[gmx-users] RDF calculations for certain atoms in a polymer

[Mahsa]

Hello,

I would like to calculate the RDF between some ions and oxygen atoms of a
polymer. The polymer has ester functional group with two different atom
types O and OS according to GAFF. It has also a side chain with ethereal
oxygen which has the same atom type as the ester part. How can I specify
the O of the side chain in the index file to tell it apart from the other O
atoms?
Besides there are around 500 of this atom in the system so changing
something manually is not a good option either.

Best regards,
Mahsa