I thought I would ask if second-order vibrational perturbation theory (VPT2) is available in Psi4. Specifically, I would like to compute a polyatomic molecule’s rotational constant. I didn’t see it listed in the manual or webpages.

Thanks for the reply. My goal is to validate a theory level for geometry optimization by comparing computed rotational constants (RCs) to known experimental values. Someone suggested that I use VPT2 (which is new to me) to obtain vibrationally average RCs. I have compared the values given by Psi4 (via the moments of inertia), but I wanted to also try this other way too.

If VPT2 is unfamiliar to you, I should point out that it requires a series of displacements along each vibrational normal coordinate and then a full frequency calculation at each of those displacements. How big is your molecular system?