Vibrational perturbation theory: VPT2

A VPT2 program based on a semi-quartic force field (QFF) has been implemented, which allows for low-cost evaluations of anharmonic vibrational frequencies and associated vibrational constants. The determination of the QFF does not require analytical gradients (see the SURF program) and can thus been used in combination with any electronic structure method. 2020-04-18