Clarification on gmx select moltype syntax in GROMACS 2025.3

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1

I am running a molecular dynamics simulation using GROMACS 2025.3 (CUDA build) on Linux, and I am trying to extract index groups for each molecule type from my system.

From gmx dump -s md_production.tpr, my topology contains:

moltype 0 = "Protein_chain_A"
moltype 1 = "Protein_chain_B"
moltype 2 = "SOL"
moltype 3 = "NA"

However, every attempt to use gmx select with the moltype keyword fails with a syntax error. For example:

gmx select -s md_production.tpr -on pep.ndx -select "moltype 'Protein_chain_A'"

or

gmx select -s md_production.tpr -on pep.ndx -select 'moltype == "Protein_chain_A"'

Both return:

Invalid selection 'moltype ...' — syntax error near quotes

I have reviewed the online manual for GROMACS 2025.1 and 2025.3, but it appears that the moltype keyword syntax or accepted grammar may have changed. Could you please confirm the correct way to generate an index group for a specific molecule type (e.g., only “Protein_chain_A”) in current versions of GROMACS?