Grompp name matching error

GROMACS version:2020
GROMACS modification: Yes/No
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Dear GROMACS users,
I got the following error.

group Protein_chain_A referenced in the .mdp file was not found in the index
Group names must match either [moleculetype] names or custom index, group
names, in which case you must supply an index file to the ‘-n’ option
of grompp.

Group names are the same in the mdp file and the top file [moleculetype]
Should I create an index file in this case? If yes, how to generate an index file that has 13 groups?

Thank you so much

you can generate an index file using gmx make_ndx tools ( gmx make_ndx — GROMACS 2021.2 documentation) . There is no restriction in the number of group you can generate.
For what you need the group in mdp?
best regards

Dear Alessandra,

Thank you so much for your reply.
I want to make the groups in mdp to define the pull groups to do AWH