text: user9890: What are the units of force, velocity, and coordinates output by gromacs trajectory? In .mdp, they are specified by nstfout,nstvout,nstxout.
  
  user12054: nstxout, nstvout and nstfout are the number of steps between coordinates, speed and forces written in the output trajectory (Output control documentation).
  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).
  Note that .xtc files multiply coordinates by 1000 to compress them, so they are stored in pm (see GROMACS xtc documentation). Given MDAnalysis documentation 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.
  
original_text: 9890: <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>
   12054: <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>
  