Analytic Non-adiabatic Couplings for Selected Configuration Interaction via Approximate Degenerate Coupled Perturbed...

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Non-adiabatic couplings are crucial for understanding molecular dynamics and photochemistry.
• Accurate computation of these couplings is often computationally expensive, especially for large systems or complex electronic states.

Purpose of the Study:

• To develop an efficient and accurate analytic method for calculating non-adiabatic couplings.
• To enable accurate calculations for selected configuration interaction (CI) methods.

Main Methods:

• Utilized degenerate perturbation theory and coupled perturbed Hartree-Fock (CPHF) theory.
• Developed an analytic approach for non-adiabatic couplings, applicable even with degenerate orbitals.
• Benchmarked against full CI calculations for excited states of carbon monoxide and ammonia.

Main Results:

• The analytic method requires only a minimal modification to the original molecular orbital basis.
• The method demonstrates high accuracy when compared to full CI.
• A semi-numerical approach confirmed the analytic method's validity even when symmetry is broken, leading to near-degeneracies.

Conclusions:

• The developed analytic non-adiabatic couplings for selected CI are efficient and accurate.
• The method is robust and applicable to systems with near-degenerate electronic states.
• Significant computational savings are achievable using this approach for selected CI calculations.