# Generating topology from gmx x2top

**URL:** <https://gromacs.bioexcel.eu/t/generating-topology-from-gmx-x2top/11550>\
**Category:** User discussions\
**Created:** [February 14, 2025, 6:46pm UTC](https://gromacs.bioexcel.eu/t/generating-topology-from-gmx-x2top/11550 "2025-02-14T18:46:21Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![ryanm](https://avatars.discourse-cdn.com/v4/letter/r/85f322/32.png) [@ryanm](https://gromacs.bioexcel.eu/u/ryanm)\
**Post date:** [February 14, 2025, 6:46pm UTC](https://gromacs.bioexcel.eu/t/generating-topology-from-gmx-x2top/11550/1 "2025-02-14T18:46:21Z")

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GROMACS version:2021.4  
GROMACS modification: Yes/No  
Here post your question  
I have a protein with a metal ion. After the running Acpype, the input topology used to run MD in GROMACS doesn’t work. So, I am trying to generate a new topology file using gmx x2top. I get an error saying:  
Fatal error:  
No or incorrect atomname2type.n2t file found (looking for amber99sb.ff)  
I want to use amber99sb forcefield.

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**Author:** ![obZehn](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/obzehn/32/2629_2.png) [@obZehn](https://gromacs.bioexcel.eu/u/obZehn)\
**Post date:** [February 14, 2025, 7:38pm UTC](https://gromacs.bioexcel.eu/t/generating-topology-from-gmx-x2top/11550/2 "2025-02-14T19:38:19Z")

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For AMBER99SB use [CHARMM-GUI](https://www.charmm-gui.org/).