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

### Description

`g_rms` compares two structures by computing the root mean square deviation (RMSD), the size-independent ρ similarity parameter (`rho`) or the scaled ρ (`rhosc`), see Maiorov & Crippen, Proteins 22, 273 (1995). This is selected by `-what`.

Each structure from a trajectory (`-f`) is compared to a reference structure. The reference structure is taken from the structure file (`-s`).

With option `-mir` also a comparison with the mirror image of the reference structure is calculated. This is useful as a reference for 'significant' values, see Maiorov & Crippen, Proteins 22, 273 (1995).

Option `-prev` produces the comparison with a previous frame the specified number of frames ago.

Option `-m` produces a matrix in `.xpm` format of comparison values of each structure in the trajectory with respect to each other structure. This file can be visualized with for instance `xv` and can be converted to postscript with `xpm2ps`.

Option `-fit` controls the least-squares fitting of the structures on top of each other: complete fit (rotation and translation), translation only, or no fitting at all.

Option `-mw` controls whether mass weighting is done or not. If you select the option (default) and supply a valid `.tpr` file masses will be taken from there, otherwise the masses will be deduced from the `atommass.dat` file in `GMXLIB`. This is fine for proteins, but not necessarily for other molecules. A default mass of 12.011 amu (carbon) is assigned to unknown atoms. You can check whether this happend by turning on the `-debug` flag and inspecting the log file.

With `-f2`, the 'other structures' are taken from a second trajectory, this generates a comparison matrix of one trajectory versus the other.

Option `-bin` does a binary dump of the comparison matrix.

Option `-bm` produces a matrix of average bond angle deviations analogously to the `-m` option. Only bonds between atoms in the comparison group are considered.

### Files

option | filename | type | description
--- | --- | --- | ---
`-s` | `topol.tpr` | Input | Structure+mass(db): tpr tpb tpa gro g96 pdb
`-f` | `traj.xtc` | Input | Trajectory: xtc trr trj gro g96 pdb cpt
`-f2` | `traj.xtc` | Input, Opt. | Trajectory: xtc trr trj gro g96 pdb cpt
`-n` | `index.ndx` | Input, Opt. | Index file
`-o` | `rmsd.xvg` | Output | xvgr/xmgr file
`-mir` | `rmsdmir.xvg` | Output, Opt. | xvgr/xmgr file
`-a` | `avgrp.xvg` | Output, Opt. | xvgr/xmgr file
`-dist` | `rmsd-dist.xvg` | Output, Opt. | xvgr/xmgr file
`-m` | `rmsd.xpm` | Output, Opt. | X PixMap compatible matrix file
`-bin` | `rmsd.dat` | Output, Opt. | Generic data file
`-bm` | `bond.xpm` | Output, Opt. | X PixMap compatible matrix 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)
`-tu` | enum | `ps` | Time unit: `fs`, `ps`, `ns`, `us`, `ms` or `s`
`-[no]w` | bool | `no` | View output `.xvg`, `.xpm`, `.eps` and `.pdb` files
`-xvg` | enum | `xmgrace` | xvg plot formatting: `xmgrace`, `xmgr` or `none`
`-what` | enum | `rmsd` | Structural difference measure: `rmsd`, `rho` or `rhosc`
`-[no]pbc` | bool | `yes` | PBC check
`-fit` | enum | `rot+trans` | Fit to reference structure: `rot+trans`, `translation` or `none`
`-prev` | int | `0` | Compare with previous frame
`-[no]split` | bool | `no` | Split graph where time is zero
`-skip` | int | `1` | Only write every nr-th frame to matrix
`-skip2` | int | `1` | Only write every nr-th frame to matrix
`-max` | real | `-1` | Maximum level in comparison matrix
`-min` | real | `-1` | Minimum level in comparison matrix
`-bmax` | real | `-1` | Maximum level in bond angle matrix
`-bmin` | real | `-1` | Minimum level in bond angle matrix
`-[no]mw` | bool | `yes` | Use mass weighting for superposition
`-nlevels` | int | `80` | Number of levels in the matrices
`-ng` | int | `1` | Number of groups to compute RMS between

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