Optimizing Conical Intersections without Derivative Coupling Vectors:Application to Multistate Multireference Second-Order Perturbation Theory (MS-CASPT2)

摘要

We introduce a new method for optimizing minimal energy conical intersections (MECIs),based on a sequential penalty constrained optimization in conjunction with a smoothing function.The method is applied to optimize MEG geometries using the multistate formulation of second-order multireference perturbation theory (MS-CASPT2).Resulting geometries and energetics for conjugated molecules including ethylene,butadiene,stilbene,and the green fluorescent protein chromophore are compared with state-averaged complete active space self-consistent field (SA-CASSCF) and,where possible,benchmark multireference single- and double-excitation configuration interaction (MRSDCI) optimizations.Finally,we introduce the idea of "minimal distance conical intersections",which are points on the intersection seam that lie closest to some specified geometry such as the Franck-Condon point or a local minimum on the excited state.