# Average velocities **URL:** \ **Category:** User discussions\ **Created:** [January 15, 2021, 1:24pm UTC](https://gromacs.bioexcel.eu/t/average-velocities/1445 "2021-01-15T13:24:01Z")\ **Posts on this page:** 9\ **Page:** 1
**Author:** ![yasser.almeida](https://avatars.discourse-cdn.com/v4/letter/y/76d3ee/32.png) [@yasser.almeida](https://gromacs.bioexcel.eu/u/yasser.almeida)\ **Post date:** [January 15, 2021, 1:24pm UTC](https://gromacs.bioexcel.eu/t/average-velocities/1445/1 "2021-01-15T13:24:01Z")
GROMACS version: 2020.1 GROMACS modification: No Hi all, I want to calculate the average velocity of a particle group in different systems. I am using: `gmx traj -s system_1.tpr -f system_1.trr -av system_1.xvg -dt 50 -b 250 -nojump` … but I get: ``` WARNING: No coordinate frames found for option -cv or -cf No frames found for average velocity, will not write veloc.pdb ``` What am I missing? The `.trr` file should also contain coordinates, right? Thanks in advance. Best -Yasser ---
**Author:** ![cblau](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/cblau/32/5_2.png) [@cblau](https://gromacs.bioexcel.eu/u/cblau)\ **Post date:** [January 15, 2021, 1:29pm UTC](https://gromacs.bioexcel.eu/t/average-velocities/1445/2 "2021-01-15T13:29:44Z")
Hi Yasser, The `trr` file only contains velocities and forces if the `nstfout` and `nstvout` are not set to zero (as they are by default). [https://manual.gromacs.org/documentation/current/user-guide/mdp-options.html#output-control](https://manual.gromacs.org/documentation/current/user-guide/mdp-options.html#output-control) ---
**Author:** ![yasser.almeida](https://avatars.discourse-cdn.com/v4/letter/y/76d3ee/32.png) [@yasser.almeida](https://gromacs.bioexcel.eu/u/yasser.almeida)\ **Post date:** [January 15, 2021, 1:35pm UTC](https://gromacs.bioexcel.eu/t/average-velocities/1445/3 "2021-01-15T13:35:17Z")
Hi, Thanks for the reply. In my case `nstfout` = `nstvout` = 5000 (otherwise the file would be huge). I am only interested in the velocities, not in the `veloc.pdb`. It is possible to get it without the coordinates? ---
**Author:** ![cblau](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/cblau/32/5_2.png) [@cblau](https://gromacs.bioexcel.eu/u/cblau)\ **Post date:** [January 15, 2021, 1:38pm UTC](https://gromacs.bioexcel.eu/t/average-velocities/1445/4 "2021-01-15T13:38:17Z")
Ah, I see - sorry , I misunderstood the setup. Let me have a look, there should be a way. I’ll reply as soon as I figured. ---
**Author:** ![cblau](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/cblau/32/5_2.png) [@cblau](https://gromacs.bioexcel.eu/u/cblau)\ **Post date:** [January 15, 2021, 1:42pm UTC](https://gromacs.bioexcel.eu/t/average-velocities/1445/5 "2021-01-15T13:42:16Z")
Hi Yasser, Does `-av` and `-af` work for you? The `trr` does indeed not contain coordinates if you set `nstxout=0`. ---
**Author:** ![yasser.almeida](https://avatars.discourse-cdn.com/v4/letter/y/76d3ee/32.png) [@yasser.almeida](https://gromacs.bioexcel.eu/u/yasser.almeida)\ **Post date:** [January 15, 2021, 1:49pm UTC](https://gromacs.bioexcel.eu/t/average-velocities/1445/6 "2021-01-15T13:49:26Z")
Hi, No, that’s what I tried before and got the error (see the first post). In my case `nstxout=5000`. I tried the `-ov` option and works, but the file is huge and messy, since it reports the velocity for each particle at each frame. ---
**Author:** ![cblau](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/cblau/32/5_2.png) [@cblau](https://gromacs.bioexcel.eu/u/cblau)\ **Post date:** [January 21, 2021, 11:56am UTC](https://gromacs.bioexcel.eu/t/average-velocities/1445/7 "2021-01-21T11:56:55Z")
Hi Yasser, I found some time to have a closer look at the source code - it was written overlooking the possiblity that one might have only velocities and forces on hand, but not the coordinates. I was trying to re-write the tool, but could not do a quick-fix and it’s part of future refactoring efforts to provide tools that don’t have that type of issue (the code of `gmx traj` is at least 17 years old). Does using `gmx trajectory -seltype res_com` help to get averaged numbers per residue? I did not try myself, but it might be an easier way forward… Best, Christian ---
**Author:** ![yasser.almeida](https://avatars.discourse-cdn.com/v4/letter/y/76d3ee/32.png) [@yasser.almeida](https://gromacs.bioexcel.eu/u/yasser.almeida)\ **Post date:** [January 25, 2021, 8:57am UTC](https://gromacs.bioexcel.eu/t/average-velocities/1445/8 "2021-01-25T08:57:50Z")
Hi Christian, I will try with `gmx trajectory`. I used `gmx msd` as a turn around. Thanks, -Yasser ---
**Author:** ![cblau](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/cblau/32/5_2.png) [@cblau](https://gromacs.bioexcel.eu/u/cblau)\ **Post date:** [January 25, 2021, 12:25pm UTC](https://gromacs.bioexcel.eu/t/average-velocities/1445/9 "2021-01-25T12:25:00Z")
Good - I did not think of `gmx msd`. I’m tagging @pbauer as this is closely related to the work he had planned (as far as I know) on converting and extracting information from trajectories.