Suppose I have a napthalene-like molecule (attached), and I would like to scan (relaxed) its aromatic dihedral (highlighted atoms). I used CGENFF to obtain its dihedral parameters as follows,
HGA4 CG2DC2 CG311 CG331 0.2000 3 0.00
As shown, the dihedral force constant is 0.2, periodicity is 3, and angle is 0.
However, the penalty factor is high >50. Hence, in order to obtain new parameters and following the typical charmm protocol using MP2/6-31G*, should I perform a full scan +/- 180 degrees? +/- 180 seems unphysical (as compared to a rotatable bond) for a dihedral that belongs to a ring. If so, how much should I lower this value to?