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
That part is pretty obvious, but how do I factor in the fact that amber expects a unit of angle (radians^2) yet the hessian is in a unit of displacement (bohr radius)?
Dear Dr. Nash,
Have you managed to find an answer to this question? I am trying to do the same thing and so far I haven't had much success in converting from Eh/Bohr^2 to kcal/mol*rad^2...
- Badges
- Science topic