Conical Intersections from Particle-Particle Random Phase and Tamm-Dancoff Approximations.

The particle-particle random phase approximation (pp-RPA) and the particle-particle Tamm-Dancoff approximation (pp-TDA) are applied to the challenging conical intersection problem. Because they describe the ground and excited states on the same footing and naturally take into account the interstate interaction, these particle-particle methods, especially the pp-TDA, can correctly predict the dimensionality of the conical intersection seam as well as describe the potential energy surface in the vicinity of conical intersections. Though the bond length of conical intersections is slightly underestimated compared with the complete-active-space self-consistent field (CASSCF) theory, the efficient particle-particle methods are promising for conical intersections and nonadiabatic dynamics.