CHARMM Development Project Forums User Discussion & Questions Parameter Set Discussion How to generate topology file and parameter file

For those residues not defined in CHARMm, especially, those small molecules, such as, TERT-BUTYLOXYCARBONYL GROUP(BOC) ...
Anybody suggest me any tips?

Thanks in advance!

I'd either use amino-acid fragments with similar properties and consequently the same atom-types for the parameters, as suggested in the MacKerrel paper. Guess you use a graphical program (e.g. quanta or insight) with an in-buildt CHARMM facility. If so, then I believe the topology file can be generated automatically.

An alternative is to use semi - or ab initio methods and perform a gasphase optimization geometry. In that case you'll have to rescale the frequencies to assign the empirical force-constants (i.e. kb, ka and so on) within your molecule, and use existing non-bonded L-J and charges from an analog (protein) residue.

Good luck!

Yes, using those similar structure may work, I'll try it. Actually, there are some molecules that I can't easily find any similar amino-acid fragment, eventually, I have to face this problem again.
I don't have InsightII or Quanta, I am poor enough. :-(
Are there any package for those semi- or ab initio methods you mentioned? Any tips will be welcome!
Thanks in advance!