CASSCF Calculations for Excited States of Large Molecules: Choosing when to use the RASSCF, ONIOM and MMVB Approximations

Abstract

We compare and contrast the RASSCF, ONIOM and MMVB electronic structure methods for calculating relaxation paths on potential energy surfaces of the excited states of large molecules, and for locating any resulting conical intersections at which nonadiabatic decay can take place. Each method is treated as an approximation to CASSCF, which we choose as our reference level of theory, but which becomes prohibitively expensive computationally for large molecules. Both MMVB and ONIOM are hybrid computational methods—combining different levels of theory in an energy plus derivatives calculation at a particular molecular geometry—but they differ fundamentally in that MMVB is a hybrid‐atom method, whereas ONIOM is a hybrid‐molecule method. We explain this distinction through representative applications, and show that each method is more appropriate for a particular type of photochemical problem.