Charmm/GROMACS Residue 'ZN' not found in residue topology database?

I am trying to do some MD simulations of a protein with zinc ions. I am trying to use a charmm forcefield (I have tried 2017 and 2021) and Gromacs. I get an error when doing pdb2gmx Residue 'ZN'...

13 February 2021 9,645 2 View