Single-reference coupled cluster theory for multi-reference problems.

Area of Science:

• Quantum chemistry
• Computational chemistry
• Theoretical chemistry

Background:

• Coupled Cluster (CC) theory is a leading quantum chemistry method for accurate electronic structure calculations.
• Standard CC methods often rely on single Slater-determinant references, such as Hartree-Fock (HF).
• Poor reference determinant choice can lead to unreliable CC results, especially in systems with significant static correlation.

Purpose of the Study:

• To investigate the performance of CC methods for singlet/triplet splitting in phenyldinitrene molecules.
• To analyze the impact of reference determinant choice on CC calculations.
• To explore alternative methods for systems where standard CC fails.

Main Methods:

• Coupled Cluster with singles, doubles, and perturbative triples [CCSD(T)] calculations using various reference determinants.
• Higher-level CC calculations including CCSDT-3 and CCSDT.
• Doubly electron attached and doubly ionized equation-of-motion (DEA/DIP-EOM) methods.

Main Results:

• CCSD(T) performance was sensitive to the reference determinant choice for the studied phenyldinitrene systems.
• Higher-level CC calculations confirmed the challenges associated with standard reference determinants.
• DEA/DIP-EOM methods successfully described the multi-reference character of the molecules, providing accurate singlet/triplet splittings.

Conclusions:

• The "failures" of CC methods are often due to poor reference determinant selection, not intrinsic limitations.
• DEA/DIP-EOM approaches offer a robust alternative for systems exhibiting multi-reference character.
• CC theory remains a powerful tool for static and dynamic correlation when reference determinant pitfalls are avoided.