On the accuracy of retinal protonated Schiff base models

TL;DR

This study compares different computational methods for modeling the molecular geometries of retinal protonated Schiff base models, revealing that advanced XMS-CASPT2 calculations suggest certain isomerization pathways are thermally inaccessible, unlike traditional SA-CASSCF results.

Contribution

It provides a detailed comparison of XMS-CASPT2 and SA-CASSCF methods for retinal models, highlighting the importance of dynamical electron correlation in predicting conical intersection geometries.

Findings

XMS-CASPT2 indicates the 13-14 isomerization conical intersection is thermally inaccessible.

Differences between methods are due to stabilization of charge transfer states by dynamical correlation.

Active space, basis set, and state averaging choices significantly affect results.

Abstract

We investigate the molecular geometries of the ground state and the minimal energy conical intersections (MECIs) between the ground and first excited states of the models for the retinal protonated Schiff base in the gas phase using the extended multistate complete active space second-order perturbation theory (XMS-CASPT2). The biggest model in this work is the rhodopsin chromophore truncated between the {\epsilon} and {\delta} carbon atoms, which consists of 54 atoms and 12-orbital {\pi} conjugation. The results are compared with those obtained by the state-averaged complete active space self-consistent field (SA-CASSCF). The XMS-CASPT2 results suggest that the minimum energy conical intersection associated with the so-called 13-14 isomerization is thermally inaccessible, which is in contrast to the SA-CASSCF results. The differences between the geometries of the conical intersections computed by SA-CASSCF and XMS-CASPT2 are ascribed to the fact that the charge transfer states are more stabilized by dynamical electron correlation than the diradicaloid states. The impact of the various choices of active spaces, basis sets, and state averaging schemes is also examined.