Hi all,

I am trying to work out the angular force constant from the frequency calculation of an ab initio Hessian for a mechanical force field (amber). The force constant of bond stretching is relatively easy, however, when it comes to the units for the angle I am a little stuck. 

The Hessian in Gaussian is stored as Hartree/bohr^2 

The angular force constant in Amber is kcal/(mol.rad^2). Then the difference between the angle and the equilibrium angle is calculated, the units are reduced to kcal/(mol.rad), hence the original need for rad^2.

The maths are in the attached file. I simply don't know how to work the units. Distance (d) is in angstroms which must suggest that the final 1/k has at least a unit of distance, which makes very little sense when the amber force field expects units of angle.

Thanks

Anthony