The ONIOM (our own N-layered integrated molecular orbital plus molecular mechanics) method for the first singlet excited (S-1) state photoisomerization path of a retinal protonated Schiff base

By testing a large number of ONIOM (our own N-layered integrated molecular orbital + molecular mechanics) combinations against the standard complete a ctive space self-consistent field (CASSCF) method with the 6-31G(d) basis s et, we have investigated the suitability of the ONIOM (molecular orbital molecular orbital) method for the investigation of the first singlet excite d state (S-1) photoisomerization pathways in protonated Schiff bases (PSBs) . For the isomerization reaction of an 11-nonhydrogen (H) PSB (10-non-H Sch iff base plus one methyl group), ONIOM can accurately reproduce the standar d CASSCF(10e/10o) (10 active electrons in 10 orbitals) results for only 10% of the computer time. The model system, which includes the protonated Schi ff base group as well as the isomerization bond, was always treated at the CASSCF level. With the unrestricted Hartree-Fock first triplet state (T-1) in the low level, the S-1 energy profile is reproduced accurately, while ti me-dependent Hartree-Fock or single excitation configuration interaction in the low level reproduces the difference between the singlet ground state ( S-0) and S-1 states very well. Using our ONIOM method, we also computed the first S-1 isomerization energy profile of the entire retinal protonated Sc hiff base. (C) 2000 American Institute of Physics. [S0021-9606(00)30532-3].