An Uncontracted Epstein-Nesbet Perturbation Theory Approximation to SC-NEVPT2 Based on Spin-Pure Selected CI Wave...

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Accurately describing electron correlation is crucial for predicting molecular properties.
• Selected configuration interaction (SCI) methods capture static correlation but often neglect dynamic correlation.
• Large active spaces pose computational challenges for traditional quantum chemistry methods.

Purpose of the Study:

• To develop methods for incorporating dynamical electron correlation into SCI calculations.
• To enable accurate calculations for larger active spaces than previously possible.
• To improve the efficiency of quantum chemical calculations for complex systems.

Main Methods:

• Implemented a strongly contracted N-electron valence state perturbation theory (SC-NEVPT2) method with residual terms.
• Developed a hybrid Epstein-Nesbet (EN)-SC-NEVPT2 approach to address challenging perturber classes.
• Utilized symmetries, selection procedures, and parallelization to reduce computational costs.

Main Results:

• The implemented SC-NEVPT2 method effectively incorporates dynamical correlation effects.
• The hybrid EN-SC-NEVPT2 method significantly reduces computational costs compared to SC-NEVPT2.
• The new methods can handle active spaces up to 30 orbitals.

Conclusions:

• The developed methods provide accurate and efficient ways to include dynamical correlation in SCI calculations.
• These advancements enable the study of larger and more complex molecular systems.
• The computational cost reductions make these methods practical for routine use.