Shape of Multireference, Equation-of-Motion Coupled-Cluster, and Density Functional Theory Potential Energy Surfaces...

Area of Science:

• Computational Chemistry
• Theoretical Chemistry
• Photochemistry

Background:

• Conical intersections (CIs) are critical for understanding photochemical reactions, particularly photoisomerization in retinal chromophores.
• Previous studies evaluated electronic structure methods along reaction paths but not the CI branching plane.
• The penta-2,4-dieniminium cation (PSB3) serves as a model system for studying these phenomena.

Purpose of the Study:

• To investigate the ability of various electronic structure methods to accurately describe the potential energy surface shape along the branching plane of a conical intersection (CI).
• To compare the performance of different computational methods in characterizing the topology of CIs in the PSB3 model.
• To assess the impact of different CI topologies (linear vs. conical) on the photoisomerization efficiency of PSB3.

Main Methods:

• Characterization of ground- and excited-state potential energy profiles using multiple electronic structure methods.
• Focus on the branching plane associated with a conical intersection (CI) in the penta-2,4-dieniminium cation (PSB3) model.
• Computation of 100 semiclassical trajectories using CASSCF and SS-CASPT2(IPEA=0.25) to analyze photoisomerization dynamics.

Main Results:

• Standard time-dependent DFT (TDDFT) and several other methods (SS-CASPT2, spin-projected SF-TDDFT, EOM-SF-CCSD, MRCISD+Q) incorrectly yield a 1D (linear) crossing instead of a 2D (conical) crossing.
• Methods such as MRCISD, CASSCF, MS-CASPT2, XMCQDPT2, QD-NEVPT2, non-spin-projected SF-TDDFT, and SI-SA-REKS correctly describe the conical crossing.
• Semiclassical dynamics simulations show similar photoisomerization efficiencies for CASSCF and SS-CASPT2(IPEA=0.25) despite differences in CI topology description, with quantum yields differing by only a few percent.

Conclusions:

• The choice of electronic structure method significantly impacts the description of conical intersection topology.
• While some methods incorrectly predict linear crossings, others accurately capture the conical nature essential for photochemical processes.
• Despite differing CI topology predictions, the simulated photoisomerization dynamics and quantum yields for PSB3 were comparable, suggesting robustness in dynamics calculations under certain conditions.