# Problem demuxing a PTReplica Exchange

**URL:** <https://gromacs.bioexcel.eu/t/problem-demuxing-a-ptreplica-exchange/502>\
**Category:** User discussions\
**Created:** [July 15, 2020, 3:32pm UTC](https://gromacs.bioexcel.eu/t/problem-demuxing-a-ptreplica-exchange/502 "2020-07-15T15:32:23Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![luisbustom](https://avatars.discourse-cdn.com/v4/letter/l/e47774/32.png) [@luisbustom](https://gromacs.bioexcel.eu/u/luisbustom)\
**Post date:** [July 15, 2020, 3:32pm UTC](https://gromacs.bioexcel.eu/t/problem-demuxing-a-ptreplica-exchange/502/1 "2020-07-15T15:32:23Z")

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GROMACS version: 2019.4  
GROMACS modification: No  
Hey! I am running a 20 replica exchange dynamics, generated the replica\_index.xvg file, an I was now trying to demux my trajectories in order to follow individual replicas trajectories. I tried to use the following command:  
gmx trjcat -f traj0.xtc traj1.xtc traj2.xtc traj3.xtc traj4.xtc traj5.xtc traj6.xtc traj7.xtc traj8.xtc traj9.xtc traj10.xtc traj11.xtc traj12.xtc traj13.xtc traj14.xtc traj15.xtc traj16.xtc traj17.xtc traj18.xtc traj19.xtc -demux replica\_index.xvg

But I receive the following error:  
Fatal error:  
Sorry, write\_trxframe can not write unknown

Any ideas of what might be causing this? Is it because I am using xtc instead of trr files?

Thanks!

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**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:** [September 4, 2020, 8:00am UTC](https://gromacs.bioexcel.eu/t/problem-demuxing-a-ptreplica-exchange/502/2 "2020-09-04T08:00:21Z")

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Hi luisbustom,

Sorry it took so long to reply to this thread - the problem has come up again here

> [@Gmx trjcat -demux command causes fatal error: "Sorry, write\_trxframe can not write unknown"](https://gromacs.bioexcel.eu/t/gmx-trjcat-demux-command-causes-fatal-error-sorry-write-trxframe-can-not-write-unknown/761):
>
> GROMACS version: 2020.1 GROMACS modification: No Hello, I’ve used GROMACS to perform a short REMD simulation (200 ps) in order to test whether my choice of temperature range allowed sufficient exchanges between replicas. The REMD simulations progressed and finished without problems and produced the following files in each replica directory: remd.part0001.log, remd.part0001.xtc, remd.part0001.edr, remd.part0001.trr, remd.part0001.gro using -deffnm remd and -noappend to run the REMD simula…

please follow our discussion there