Single-State Single-Reference and Multistate Multireference Zeroth-Order Hamiltonians in MS-CASPT2 and Conical...

Area of Science:

• Quantum Chemistry
• Computational Photochemistry
• Theoretical Spectroscopy

Background:

• Multistate complete active space second-order perturbation theory (MS-CASPT2) is crucial for modeling static and dynamical correlations in photochemistry.
• Two primary definitions of the zeroth-order Hamiltonian exist: multistate multireference (MS-MR-MS-CASPT2) and single-state single-reference (SS-SR-MS-CASPT2).

Purpose of the Study:

• Implement analytical gradient and derivative coupling for SS-SR-MS-CASPT2.
• Compare SS-SR-MS-CASPT2 with MS-MR-(X)MS-CASPT2 for optimizing molecular geometries and minimum energy conical intersections (MECIs).

Main Methods:

• Analytical gradient and derivative coupling implementation for SS-SR-MS-CASPT2.
• Geometry optimization of critical points (stable geometries and MECIs) for rhodopsin and green fluorescent protein chromophore models.
• Comparative analysis of potential energy surfaces (PESs) and MECI characteristics between the two methods.

Main Results:

• MECIs in MS-MR-XMS-CASPT2 exhibited more out-of-plane bridge hydrogen bonds compared to SS-SR-MS-CASPT2.
• Distinct PES topologies near conical intersections were observed for the PSB3 model.
• MS-MR-XMS-CASPT2 consistently produced smooth PESs near MECIs, unlike SS-SR-MS-CASPT2.

Conclusions:

• Different zeroth-order Hamiltonians and state-averaging schemes necessitate careful interpretation of simulation results.
• MS-MR-XMS-CASPT2 is recommended for conical intersection simulations requiring smooth potential energy surfaces.