I am trying to generate new force field parameters based on quantum mechanical calculation. I used amber manual "parameter development" title for this and I calculated bond-angle-non bond parameters but I am having a hard time obtaning dihedral parameters for aromatics Let's consider about pyrazin. These are gaff2 pyrazin parameters :
ca-ca-nb-ca 2 9.600 180.000 2.000 h4-ca-nb-ca 2 9.600 180.000 2.000 nb-ca-ca-nb 4 14.500 180.000 2.000 h4-ca-ca-nb 4 14.500 180.000 2.000 h4-ca-ca-h4 4 14.500 180.000 2.000
When I scanned "ca-ca-nb-ca" dihedral angels, I found that dihedral bond is breaking at 100 degre and 111 kcal/mol energy. According to manual, if this is an ordinary dihedral ca-ca-nb-ca should have been (111/2=)55,5 kcal/mol V/2 value. But in gaff2 it is just 9.60kcal/mol. There is a huge difference. So, how to obtain such aromatic ring dihedral parameters by using QM calculation? My advisors said: "you should use twist scan not rotational scan" but I am not sure even if "twist-scan" exists and I am not sure "rigid" scan is the true answer. Thank you.
NOTE: Here is the picture.every step is 10 degree.