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

### Description

`g_helix` computes all kinds of helix properties. First, the peptide is checked to find the longest helical part, as determined by hydrogen bonds and φ/ψ angles. That bit is fitted to an ideal helix around the z-axis and centered around the origin. Then the following properties are computed:

1. Helix radius (file `radius.xvg`). This is merely the RMS deviation in two dimensions for all Cα atoms. it is calculated as sqrt((sum_i (x^2(i)+y^2(i)))/N) where N is the number of backbone atoms. For an ideal helix the radius is 0.23 nm
2. Twist (file `twist.xvg`). The average helical angle per residue is calculated. For an α-helix it is 100 degrees, for 3-10 helices it will be smaller, and for 5-helices it will be larger.
3. Rise per residue (file `rise.xvg`). The helical rise per residue is plotted as the difference in z-coordinate between Cα atoms. For an ideal helix, this is 0.15 nm
4. Total helix length (file `len-ahx.xvg`). The total length of the helix in nm. This is simply the average rise (see above) times the number of helical residues (see below).
5. Helix dipole, backbone only (file `dip-ahx.xvg`).
6. RMS deviation from ideal helix, calculated for the Cα atoms only (file `rms-ahx.xvg`).
7. Average Cα - Cα dihedral angle (file `phi-ahx.xvg`).
8. Average φ and ψ angles (file `phipsi.xvg`).
9. Ellipticity at 222 nm according to Hirst and Brooks.

### Files

option | filename | type | description
--- | --- | --- | ---
`-s` | `topol.tpr` | Input | Run input file: tpr tpb tpa
`-n` | `index.ndx` | Input | Index file
`-f` | `traj.xtc` | Input | Trajectory: xtc trr trj gro g96 pdb cpt
`-to` | `gtraj.g87` | Output, Opt. | Gromos-87 ASCII trajectory format
`-cz` | `zconf.gro` | Output | Structure file: gro g96 pdb etc.
`-co` | `waver.gro` | Output | Structure file: gro g96 pdb etc.

### 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
`-r0` | int | `1` | The first residue number in the sequence
`-[no]q` | bool | `no` | Check at every step which part of the sequence is helical
`-[no]F` | bool | `yes` | Toggle fit to a perfect helix
`-[no]db` | bool | `no` | Print debug info
`-prop` | enum | `RAD` | Select property to weight eigenvectors with. WARNING experimental stuff: `RAD`, `TWIST`, `RISE`, `LEN`, `NHX`, `DIP`, `RMS`, `CPHI`, `RMSA`, `PHI`, `PSI`, `HB3`, `HB4`, `HB5` or `CD222`
`-[no]ev` | bool | `no` | Write a new 'trajectory' file for ED
`-ahxstart` | int | `0` | First residue in helix
`-ahxend` | int | `0` | Last residue in helix

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