Multiconfigurational Methods Including XMCQDPT2 Theory for Excited States of Light-Sensitive Biosystems

Abstract

Multiconfigurational methods are often required for treating extended regions of potential energy surfaces of excited and ground electronic states and their crossings. Among various multiconfigurational approaches, multireference perturbation theory (MR-PT) methods, which are based on multiconfigurational reference wave functions, are widely employed in the studies of photochemistry of large molecular systems. This is due to their favorable cost-to-performance ratio as compared to more accurate MR approaches. In this chapter, we discuss a hierarchy of MR-PT2 methods, which use both single-state (SS) and multi-state (MS) strategies with emphasis on their theoretical foundations, relations to each other and applicability to specific problems. Formulation of the MS-MR-PT2 methods are directly related to the theory of effective Hamiltonians, which is briefly described here. We underline the importance of invariance properties and discuss the XMCQDPT2 method in more detail. As a representative example, we also provide its application to the studies of the photoresponse of retinal protonated Schiff base – a chromophore of vision.