title: Conical Intersection and Potential Energy Surface Features of a Model Retinal Chromophore: Comparison of EOM-CC and Multireference Methods.
abstract: This work investigates the performance of equation-of-motion coupled-cluster (EOM-CC) methods for describing the changes in the potential energy surfaces of the penta-2,4-dieniminium cation, a reduced model of the retinal chromophore of visual pigments, due to dynamical electron correlation effects. The ground-state wave function of this model includes charge-transfer and diradical configurations whose weights vary along different displacements and are rapidly changing at the conical intersection between the ground and the first excited states, making the shape of the potential energy surface sensitive to a balanced description of nondynamical and dynamical correlation. Recently, variational (MRCISD) and perturbative (MRPT2) approaches for including dynamical correlation in CASSCF-based calculations were tested along three representative ground state paths. Here, we use the same three paths to compare the performance of single-reference EOM-CC methods against MRCISD and MRCISD+Q. We find that the spin-flip variant of EOM-CCSD with perturbative inclusion of triple excitations (dT or fT) produces potential energy profiles of the two lowest electronic states in quantitative agreement with MRCISD+Q (our highest-quality reference method). The nonparallelity errors and differences in vertical energy differences of the two surfaces along these scans are less than 1.4 kcal/mol (EOM-SF-CCSD(dT) versus MRCISD+Q). For comparison, the largest error of MRCISD versus MRCISD+Q is 1.7 kcal/mol. Our results show that the EOM-CC methods provide an alternative to multireference approaches and may be used to study photochemical systems like the one used in this work.
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: 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: Department of Chemistry, Bowling Green State University, Bowling Green, Ohio 43403, United States.|Dipartimento di Chimica, Università di Siena, via De Gasperi 2, I-53100 Siena, Italy.
authors.forename: Massimo
authors.initials: M
authors.lastname: Olivucci