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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?

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  • $\begingroup$ That is a molecule with far more saturation than naphthalene, but it will still be pretty rigid, so exactly how far do you think it could deform without breaking any bonds? $\endgroup$
    – matt_black
    Feb 17 at 1:12
  • $\begingroup$ @matt_black not very far at all, I suppose. Any ideas? $\endgroup$
    – kwaldner
    Feb 17 at 4:47

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