I am running a QM/MM calculation using the Q-Chem stand-alone QM/MM features. I am using the CHARMM27 force field and need to add additional parameters for my system. The section of the Q-Chem manual discussing the input of parameters into the $force_field_params section of the Q-Chem input file specifies that the harmonic force constant for angles should be in units of kcal/mol/deg^2. However, CHARMM parameter files list this parameter in units of kcal/mol/rad^2. Do I need to perform the unit conversion from kcal/mol/rad^2 to kcal/mol/deg^2 or is this built in to the implementation of the CHARMM27 force field?
I am not sure, but if you try something simple (like optimizing the geometry of a single water molecule), you should be able to figure this out. If the manual is incorrect, Iet me know and I will change it.
I ended up optimizing a cluster of 3 waters using the CHARMM27 force field. The angle harmonic force constant parameter must be entered in units of kcal/mol/rad^2, as listed in the CHARMM parameter file.
Units should indeed be kcal/mol/radian/radian, not only for CHARMM but also for AMBER and OPLSAA force fields. I have updated the manual to reflect this fact and this change will appear in the manual for Q-Chem v. 6.1. Thanks for pointing out the error.