# velocityautocorr

Calculate velocity autocorrelation function.

velocityautocorr [<set name>] [<mask>] [usevelocity] [out <filename>] [diffout <file>] [maxlag <frames>] [tstep <timestep>] [direct] [norm]

`[<set name>]`

Data set name.

`[<mask>]`

Atoms(s) to calculate velocity autocorrelation (VAC) function for.

`[usevelocity]`

Use velocity information in frame if present. This will only give sensible results if the velocities are recorded close to the order of the

simulation time step.

`[out <filename>]`

Write VAC function to <filename>.

`[diffout <file>]`

File to write diffusion constants to.

`[maxlag <frames>]`

Maximum lag in frames to calculate VAC function for. Default is half the total number of frames.

`[tstep <timestep>]`

Time between frames in ps (default 1.0).

`[direct]`

Calculate VAC function directly instead of via FFT (will be much slower).

`[norm]`

Normalize resulting VAC function to 1.0.

**DataSet Aspects:**

`[D]`

Diffusion constant calculated from integral over VAC function in 1×10-5 cm2/s.

Calculate the velocity autocorrelation (VAC) function averaged over the atoms in <mask>. Pseudo-velocities are calculated using coordinates and the specified time step. As with all time correlation functions the statistical noise will increase if the maximum lag is greater than half the total number of frames. In addition to calculating the velocity autocorrelation function, the self-diffusion coefficient will be reported in the output, calculated from the

integral over the VAC function.