Multireference Approaches for Excited States of Molecules

Abstract

Understanding the properties of electronically excited states is a challenging task that becomes increasingly important for numerous applications in chemistry, molecular physics, molecular biology, and materials science. A substantial impact is exerted by the fascinating progress in time-resolved spectroscopy, which leads to a strongly growing demand for theoretical methods to describe the characteristic features of excited states accurately. Whereas for electronic ground state problems of stable molecules the quantum chemical methodology is now so well developed that informed nonexperts can use it efficiently, the situation is entirely different concerning the investigation of excited states. This review is devoted to a specific class of approaches, usually denoted as multireference (MR) methods, the generality of which is needed for solving many spectroscopic or photodynamical problems. However, the understanding and proper application of these MR methods is often found to be difficult due to their complexity and their computational cost. The purpose of this review is to provide an overview of the most important facts about the different theoretical approaches available and to present by means of a collection of characteristic examples useful information, which can guide the reader in performing their own applications.

The DFT/MRCI method

“…Moreover, as a consequence of near‐degeneracy effects, most electronically excited states exhibit genuine multiconfiguration character. Ab initio multiconfiguration electron correlation methods, that is, electronic structure methods that do not rely on empirical knowledge, can only be applied to small‐ and medium‐sized molecules due to their high technical and computational complexity . To address electronically excited states of larger molecular systems, approximations are inevitable.…”

Visualisation of Electronic Excited‐State Correlation in Real Space

“…Such cases are widespread and two notorious failures of the MO picture relate to excitons in extended systems [24][25][26] and to ionic/ covalent wavefunction character in alternant hydrocarbons. [28] This is particularly troublesome as it has been mentioned repeatedly that the ionic/covalent character has a strong but hard-to-pin-down impact on commonly used electronic structure methods such as multiconfigurational self-consistent field [44][45][46][47] and timedependent density functional theory (TDDFT). [26,[29][30][31] Previous attempts of visualising the ensuing correlated electron-hole distribution have relied on coarsegrained correlation plots [32][33][34][35][36][37][38] and other abstract visualisation techniques [39,40] but the goal of visualising correlated wavefunctions in real space has remained elusive.…”

“…[27,28] These molecules possess two distinct classes of states, denoted " + " and "À ", [41] which are understood as ionic and covalent states within valence bond (VB) theory. [28] This is particularly troublesome as it has been mentioned repeatedly that the ionic/covalent character has a strong but hard-to-pin-down impact on commonly used electronic structure methods such as multiconfigurational self-consistent field [44][45][46][47] and timedependent density functional theory (TDDFT). [28] This is particularly troublesome as it has been mentioned repeatedly that the ionic/covalent character has a strong but hard-to-pin-down impact on commonly used electronic structure methods such as multiconfigurational self-consistent field [44][45][46][47] and timedependent density functional theory (TDDFT).…”

RASPT2 study of the valence excited states of an iron–porphyrin–carbonyl model complex

“…Multireference perturbative methods are commonly used for calculations of excited states (see recent review in Ref. ). The introduction of dynamical correlation thanks to methods like CASPT2 in 1990 and then RASPT2 in 2008 represented an important step toward the study of excited states of large systems.…”