title: Mapping the Excited State Potential Energy Surface of a Retinal Chromophore Model with Multireference and Equation-of-Motion Coupled-Cluster Methods.
abstract: The photoisomerization of the retinal chromophore of visual pigments proceeds along a complex reaction coordinate on a multidimensional surface that comprises a hydrogen-out-of-plane (HOOP) coordinate, a bond length alternation (BLA) coordinate, a single bond torsion and, finally, the reactive double bond torsion. These degrees of freedom are coupled with changes in the electronic structure of the chromophore and, therefore, the computational investigation of the photochemistry of such systems requires the use of a methodology capable of describing electronic structure changes along all those coordinates. Here, we employ the penta-2,4-dieniminium (PSB3) cation as a minimal model of the retinal chromophore of visual pigments and compare its excited state isomerization paths at the CASSCF and CASPT2 levels of theory. These paths connect the cis isomer and the trans isomer of PSB3 with two structurally and energetically distinct conical intersections (CIs) that belong to the same intersection space. MRCISD+Q energy profiles along these paths provide benchmark values against which other ab initio methods are validated. Accordingly, we compare the energy profiles of MRPT2 methods (CASPT2, QD-NEVPT2, and XMCQDPT2) and EOM-SF-CC methods (EOM-SF-CCSD and EOM-SF-CCSD(dT)) to the MRCISD+Q reference profiles. We find that the paths produced with CASSCF and CASPT2 are topologically and energetically different, partially due to the existence of a "locally excited" region on the CASPT2 excited state near the Franck-Condon point that is absent in CASSCF and that involves a single bond, rather than double bond, torsion. We also find that MRPT2 methods as well as EOM-SF-CCSD(dT) are capable of quantitatively describing the processes involved in the photoisomerization of systems like PSB3.
authors.affiliation: Department of Chemistry, Bowling Green State University , Bowling Green, Ohio 43403, United States.
authors.forename: Samer
authors.initials: S
authors.lastname: Gozem
authors.affiliation: Dipartimento di Biotecnologie, Chimica e Farmacia, Università di Siena , via A. Moro 2, I-53100 Siena, Italy.
authors.forename: Federico
authors.initials: F
authors.lastname: Melaccio
authors.affiliation: Department of Chemistry, Ångström, the Theoretical Chemistry Programme , POB 518, SE-751 20 Uppsala, Sweden.
authors.forename: Roland
authors.initials: R
authors.lastname: Lindh
authors.affiliation: Department of Chemistry, University of Southern California , Los Angeles, California 90089-0482, United States.
authors.forename: Anna I
authors.initials: AI
authors.lastname: Krylov
authors.affiliation: Firefly Project , Moscow 117593, Russia.
authors.forename: Alexander A
authors.initials: AA
authors.lastname: Granovsky
authors.affiliation: Dipartimento di Chimica, Università di Ferrara , via Borsari 46, I-44121 Ferrara, Italy.
authors.forename: Celestino
authors.initials: C
authors.lastname: Angeli
authors.affiliation: Department of Chemistry, Bowling Green State University , Bowling Green, Ohio 43403, United States.|Dipartimento di Biotecnologie, Chimica e Farmacia, Università di Siena , via A. Moro 2, I-53100 Siena, Italy.
authors.forename: Massimo
authors.initials: M
authors.lastname: Olivucci