Geometries and Vertical Excitation Energies in Retinal Analogues Resolved at the CASPT2 Level of Theory: Critical Assessment of the Performance of CASSCF, CC2, and DFT Methods

Abstract

A systematic investigation of structural properties and vertical excitation energies of a series of structurally modified 11-cis-retinal chromophores in vacuo was performed by means of multiconfigurational second-order perturbation theory (CASPT2). CASPT2-based geometries agree reasonably well with Møller-Plesset second-order perturbation theory (MP2), local second-order approximate coupled cluster singles and doubles (LCC2), and density functional theory (DFT) geometries, while the complete active space self-consistent field (CASSCF) method exaggerates dramatically the bond length pattern in the polyene chain. The quality of the resulting vertical excitation energies obtained by employing CASSCF, second-order approximate coupled cluster singles and doubles (CC2), LCC2, and time-dependent density functional theory (TD-DFT) approaches is assessed with respect to the CASPT2 data. We show that the commonly used CASSCF/CASPT2 approach works reasonably well in the case of vertical excitation energies of planar structures, but lack of dynamic correlation leads to large errors in energetics for strongly strained structures. For example, the highly twisted conformers of 9,10-dimethyl and 9,10,13-trimethyl species are found as global minima at the CASSCF level, whereas they turn almost planar at the CASPT2, MP2, LCC2, and DFT levels of theory. The CC2 method has shown a remarkable performance, manifested by a maximum deviation of 0.05 eV from the reference CASPT2 results, whereas the local version of CC2 seems to fail to describe the charge-transfer character of the S0 → S1 transitions correctly. We believe that our CASPT2 benchmark set will provide a reference that can be utilized for validation and development studies on 11-cis-retinal protonated Schiff base chromophore analogues.

Modeling red coral (Corallium rubrum) and African snail (Helixia aspersa) shell pigments: Raman spectroscopy versus DFT studies

“…Biological systems containing conjugated CC bonds are also sensitive to light. Theoretical modeling of simplified systems sensitive to light has been reported recently by Andruniow and coworkers . The degree of electron delocalization in such molecular systems is reflected by a parameter called bond length alternation (BLA).…”

Impacts of retinal polyene (de)methylation on the photoisomerization mechanism and photon energy storage of rhodopsin

“…† All (de)methylated chromophore models have a spiral-like structure with a negative (counterclockwise) helicity, alike native 11-cis-RPSB. 43 It is evident from Fig. Bond lengths are decreased by 0.01-0.025 Å (0.01-0.015 Å at B3LYP), and dihedral angles are changed by up to 281 (up to 181 at B3LYP).…”

“…52 Here we focus on the description of the Franck-Condon (FC) structure of the native Rh and its analogues with a modified RPSB chromophore. It has been previously found that CASSCF method overestimates the difference in single and double bond lengths in comparison to accurate Multi-Reference Configuration Interaction (MRCI) 66 and CASPT2 41,43,67,68 methods. 3 and 4, and in Tables S1-S4 in the ESI.…”

“…The enhanced torsional strain in the 10-m-Rh chromophore has also been evidenced in UV-Vis and CD spectra. In our previous study 43 we found that B3LYP is a reliable method for calculating ground-state equilibrium geometries for RPSB models in the gas phase. These interactions increase the distance between C10 and C20 from 3.21 Å in Rh to 3.25 Å in the 10-m-Rh analogue (from 3.14 to 3.24 Å for B3LYP-based geometries) in agreement with experimental data.…”

“…It has been shown that the excited state relaxation process of RPSB models needs a proper description of static and dynamic correlation. 43 Four-double-bond RPSB models with different methyl substitution patterns at C10 and C13 have been studied with CASSCF excited state molecular dynamics simulations. In addition, CASPT2//CASSCF (CASSCF -Complete Active Space Self Consistent Field) and CASPT2//B3LYP schemes of calculations yield excitation energies with good agreement with CASPT2//CASPT2 values.…”