Orbital Optimization in Selected Configuration Interaction Methods.

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Orbital optimization is essential in electronic structure methods.
• Selected configuration interaction (SCI) methods offer a balance between accuracy and computational cost.
• Perturbation theory is often used to refine SCI calculations.

Purpose of the Study:

• To investigate and compare different orbital optimization strategies within SCI methods.
• To analyze the similarities and differences between orbital optimization in SCI and complete active space self-consistent field (CASSCF).
• To identify efficient methods for achieving fast convergence in SCI calculations.

Main Methods:

• Utilized the semistochastic heat-bath configuration interaction (SHCI) method.
• Tested optimization approaches on ground and excited states of three molecules.
• Classified optimization methods into uncoupled, fully coupled, and quasi-fully coupled based on parameter interactions.

Main Results:

• Demonstrated that incorporating coupling between configuration interaction coefficients and orbital parameters is vital for rapid convergence.
• Evaluated the performance of various orbital optimization techniques.
• Identified specific methods that significantly enhance computational efficiency.

Conclusions:

• Coupling effects are critical for efficient orbital optimization in SCI.
• Recommends two quasi-fully coupled methods: accelerated diagonal Newton and Broyden-Fletcher-Goldfarb-Shanno (BFGS).
• These methods provide a pathway for faster and more reliable electronic structure calculations.