# Couldn't run gmx trjconv

**URL:** <https://gromacs.bioexcel.eu/t/couldnt-run-gmx-trjconv/1538>\
**Category:** User discussions\
**Created:** [February 4, 2021, 2:41am UTC](https://gromacs.bioexcel.eu/t/couldnt-run-gmx-trjconv/1538 "2021-02-04T02:41:55Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![sam](https://avatars.discourse-cdn.com/v4/letter/s/5f9b8f/32.png) [@sam](https://gromacs.bioexcel.eu/u/sam)\
**Post date:** [February 4, 2021, 2:41am UTC](https://gromacs.bioexcel.eu/t/couldnt-run-gmx-trjconv/1538/1 "2021-02-04T02:41:56Z")

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GROMACS version: 2020.4  
i tried to run

> gmx trjconv -f file.xtc -o file.pdb

but got

> Program: gmx trjconv, version 2020.4  
> Source file: src/gromacs/utility/futil.cpp (line 480)
> 
> File input/output error:  
> topol.tpr

the file.xtc exists but couldn’t resolve this problem

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<div class="post-metadata">

**Author:** ![jalemkul](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/jalemkul/32/18_2.png) [@jalemkul](https://gromacs.bioexcel.eu/u/jalemkul)\
**Post date:** [February 4, 2021, 2:56am UTC](https://gromacs.bioexcel.eu/t/couldnt-run-gmx-trjconv/1538/2 "2021-02-04T02:56:12Z")

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You need to supply a `.tpr` file if you want to write output in PDB format from a trajectory (which does not have any concept of atom or residue names, numbers, etc). You did not specify one, so `trjconv` is looking for the default file name. It didn’t find it, hence the error.

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<div class="post-metadata">

**Author:** ![sam](https://avatars.discourse-cdn.com/v4/letter/s/5f9b8f/32.png) [@sam](https://gromacs.bioexcel.eu/u/sam)\
**Post date:** [February 4, 2021, 3:17am UTC](https://gromacs.bioexcel.eu/t/couldnt-run-gmx-trjconv/1538/3 "2021-02-04T03:17:43Z")

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Thanks for helping … it’s done