Prepare HEME topology

GROMACS version:2020-5
GROMACS modification: No
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I have to simulate Cytochrome C with HEME group. I am trying using force-filed charmm36-feb2021. However, gmx pdb2gmx fails because it can’t, because it can not fix HEME group. The merged.rtp has parameters for Heme still it is not working.
Can some expert help me to fix this problem.
Best regards