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

### Description

`g_vanhove` computes the Van Hove correlation function. The Van Hove G(r,t) is the probability that a particle that is at r_0 at time zero can be found at position r_0+r at time t. `g_vanhove` determines G not for a vector r, but for the length of r. Thus it gives the probability that a particle moves a distance of r in time t. Jumps across the periodic boundaries are removed. Corrections are made for scaling due to isotropic or anisotropic pressure coupling.

With option `-om` the whole matrix can be written as a function of t and r or as a function of sqrt(t) and r (option `-sqrt`).

With option `-or` the Van Hove function is plotted for one or more values of t. Option `-nr` sets the number of times, option `-fr` the number spacing between the times. The binwidth is set with option `-rbin`. The number of bins is determined automatically.

With option `-ot` the integral up to a certain distance (option `-rt`) is plotted as a function of time.

For all frames that are read the coordinates of the selected particles are stored in memory. Therefore the program may use a lot of memory. For options `-om` and `-ot` the program may be slow. This is because the calculation scales as the number of frames times `-fm` or `-ft`. Note that with the `-dt` option the memory usage and calculation time can be reduced.

### Files

option | filename | type | description
--- | --- | --- | ---
`-f` | `traj.xtc` | Input | Trajectory: xtc trr trj gro g96 pdb cpt
`-s` | `topol.tpr` | Input | Structure+mass(db): tpr tpb tpa gro g96 pdb
`-n` | `index.ndx` | Input, Opt. | Index file
`-om` | `vanhove.xpm` | Output, Opt. | X PixMap compatible matrix file
`-or` | `vanhove_r.xvg` | Output, Opt. | xvgr/xmgr file
`-ot` | `vanhove_t.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`
`-sqrt` | real | `0` | Use sqrt(t) on the matrix axis which binspacing # in sqrt(ps)
`-fm` | int | `0` | Number of frames in the matrix, 0 is plot all
`-rmax` | real | `2` | Maximum r in the matrix (nm)
`-rbin` | real | `0.01` | Binwidth in the matrix and for `-or` (nm)
`-mmax` | real | `0` | Maximum density in the matrix, 0 is calculate (1/nm)
`-nlevels` | int | `81` | Number of levels in the matrix
`-nr` | int | `1` | Number of curves for the `-or` output
`-fr` | int | `0` | Frame spacing for the `-or` output
`-rt` | real | `0` | Integration limit for the `-ot` output (nm)
`-ft` | int | `0` | Number of frames in the `-ot` output, 0 is plot all

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