New variants of the multistate CASPT2 method

The accuracy of multistate CASPT2 (MS-CASPT2) for vertical transition energies and the robustness of extended MS-CASPT2 (XMS-CASPT2) in case of near-degenerate CASSCF states are combined in a new complete active space second-order perturbation theory approach.
The new method, building on top of the well-established machinery of the original CASPT2 approach, uses a transformation akin to XMS-CASPT2 to ensure approximate invariance under unitary rotations of the zeroth-order states and smoothly interpolates the Fock operator used in the Hamiltonian partition between state-specific and state-average regimes according to a dynamic weighting scheme.
The resulting methodology possesses the most desirable features of both MS-CASPT2 and XMS-CASPT2, that is, the ability to provide accurate transition energies and correctly describe avoided crossings and conical intersections.
In this contribution we present the results obtained with different weighting schemes and parameters, assessing the reliability on typical benchmark systems of photochemical relevance. The current limitations of the new method are discussed, especially from the point of view of excited states dynamics and recent developments addressing them are reported.