Crossing conditions in coupled cluster theory.

Area of Science:

• Quantum Chemistry
• Theoretical Chemistry
• Computational Chemistry

Background:

• Conical intersections are crucial for understanding non-adiabatic processes in molecules.
• Coupled cluster theory is a high-level ab initio method for electronic structure calculations.
• Accurate description of conical intersections is vital for predicting molecular reaction dynamics.

Purpose of the Study:

• To derive and analyze the crossing conditions at conical intersections within coupled cluster (CC) theory.
• To investigate the impact of Jacobian matrix defects on the description of conical intersections.
• To identify necessary modifications in CC theory for accurate conical intersection treatment.

Main Methods:

• Derivation of crossing conditions for conical intersections in coupled cluster theory.
• Application of coupled cluster theory to model systems: hypofluorous acid and hydrogen sulfide.
• Analysis of the coupled cluster Jacobian matrix and its properties.

Main Results:

• Nondefective coupled cluster Jacobian matrices yield correct crossing conditions for real and complex Hamiltonians.
• Defects in the Jacobian matrix at accidental same-symmetry intersections lead to nonphysical artifacts, such as folded intersection seams.
• An accidental symmetry-allowed intersection in hydrogen sulfide is accurately described by the theory, showing no artifacts.

Conclusions:

• The coupled cluster theory requires minor modifications to accurately describe conical intersections in general.
• The study provides a framework for understanding and correcting artifacts in the treatment of conical intersections.
• Accurate handling of conical intersections is essential for reliable predictions in quantum chemistry.