I am working on my PTM project. I have some peptide. I got the result of doing MD of those peptide without PTM in different force field like Charm-GUI, Amber, Gromacs. Now I have to do MD of those peptide with PTM in mentioned force field. I found trouble to run MD of my peptide with PTM in different force field. In Charm-GUI, it cant recognize the engineering residue. Similar problem i found in Gromacs ,Amber. How can I get out of it?