Complete Active Space Iterative Coupled Cluster Theory

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Multireference-driven coupled cluster (CC) methods are essential for accurately describing complex electronic structures.
• Existing methods face challenges in balancing accuracy and computational cost, particularly in strongly correlated systems.

Purpose of the Study:

• To develop and evaluate a novel algorithm, complete active space iterative coupled cluster (CASiCC), for improving multireference CC calculations.
• To assess the efficiency and accuracy of CASiCC compared to existing methods for prototypical molecular systems.

Main Methods:

• An iterative feedback loop was established between complete active space (CAS) and coupled cluster (CC) calculations.
• A similarity transformation of the Hamiltonian was employed using CC amplitudes outside the active space.
• The CASiCC ansatz was tested in the singles and doubles approximation on H4, H8, H2O, and N2.

Main Results:

• CASiCC systematically enhanced the accuracy of single-reference coupled cluster doubles (CCSD) and externally corrected CCSD across potential energy curves.
• The method demonstrated modest computational costs.
• Tailored coupled cluster (TCC) showed superior performance in the strong correlation regime, indicating potential error compensation mechanisms.

Conclusions:

• The CASiCC approach offers a systematic improvement over standard and externally corrected CC methods.
• Iterative versions of externally corrected and tailored coupled cluster methods were found to converge to identical results.
• CASiCC presents a promising avenue for accurate and efficient quantum chemical calculations.