands

[compute centro/atom](compute_centro_atom)

## Default

none

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

::: {#Faken}
**(Faken)** Faken, Jonsson, Comput Mater Sci, 2, 279 (1994).
:::

::: {#Tsuzuki1}
**(Tsuzuki)** Tsuzuki, Branicio, Rino, Comput Phys Comm, 177, 518
(2007).
:::