I calculated the SAPT interaction energy for various configurations of pairs of a set of molecules.

My question is: is it possible to calculate the acting force on each atom of each of the molecules in the pair by the energy of the SAPT that I calculated? Could someone kindly tell me how I could do this? Or if this function is available on psi4?

Sorry if my question doesn’t make much sense, I didn’t explore quantum chemistry much in my engineering degree.

AFAIK there is no gradient theory formulated for SAPT.
SAPT is an intermolecular theory and not supermolecular like HF or MP2 so the situation is different and I dont know how one would approach a gradient/forces with finite differences.

What do you what to know about the system that you need those forces?

I’m trying to fit parameters of the non-bonded potential of force fields through the energy of dimer interaction. I know there are supermolecular and perturbative approaches (SAPT).

According to the literature (which usually considers the supermolecular approach), one of the ways to do this would be to calculate the energy gradient and obtain the forces experienced by each atom.

With the forces experienced by each atom by the quantum mechanics data, I would have to vary parameters of the non-bonded potential of the force field until I could reproduce the forces observed in quantum mechanics by molecular dynamics simulations. These are called force fields based on quantum mechanics (a bottom-up approach).

My idea would be to compare supermolecular and perturbative approaches (of course, if it makes sense) in obtaining parameters of the non-bonded potential of force fields. However, as it is apparently not possible to calculate forces by the perturbative approach, in this method of study the approaches would not be comparable I think now.

If your FF is not an intramolecular FF, I think the SAPT gradient would not help much since you cannot map them (directly) to the FF. The degrees of freedom different.