# The compute dos keyword

## Introduction

- This keyword computes the phonon density of states (PDOS) using the mass-weighted velocity autocorrelation (VAC). The output is normalized such that the integral of the PDOS over all frequencies equals [math]3N[/math], where [math]N[/math] is the number of atoms. If this keyword appears in a run, the mass-weighted VAC will be computed and directly used to compute the PDOS.

## Grammar

- For this keyword, the command looks like:

compute_dos sample_interval Nc omega_max <optional_args>

with parameters defined as:

`sample_interval`

: Sampling interval of the velocity data`Nc`

: Maximum number of correlation steps`omega_max`

: Maximum angular frequency [math]\omega_{max}=2\pi\nu_{max}[/math] used in the PDOS calculations.

The `optional_args`

provide additional functionality by allowing special keywords. The keywords for this function are `group`

and `num_dos_points`

. These keywords can be used in any order, but the parameters associated with each must follow exactly. The parameters are:

`group group_method group`

`group_method`

: The grouping method to use for computation`group`

: The group in the grouping method to use

`num_dos_points points`

`points`

: Number of frequency points to be used in the DOS calculation (`Nc`

if option not selected).

## Examples

### Example 1

- An example of this keyword is:

compute_dos 5 200 400.0 group 1 1 num_dos_points 300

This means that you (1) want to calculate the PDOS; (2) the velocity data will be recorded every 5 steps; (3) the maximum number of correlation steps is 200; (4) the maximum angular frequency you want to consider is [math]\omega_{max} = 2\pi\nu_{max} =[/math] 400 THz; (5) You would like to compute only over group 1 in group method 1; (5) You would like the maximum angular frequency to be cut in to 300 points for output. The results will be written into files named mvac.out for the mass-normalized VAC and dos.out for the DOS output.

## Caveats

- Cannot be used in the same run as the compute_sdc keyword.