Analytical SA-HCISCF Nuclear Gradients from Spin-Adapted Heat-Bath Configuration Interaction

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Accurate calculation of molecular properties requires advanced quantum chemical methods.
• Excited-state geometry optimization is crucial for understanding photochemistry and photophysics.
• Efficient computation of nuclear gradients and nonadiabatic couplings is essential for dynamics simulations.

Purpose of the Study:

• To implement analytical nuclear gradients and nonadiabatic couplings for state-averaged selected configuration interaction (SCI) wave functions.
• To develop a fully internally contracted multireference configuration interaction (CI) coupled second-order orbital optimization method.
• To demonstrate the application of these methods for excited-state geometry optimizations.

Main Methods:

• Implementation of analytical nuclear gradients and nonadiabatic couplings using state-averaged SCI wave functions.
• Development of a fully CI-coupled second-order orbital optimization method.
• Utilization of density fitting to accelerate the calculation of two-electron integrals.

Main Results:

• Successful implementation of analytical nuclear gradients and nonadiabatic couplings for SCI.
• Demonstration of excited-state geometry optimizations for conjugated molecules.
• Optimization of the first triplet excited-state geometry for the Fe(PDI) transition-metal catalyst.

Conclusions:

• The implemented methods provide efficient tools for excited-state calculations.
• The approach is applicable to both organic conjugated molecules and transition-metal complexes.
• Future work will focus on further method development and application to larger systems.