I need some help with the standalone QM/MM interface in Q-Chem (using 4.3.2). I have read in the manual that there is a limitation in the type of carbon atoms that the QM system must end on, but there is no explanation why this is. I understand that the QM/MM link must be done between two carbon atoms, but it seems that Q-Chem will only accept a carbon of type 26 (Tinker numbering) as the end of the QM system. I don't know if this is only a limitation when using the CHARMM27 FF. I have tried with the OPLSAA force field but I get a segmentation fault.

I'm trying to simulate an enzyme active site with multiple QM/MM boundaries across peptide bonds (between the CA and C carbons). I have tried contacting the Q-Chem support, but I have not gotten a response.

Thanks for any help.

Juan Vanegas
Post-doctoral Researcher
Center for Biological and Materials Sciences
Sandia National Laboratories