Journal
JOURNAL OF CHEMICAL THEORY AND COMPUTATION
Volume 9, Issue 10, Pages 4495-4506Publisher
AMER CHEMICAL SOC
DOI: 10.1021/ct400460h
Keywords
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Funding
- Bowling Green State University
- Center for Photochemical Sciences of Bowling Green State University
- Human Frontier Science Program Organization [RGP0049/385]
- National Science Foundation [CHE-1152070]
- Swedish Research Council (VR)
- Department of Energy [DE-FG02-05ER15685]
- Russian Ministry of Education and Science [07.514.11.4128]
- RFBR [11-03-01214]
- Italian MIUR
- Direct For Mathematical & Physical Scien
- Division Of Chemistry [1152070] Funding Source: National Science Foundation
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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.
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