# 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 $3N$, where $N$ 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 $\omega_{max}=2\pi\nu_{max}$ 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 $\omega_{max} = 2\pi\nu_{max} =$ 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.