# How to fix this?
**URL:** https://gromacs.bioexcel.eu/t/how-to-fix-this/13262
**Category:** User discussions
**Created:** [March 3, 2026, 10:06am UTC](https://gromacs.bioexcel.eu/t/how-to-fix-this/13262 "2026-03-03T10:06:24Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@Samhitha](https://gromacs.bioexcel.eu/u/Samhitha)
#### Post date: [March 3, 2026, 10:06am UTC](https://gromacs.bioexcel.eu/t/how-to-fix-this/13262/1 "2026-03-03T10:06:24Z")
GROMACS version: version 2023.3-Ubuntu\_2023.3
GROMACS modification: Yes/No
Here post your question:
Fatal error:
Residue 156 named SER of a molecule in the input file was mapped
to an entry in the topology database, but the atom OG used in
that entry is not found in the input file. Perhaps your atom
and/or residue naming needs to be fixed.
Tried using all the force fields, but met with same error.
PDB ID: 966C
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### Author:  [@discourse\_ai\_spam](https://gromacs.bioexcel.eu/u/discourse_ai_spam)
#### Post date: [March 3, 2026, 10:06am UTC](https://gromacs.bioexcel.eu/t/how-to-fix-this/13262/2 "2026-03-03T10:06:36Z")
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### Author:  [@system](https://gromacs.bioexcel.eu/u/system)
#### Post date: [March 3, 2026, 1:20pm UTC](https://gromacs.bioexcel.eu/t/how-to-fix-this/13262/3 "2026-03-03T13:20:01Z")
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### Author:  [@lmullender](https://gromacs.bioexcel.eu/u/lmullender)
#### Post date: [March 4, 2026, 2:02pm UTC](https://gromacs.bioexcel.eu/t/how-to-fix-this/13262/4 "2026-03-04T14:02:34Z")
Sadly, in these cases you can’t to anything but look into the topology files and try to see what’s going on. `pdb2gmx` will go through your PDB file and try and match the contained residues with those that the given forcefield parametrizes. Specifically, look at the Serine residue 156 that’s been giving you trouble in the input PDB file, and see if any atoms might be missing or wrongly named. Compare it to the SER entry in the file $GMXDATA/top//aminoacids.rtp. If there’s any missing atoms, you will need to model them with external software.