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

### Description

This is simple tool to compute SANS spectra using Debye formula It currently uses topology file (since it need to assigne element for each atom)

Parameters:

`-pr` Computes normalized g(r) function averaged over trajectory

`-prframe` Computes normalized g(r) function for each frame

`-sq` Computes SANS intensity curve averaged over trajectory

`-sqframe` Computes SANS intensity curve for each frame

`-startq` Starting q value in nm

`-endq` Ending q value in nm

`-qstep` Stepping in q space

Note: When using Debye direct method computational cost increases as 1/2 * N * (N - 1) where N is atom number in group of interest

WARNING: If sq or pr specified this tool can produce large number of files! Up to two times larger than number of frames!

### Files

option | filename | type | description
--- | --- | --- | ---
`-s` | `topol.tpr` | Input | Run input file: tpr tpb tpa
`-f` | `traj.xtc` | Input | Trajectory: xtc trr trj gro g96 pdb cpt
`-n` | `index.ndx` | Input, Opt. | Index file
`-d` | `nsfactor.dat` | Input, Opt. | Generic data file
`-pr` | `pr.xvg` | Output | xvgr/xmgr file
`-sq` | `sq.xvg` | Output | xvgr/xmgr file
`-prframe` | `prframe.xvg` | Output, Opt. | xvgr/xmgr file
`-sqframe` | `sqframe.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)
`-tu` | enum | `ps` | Time unit: `fs`, `ps`, `ns`, `us`, `ms` or `s`
`-xvg` | enum | `xmgrace` | xvg plot formatting: `xmgrace`, `xmgr` or `none`
`-mode` | enum | `direct` | Mode for sans spectra calculation: `direct` or `mc`
`-mcover` | real | `-1` | Monte-Carlo coverage should be -1(default) or (0,1]
`-[no]pbc` | bool | `yes` | Use periodic boundary conditions for computing distances
`-startq` | real | `0` | Starting q (1/nm)
`-endq` | real | `2` | Ending q (1/nm)
`-qstep` | real | `0.01` | Stepping in q (1/nm)
`-seed` | int | `0` | Random seed for Monte-Carlo
`-nt` | int | `8` | Number of threads to start

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