Block correlated coupled cluster theory with a complete active-space self-consistent-field reference function: the...

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Describing multireference character in electronic wave functions is crucial for accurate molecular modeling.
• Traditional coupled cluster methods struggle with systems exhibiting strong static correlation.

Purpose of the Study:

• Introduce and validate a novel multireference coupled cluster theory, Block Correlated Coupled Cluster (BCCC).
• Assess the performance of BCCC with a Complete Active Space Self-Consistent Field (CASSCF) reference function for electronic structure and bond dissociation.

Main Methods:

• Developed Block Correlated Coupled Cluster (BCCC) theory using a CASSCF reference function.
• Truncated cluster operators to the four-block correlation level, creating the CAS-BCCC4 scheme.
• Applied CAS-BCCC4 to model systems (H4, P4, BeH2) and bond breaking processes (F2, HF, BH, CH4).

Main Results:

• CAS-BCCC4 demonstrates computational scaling similar to single-reference coupled cluster singles and doubles for single bond breaking.
• Achieved quantitative agreement with full configuration interaction calculations for all studied systems.
• Exhibited minimal size-consistency errors in the dissociation limits of diatomic molecules.

Conclusions:

• The presented CAS-BCCC4 method provides a robust and accurate multireference coupled cluster framework.
• This approach effectively captures nondynamic correlation essential for describing challenging electronic systems.
• CAS-BCCC4 offers a computationally efficient alternative for studying static correlation and bond dissociation.