gmx rms does not recognize certain ions in PDB files

Summary

When running gmx rms on PDB files that contain Ions such as Magnesium or Zinc, the following error message is shown:

Can not find mass in database for atom ZN in residue 719 ZN
Can not find mass in database for atom MG in residue 721 MG

Exact steps to reproduce

1. Obtain structure from 6ZPQ as PDB file from the RCSB PDB or the file attached here.
2. Run gmx rms -s 6ZPQ.pdb -f 6ZPQ.pdb

If this is a bug, (1) what happens, and (2) what did you expect to happen?

(1) The following output is generated:

Can not find mass in database for atom ZN in residue 719 ZN
Can not find mass in database for atom MG in residue 721 MG
Can not find mass in database for atom ZN in residue 715 ZN
Can not find mass in database for atom MG in residue 717 MG
Can not find mass in database for atom ZN in residue 712 ZN
Can not find mass in database for atom MG in residue 714 MG
Can not find mass in database for atom ZN in residue 715 ZN

(2) Expected: RMS is calculated between files

Relevant input files, logs and/or screenshots

6ZPQ.pdb

Possible fixes

It seems that gmx rms uses case sensitive comparison to look for the atoms in the atommass.dat file, so "MG" and "ZN" can not be found despite "Mg" and "Zn" being present.

When searching the atommass.dat file for atom names, use a case insensitive comparison.