Simplified Multireference Coupled-Cluster Methods: Hybrid Approaches With Averaged Coupled Pair Theories

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Accurate calculation of electron correlation energy is crucial for predicting molecular properties.
• Internally contracted multireference coupled-cluster (ICMRCC) methods are computationally demanding due to non-commuting operators.
• Approximation methods are needed to balance accuracy and computational cost.

Purpose of the Study:

• To develop and evaluate a hybrid approximation to the ICMRCC method.
• To assess the performance of different linearized approaches for internal and semi-internal contributions.
• To investigate the accuracy and stability of the proposed methods for various molecular systems.

Main Methods:

• A hybrid approach combining coupled-cluster (CC) for external pairs and linearized coupled-electron pair approximation (CEPA) for internal/semi-internal pairs.
• Testing of CEPA(0), averaged coupled pair functional (ACPF), averaged quadratic coupled-cluster (AQCC), and averaged CEPA methods.
• Evaluation of size consistency, potential energy curves (C2, N2, CN, O3), and singlet-triplet splitting of benzynes.

Main Results:

• The averaged quadratic coupled-cluster (AQCC) method exhibited the most accurate and stable performance among the tested methods.
• The hybrid approach effectively handles the complexity of non-commuting operators in ICMRCC.
• Small violations of size consistency were observed for the AQCC method.

Conclusions:

• The hybrid approach, particularly AQCC, offers a promising balance between accuracy and computational feasibility for electronic structure calculations.
• Further refinements may be needed to address minor size consistency issues.
• The developed method is suitable for studying potential energy curves and electronic properties of small molecules and reactive intermediates.