| ## g_velacc Main Table of Contents | VERSION 4.6 Sat 19 Jan 2013

### Description

`g_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.

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.

### Files

option | filename | type | description
--- | --- | --- | ---
`-f` | `traj.trr` | Input | Full precision trajectory: trr trj cpt
`-s` | `topol.tpr` | Input, Opt. | Structure+mass(db): tpr tpb tpa gro g96 pdb
`-n` | `index.ndx` | Input, Opt. | Index file
`-o` | `vac.xvg` | Output | xvgr/xmgr file
`-os` | `spectrum.xvg` | Output, Opt. | xvgr/xmgr file

### Other options

option | type | default | description
--- | --- | --- | ---
`-[no]h` | bool | `no` | Print help info and quit
`-[no]version` | bool | `no` | Print version info and quit
`-nice` | int | `19` | Set the nicelevel
`-b` | time | `0` | First frame (ps) to read from trajectory
`-e` | time | `0` | Last frame (ps) to read from trajectory
`-dt` | time | `0` | Only use frame when t MOD dt = first time (ps)
`-[no]w` | bool | `no` | View output `.xvg`, `.xpm`, `.eps` and `.pdb` files
`-xvg` | enum | `xmgrace` | xvg plot formatting: `xmgrace`, `xmgr` or `none`
`-[no]m` | bool | `no` | Calculate the momentum autocorrelation function
`-[no]recip` | bool | `yes` | Use cm^-1 on X-axis instead of 1/ps for spectra.
`-[no]mol` | bool | `no` | Calculate the velocity acf of molecules
`-acflen` | int | `-1` | Length of the ACF, default is half the number of frames
`-[no]normalize` | bool | `yes` | Normalize ACF
`-P` | enum | `0` | Order of Legendre polynomial for ACF (0 indicates none): `0`, `1`, `2` or `3`
`-fitfn` | enum | `none` | Fit function: `none`, `exp`, `aexp`, `exp_exp`, `vac`, `exp5`, `exp7`, `exp9` or `erffit`
`-ncskip` | int | `0` | Skip this many points in the output file of correlation functions
`-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

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