How to do the PTM (post-translational modification) using OPLS-AA forcefield?

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I searched charms-gui for help, but it could not generate the forcefield about the modified amino acids using opls-aa forcefield. But the previous work were all done using OPLS-AA/L. I think there must be someone who have already made it, but it seems no existing software that could help. Thus, I really want to ask what should I do to get the opls-aa forcefield with PTM.