Force constant for S-N-S angle in NPF2/NTF2

[mattiafelice-palermo]

Dear Prof. Acevedo,

I'm currently working on a system containing FSI anion and was exploring the force field proposed in this work and validating the bonded contributions against the PES from QM calculations.

In particular, I noticed a large discrepancy between the PES from QM (B97-3c) for the S-N-S bond force constant and the PES from the MM force field derived in this work:

(angle units are in degrees, apologies for the inaccuracy in the figure)

In comparison, fitting the bond force constant to the QM PES yields a force constant of about 170 kJ/mol/rad, against the 671 kJ/mol/rad from this work.

I wanted to ask if there's a trivial explanation for this mismatch! Maybe I'm doing something wrong in my calculations.

Thank you for your help in understanding this!

Mattia