# Time seems to be not correct using "trjcat" **URL:** \ **Category:** User discussions\ **Created:** [November 9, 2021, 11:00pm UTC](https://gromacs.bioexcel.eu/t/time-seems-to-be-not-correct-using-trjcat/3117 "2021-11-09T23:00:39Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![lanselibai](https://avatars.discourse-cdn.com/v4/letter/l/e9c0ed/32.png) [@lanselibai](https://gromacs.bioexcel.eu/u/lanselibai)\ **Post date:** [November 9, 2021, 11:00pm UTC](https://gromacs.bioexcel.eu/t/time-seems-to-be-not-correct-using-trjcat/3117/1 "2021-11-09T23:00:39Z")
GROMACS version:2021 GROMACS modification: /No Here post your question I use `gmx trjcat -f trj1.xtc trj2.xtc -cat -o combine.xtc` to combine two trajectories into one. Each trajectory is from 0 to 50 ns, with 0.1 ns interval, totalling 501 frames. In the `combine.xtc`, I want the frame 1 to 501 to be trj1’s frames, and frame 502 to 1002 to be trj2’s frames. The prompt I got is shown below. I can see `Last frame written was 1001`, which means there are 1002 frames (the first frame is called “frame 0” in Gromacs), which is good. However, why it is showing `frame 910 time 40900.000 ps` ? I actually do not care about the time. I just want to simply combine the two trajectories. ```auto Command line: gmx trjcat -f trj1.xtc trj2.xtc -cat -o combine.xtc Reading frame 1 time 100.000 Summary of files and start times used: File Start time Time step --------------------------------------------------------- trj1.xtc 0.000 ps 100.000 ps trj2.xtc 0.000 ps 100.000 ps WARNING: same Start time as previous Reading frame 0 time 0.000 Note that major changes are planned in future for trjcat, to improve usability and utility. lasttime 0 Continue writing frames from trj1.xtc t=0 ps, frame=0 Last frame 500 time 50000.000 -> frame 500 time 50000.000 ps Reading frame 0 time 0.000 lasttime 50000 Continue writing frames from trj2.xtc t=0 ps, frame=501 Last frame 500 time 50000.000 -> frame 910 time 40900.000 ps Last frame written was 1001, time 50000.000000 ps ``` ---
**Author:** ![pjohansson](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/pjohansson/32/316_2.png) [@pjohansson](https://gromacs.bioexcel.eu/u/pjohansson)\ **Post date:** [November 11, 2021, 8:45am UTC](https://gromacs.bioexcel.eu/t/time-seems-to-be-not-correct-using-trjcat/3117/2 "2021-11-11T08:45:38Z")
Hi, You can fix the times to be continuous with the flag `-settime`. That will prompt you for how each new trajectory “relates” to the previous. Petter ---
**Author:** ![lanselibai](https://avatars.discourse-cdn.com/v4/letter/l/e9c0ed/32.png) [@lanselibai](https://gromacs.bioexcel.eu/u/lanselibai)\ **Post date:** [November 11, 2021, 8:59am UTC](https://gromacs.bioexcel.eu/t/time-seems-to-be-not-correct-using-trjcat/3117/3 "2021-11-11T08:59:09Z")
Hi, Petter, thank you. Yes, I can use `-settime` to correct it based on the prompt. But I still do not understand why `frame 910 time 40900.000 ps` appears when `-settime` is not supplied. In any case, can I confirm that the frame 1 to 501 are trj1’s frames, and frame 502 to 1002 are trj2’s frames? Thank you. ---
**Author:** ![pjohansson](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/pjohansson/32/316_2.png) [@pjohansson](https://gromacs.bioexcel.eu/u/pjohansson)\ **Post date:** [November 11, 2021, 11:25am UTC](https://gromacs.bioexcel.eu/t/time-seems-to-be-not-correct-using-trjcat/3117/4 "2021-11-11T11:25:40Z")
My best guess is that it’s an error with the text message, but that the files have been correctly cat’ed and that it means to say the correct time + frame. I’d suggest extracting the last frames from `trj2.xtc` and `combined.xtc` and compare them to see if they are identical. E.g. by loading them in `vmd` or dumping into text files through `gmx trjconv -dump