CASSCF calculations for photoinduced processes in large molecules: Choosing when to use the RASSCF, ONIOM and MMVB approximations

Abstract

In this article, 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 here 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 four representative applications: the photostability of pyracylene (studied with CASSCF, RASSCF, MMVB); large geometry changes in the singlet excited states of triangulene (studied with MMVB); a model for interstitial nickel defects in a synthetic diamond lattice (studied with ONIOM CAS:UFF); and the photochemical [4+4] cycloaddition of cyclohexadiene to naphthalene (studied with ONIOM CAS:MMVB). We show that each method is more appropriate for a particular type of photochemical problem. This article is part perspective, part review, and contains new results for three multi-state or photoinduced processes in complex systems.

A general method to describe intersystem crossing dynamics in trajectory surface hopping

“…Additionally, the quantum chemical methods should be able to give a balanced and accurate description of all states over the whole potential energy surface, which usually requires multiconfigurational wavefunctions. These requirements are currently fulfilled for several implementations [178][179][180] of the multiconfigurational self-consistent field (MCSCF) method [186,187] and for multireference configuration interaction (MRCI) as implemented in Columbus. [178] Using MCSCF or MRCI, already a number of ISC dynamics simulations have been reported using SHARC.…”

Dynamics of ultraviolet-induced DNA lesions: Dewar formation guided by pre-tension induced by the backbone

“…The calculations on the formacetal bridged T(6-4)T are based on a hybrid partitioning approach (ONIOM) [41,42], which includes the steric effects and ground state polarization effects of the complete dinucleotide on QM/QM level of theory. The first high-level quantum mechanical (QM) part refers to the photoactive chromophore 5-methyl-2-pyrimidinone (5M2P), where the electronic excitation and subsequent bond rearrangement takes place, and the second low-level QM part describes the remaining part of the bridged T(6-4)T (sugar glycol backbone+dihydrothymidine part).…”

Theoretical Design of High‐Spin Polycyclic Hydrocarbons

“…Under the label MMVB (molecular mechanics with valence‐bond), the model has been extensively exploited by Robb et al. on photochemical processes of conjugated hydrocarbons 38–44…”