<p><code>nstxout</code>, <code>nstvout</code> and <code>nstfout</code> are the number of <strong>steps</strong> between coordinates, speed and forces written in the output trajectory (<a href="https://manual.gromacs.org/current/user-guide/mdp-options.html?highlight=nstxout#output-control" rel="nofollow noreferrer">Output control documentation</a>).</p>
<p>As for the units present in your trajectory files (say a <code>.trr</code>):
coordinates are in <strong>nm</strong>, speeds in <strong>nm.μs⁻¹</strong> (so in <strong>mm.m⁻¹</strong>) and forces in <strong>kJ.mol⁻¹.nm⁻¹</strong>. These should be consistent through all GROMACS files (<a href="https://manual.gromacs.org/current/reference-manual/definitions.html" rel="nofollow noreferrer">Definitions and Units documentation</a>).</p>
<p>Note that <code>.xtc</code> files multiply coordinates by 1000 to compress them, so they are stored in <strong>pm</strong> (see <a href="https://manual.gromacs.org/current/reference-manual/file-formats.html#xtc" rel="nofollow noreferrer">GROMACS xtc documentation</a>). Given <a href="https://userguide.mdanalysis.org/1.0.0/formats/reference/xtc.html" rel="nofollow noreferrer">MDAnalysis documentation</a> on the subject, I believe <code>.xtc</code> files can <em>only</em> store coordinates, no speeds or forces.</p>
<p>I would be wary of softwares used to analyze your trajectory results (MDAnalysis, VMD, etc), as they may automatically do the conversion of these units, and for example show you coordinates in Å.</p>

CC BY-SA 4.0
2021-07-17T19:02:55.683Z
This was a great answer O.Laprevote! We have a [few unanswered GROMACS questions here](https://mattermodeling.stackexchange.com/questions/tagged/gromacs?tab=Unanswered), are you able to help us out at all with those? I'm trying to help take care of the unanswered queue.
user1271772
CC BY-SA 4.0
2021-04-22T20:23:11.407Z
2021-04-24T20:33:20.347Z
2021-04-24T20:33:20.347Z
O.Laprevote
<p>What are the units of force, velocity, and coordinates output by gromacs trajectory? In <code>.mdp</code>, they are specified by <code>nstfout</code>,<code>nstvout</code>,<code>nstxout</code>.</p>

<simulated-data><gromacs>
What are the units for the force output by nstfout, the velocity output by nstvout, and the coordinates output by nstxout in gromacs?
# What are the units for the force output by nstfout, the velocity output by nstvout, and the coordinates output by nstxout in gromacs?

(Asked by: username_0 on 2021-04-22)

What are the units of force, velocity, and coordinates output by gromacs trajectory? In `.mdp`, they are specified by `nstfout`,`nstvout`,`nstxout`.

**Tags:** simulated-data, gromacs

---

## 1 answers

### Answer by username_1 on 2021-04-22. Score: 3 | Accepted answer

`nstxout`, `nstvout` and `nstfout` are the number of **steps** between coordinates, speed and forces written in the output trajectory ([Output control documentation](https://manual.gromacs.org/current/user-guide/mdp-options.html?highlight=nstxout#output-control)).

As for the units present in your trajectory files (say a `.trr`):
coordinates are in **nm**, speeds in **nm.μs⁻¹** (so in **mm.m⁻¹**) and forces in **kJ.mol⁻¹.nm⁻¹**. These should be consistent through all GROMACS files ([Definitions and Units documentation](https://manual.gromacs.org/current/reference-manual/definitions.html)).

Note that `.xtc` files multiply coordinates by 1000 to compress them, so they are stored in **pm** (see [GROMACS xtc documentation](https://manual.gromacs.org/current/reference-manual/file-formats.html#xtc)). Given [MDAnalysis documentation](https://userguide.mdanalysis.org/1.0.0/formats/reference/xtc.html) on the subject, I believe `.xtc` files can *only* store coordinates, no speeds or forces.

I would be wary of softwares used to analyze your trajectory results (MDAnalysis, VMD, etc), as they may automatically do the conversion of these units, and for example show you coordinates in Å.

> **Comments:**
> **username_2**: This was a great answer O.Laprevote! We have a [few unanswered GROMACS questions here](https://mattermodeling.stackexchange.com/questions/tagged/gromacs?tab=Unanswered), are you able to help us out at all with those? I'm trying to help take care of the unanswered queue.  

---

