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

### Description

`g_principal` calculates the three principal axes of inertia for a group of atoms.

### 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
`-a1` | `axis1.dat` | Output | Generic data file
`-a2` | `axis2.dat` | Output | Generic data file
`-a3` | `axis3.dat` | Output | Generic data file
`-om` | `moi.dat` | Output | Generic data 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
`-[no]foo` | bool | `no` | Dummy option to avoid empty array

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