---
name: "gmx-velacc"
section: "1"
platform: "linux"
package: "gromacs-data"
version: "2025.2-1"
release: "Debian 13 (Trixie)"
platforms: ["linux"]
url: "https://new.carta.tech/gmx-velacc"
---

# gmx-velacc(1)

Calculate velocity autocorrelation functions

## NAME

gmx-velacc - Calculate velocity autocorrelation functions

## SYNOPSIS

```
gmx velacc [-f [<.trr/.cpt/...>]] [-s [<.tpr/.gro/...>]] [-n [<.ndx>]]
           [-o [<.xvg>]] [-os [<.xvg>]] [-b <time>] [-e <time>]
           [-dt <time>] [-[no]w] [-xvg <enum>] [-[no]m] [-[no]recip]
           [-[no]mol] [-acflen <int>] [-[no]normalize] [-P <enum>]
           [-fitfn <enum>] [-beginfit <real>] [-endfit <real>]
```

## DESCRIPTION

**gmx velacc** computes the velocity autocorrelation function. When the **-m** option is used, the momentum autocorrelation function is calculated.

With option **-mol** the velocity autocorrelation function of molecules is calculated. In this case the index group should consist of molecule numbers instead of atom numbers.

By using option **-os** you can also extract the estimated (vibrational) power spectrum, which is the Fourier transform of the velocity autocorrelation function. Be sure that your trajectory contains frames with velocity information (i.e. **nstvout** was set in your original *.mdp* file), and that the time interval between data collection points is much shorter than the time scale of the autocorrelation.

## OPTIONS

Options to specify input files:

- **-f \[\<.trr/.cpt/...\>\] (traj.trr)**

  Full precision trajectory: *trr* *cpt* *tng*

- **-s \[\<.tpr/.gro/...\>\] (topol.tpr) (Optional)**

  Structure+mass(db): *tpr* *gro* *g96* *pdb* brk ent

- **-n \[\<.ndx\>\] (index.ndx) (Optional)**

  Index file

Options to specify output files:

- **-o \[\<.xvg\>\] (vac.xvg)**

  xvgr/xmgr file

- **-os \[\<.xvg\>\] (spectrum.xvg) (Optional)**

  xvgr/xmgr file

Other options:

- **-b \<time\> (0)**

  Time of first frame to read from trajectory (default unit ps)

- **-e \<time\> (0)**

  Time of last frame to read from trajectory (default unit ps)

- **-dt \<time\> (0)**

  Only use frame when t MOD dt = first time (default unit ps)

- **-\[no\]w (no)**

  View output *.xvg*, *.xpm*, *.eps* and *.pdb* files

- **-xvg \<enum\> (xmgrace)**

  xvg plot formatting: xmgrace, xmgr, none

- **-\[no\]m (no)**

  Calculate the momentum autocorrelation function

- **-\[no\]recip (yes)**

  Use cm^-1 on X-axis instead of 1/ps for spectra.

- **-\[no\]mol (no)**

  Calculate the velocity acf of molecules

- **-acflen \<int\> (-1)**

  Length of the ACF, default is half the number of frames

- **-\[no\]normalize (yes)**

  Normalize ACF

- **-P \<enum\> (0)**

  Order of Legendre polynomial for ACF (0 indicates none): 0, 1, 2, 3

- **-fitfn \<enum\> (none)**

  Fit function: none, exp, aexp, exp\_exp, exp5, exp7, exp9

- **-beginfit \<real\> (0)**

  Time where to begin the exponential fit of the correlation function

- **-endfit \<real\> (-1)**

  Time where to end the exponential fit of the correlation function, -1 is until the end

## SEE ALSO

**[gmx(1)](/gmx/1.md)**

More information about GROMACS is available at \<*http://www.gromacs.org/*\>.

## COPYRIGHT

2025, GROMACS development team