Excited‐state minima and emission energies of retinal chromophore analogues: Performance of CASSCF and CC2 methods as compared with CASPT2

Abstract

This study provides gas-phase S excited-state geometries along with emission and adiabatic energies for methylated/demethylated and ring-locked analogues of protonated Schiff base retinal models comprising system of five conjugated double bonds (PSB5), using second order multiconfiguration perturbation theory (CASPT2). CASPT2 results serve as reference data to assess the performance of CC2 (second-order approximate coupled cluster singles and doubles) and a commonly used CASSCF/CASPT2 protocol, that is, complete active space self-consistent field (CASSCF) geometry optimization followed by CASPT2 energy calculation. We find that the CASSCF methodology fails to locate planar S minimum energy structures for four out of five investigated planar models in contrast to CC2 and CASPT2 methods. However, for those which were found: one planar and two twisted minima, there is an excellent agreement between CASSCF and CASPT2 results in terms of geometrical parameters, one-electron properties, as well as emission and adiabatic energies. CC2 performs well for in-plane S minima and their spectroscopic and electronic properties. However, this picture deteriorates for twisted minima. As expected, the CC2 description of the S electronic state, with strong multireference and significant double excitation character, is very poor, exhibiting errors in transition energies exceeding 1 eV. They may be substantially diminished by recalculating transition energies with CASPT2 method. Our work shows that CASSCF/CASPT2 and CC2 shortcomings may influence gas-phase retinal analogues' excited state description in a dramatic way. © 2017 Wiley Periodicals, Inc.

Initial excited‐state relaxation of locked retinal protonated schiff base chromophore. An insight from coupled cluster and multireference perturbation theory calculations

“…Also, in our recent study on PSB3 (a minimal model of RPSB with three double bonds), we noticed more pronounced double‐single bond length alternation in CASSCF ground‐state structures with respect to the third‐order corrected perturbation theory (CASPT3) and coupled cluster (CCSD(T)) data. Interestingly, a quite successful performance of CASSCF was found in the S 1 excited state of PSB5 analogs as geometrical features of S 1 true minima are well in line with CASPT2 results …”

“…In our view more studies on the excited state of various retinal systems are needed to draw general conclusions on the range of suitability of CC2 method in this field. In fact, we have recently shown that CC2 provides excited‐state geometries and emission energies for chosen PSB5 analogs in line with state‐of‐the‐art CASPT2 results for in‐plane models . Nonetheless, this good agreement usually deteriorates for twisted excited‐state minimum‐energy structures near the FC point …”

“…Based on the above evidence it is tempting to announce that CC2 method may be a good candidate to substitute much more demanding, in terms of cpu time, physical memory and disc storage, multireference perturbation theory methods, whenever the latter methods are out of reach, in mechanistic investigations of rhodopsin chromophore photoisomerization process. Even though, the pool of CC2 data regarding retinals has exploded recently, it mostly concerns studies on ground‐state structures. In our view more studies on the excited state of various retinal systems are needed to draw general conclusions on the range of suitability of CC2 method in this field.…”

Is the choice of a standard zeroth‐order hamiltonian in CASPT2 ansatz optimal in calculations of excitation energies in protonated and unprotonated schiff bases of retinal?

“…Since CC2 methodology has been often utilized in spectroscopic studies of excited states in biochromophores we assess its performance here with respect to the experimental AM and theoretical best estimates. By analyzing the results in Table , one can notice that the CC2 performance in reproducing AM is not as impressive as the one by XMCQDPT2.…”

“…Even more, for a series of RPSB models ranging from PSB3 to PSB5 (model containing five conjugated double bonds) and also for full RPSB chromophore, S‐IPEA outperforms 0‐IPEA and provides results in close agreement with quantum Monte Carlo results . Of course, there are other factors, besides the IPEA shift value, that may contribute to the CASPT2‐based vertical excitation energy error such as basis set size, the selection, and size of the active space, number of roots in the SA‐CASSCF wave function, value of level shift parameter, and the quality of the equilibrium geometry . However, among the aforementioned factors affecting vertical excitation energies (VEEs), the IPEA shift parameter does affect the excitation energies to the largest extent provided that the basis set and active space are sufficiently large.…”