# compute msd/nongauss command

## Syntax

``` LAMMPS
compute ID group-ID msd/nongauss keyword values ...
```

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

-   msd/nongauss = style name of this compute command

-   zero or more keyword/value pairs may be appended

-   keyword = *com*

        *com* value = *yes* or *no*

## Examples

``` LAMMPS
compute 1 all msd/nongauss
compute 1 upper msd/nongauss com yes
```

## Description

Define a computation that calculates the mean-squared displacement (MSD)
and non-Gaussian parameter (NGP) of the group of atoms, including all
effects due to atoms passing through periodic boundaries.

A vector of three quantities is calculated by this compute. The first
element of the vector is the total squared displacement,
$dr^2 = dx^2 + dy^2 + dz^2$, of the atoms, and the second is the fourth
power of these displacements, $dr^4 = (dx^2 + dy^2 + dz^2)^2$, summed
and averaged over atoms in the group. The third component is the
non-Gaussian diffusion parameter NGP,

$$\text{NGP}(t) = \frac{3\left\langle(r(t)-r(0))^4\right\rangle}
                     {5\left\langle(r(t)-r(0))^2\right\rangle^2} - 1.$$

The NGP is a commonly used quantity in studies of dynamical
heterogeneity. Its minimum theoretical value $(-0.4)$ occurs when all
atoms have the same displacement magnitude. $\text{NGP}=0$ for Brownian
diffusion, while $\text{NGP} > 0$ when some mobile atoms move faster
than others.

If the *com* option is set to *yes* then the effect of any drift in the
center-of-mass of the group of atoms is subtracted out before the
displacement of each atom is calculated.

See the [compute msd](compute_msd) page for further important NOTEs,
which also apply to this compute.

## Output info

This compute calculates a global vector of length 3, which can be
accessed by indices 1\--3 by any command that uses global vector values
from a compute as input. See the [Howto output](Howto_output) doc page
for an overview of LAMMPS output options.

The vector values are \"intensive\". The first vector value will be in
distance$^2$ [units](units), the second is in distance$^4$ units, and
the third is dimensionless.

## Restrictions

Compute *msd/nongauss* cannot be used with a dynamic group.

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.

## Related commands

[compute msd](compute_msd)

## Default

The option default is com = no.