# Tc-grps and xtc-grps and error

**URL:** <https://gromacs.bioexcel.eu/t/tc-grps-and-xtc-grps-and-error/7184>\
**Category:** User discussions\
**Created:** [September 13, 2023, 2:50pm UTC](https://gromacs.bioexcel.eu/t/tc-grps-and-xtc-grps-and-error/7184 "2023-09-13T14:50:16Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![fulvio](https://avatars.discourse-cdn.com/v4/letter/f/a88e57/32.png) [@fulvio](https://gromacs.bioexcel.eu/u/fulvio)\
**Post date:** [September 13, 2023, 2:50pm UTC](https://gromacs.bioexcel.eu/t/tc-grps-and-xtc-grps-and-error/7184/1 "2023-09-13T14:50:16Z")

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GROMACS version:  
GROMACS modification: Yes/No  
Dear users,  
I created an index.ndx file in which I defined the group which encompass only my protein (eventually with ions or cofactors) and the ligand of interest. Then, in the md.mdp file I set “non-Water” and “Water” as **tc-grps** groups and “non-Water” as the **xtc-grps**. The first question is: does it make sense?

In addition, now I have one more problem: when I try to bring the protein in the center of the box (by using gmx trjconv) I have the following error

Fatal error:  
Index[6942] 6943 is larger than the number of atoms in the  
trajectory file (6942). There is a mismatch in the contents  
of your -f, -s and/or -n files.

I tried with convert-tpr but it does not fix the problem.  
Any hints?  
Thanks in advance