The ONIOM (our own N-layered integrated molecular orbital + molecular mechanics) method for the first singlet excited (S1) state photoisomerization path of a retinal protonated Schiff base

Abstract

By testing a large number of ONIOM (our own N-layered integrated molecular orbital + molecular mechanics) combinations against the standard complete active space self-consistent field (CASSCF) method with the 6-31G(d) basis set, we have investigated the suitability of the ONIOM (molecular orbital + molecular orbital) method for the investigation of the first singlet excited state (S1) photoisomerization pathways in protonated Schiff bases (PSBs). For the isomerization reaction of an 11-nonhydrogen (H) PSB (10-non-H Schiff base plus one methyl group), ONIOM can accurately reproduce the standard CASSCF(10e/10o) (10 active electrons in 10 orbitals) results for only 10% of the computer time. The model system, which includes the protonated Schiff base group as well as the isomerization bond, was always treated at the CASSCF level. With the unrestricted Hartree–Fock first triplet state (T1) in the low level, the S1 energy profile is reproduced accurately, while time-dependent Hartree–Fock or single excitation configuration interaction in the low level reproduces the difference between the singlet ground state (S0) and S1 states very well. Using our ONIOM method, we also computed the first S1 isomerization energy profile of the entire retinal protonated Schiff base.