Arbitrary Order Perturbation Theory

PSI3 is capable of computing arbitrary order Møller-Plesset perturbation theory (MPn, closed-shell systems) and Z-averaged perturbation theory (ZAPTn, open-shell systems) energies, invoked with MPN = TRUE. The maximum level of perturbation theory computed is controlled by MAXNVECT. Higher order energies (2n - 1 and 2n - 2) can be computed at no additional computational cost by using WIGNER = TRUE. By default, the nth order energy is saved, but (2n - 1) or (2n - 2) order energies can be saved using SAVE_MPN2 = 1 or SAVE_MPN2 = 2, respectively.

For open-shell systems, arbitary order ZAPTn energies can be computed using WFN = ZAPTN and REF = ROHF. All other options are the same as closed-shell MPn.