Full implementation and benchmark studies of Mukherjee's state-specific multireference coupled-cluster ansatz

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• The state-specific multireference coupled-cluster (SS-MRCC) ansatz is crucial for accurately describing electronic structures.
• Previous implementations faced challenges in handling convergence and orbital transformations.

Purpose of the Study:

• To implement the SS-MRCC ansatz using string-based techniques for general applicability.
• To investigate convergence issues and explore strategies for improvement.
• To assess the impact of active orbital transformations on computational accuracy.

Main Methods:

• Implementation of SS-MRCC using string-based techniques.
• Testing with single- and multiple-bond dissociations.
• Convergence troubleshooting via amplitude thresholding and Tikhonov regularization.
• Comparison with Full Configuration Interaction (FCI) and other correlated methods (MRCI, MR-ACPF, MR-AQCC).

Main Results:

• Successful implementation of a general SS-MRCC method.
• Identified convergence issues related to small reference function weights, addressed by thresholding or regularization.
• SS-MRCC performance is generally better with localized active orbitals compared to pseudocanonical ones.
• Benchmark results show good agreement with FCI and competitive performance against other advanced CC methods.

Conclusions:

• The string-based SS-MRCC implementation provides a robust tool for quantum chemical calculations.
• Strategies for handling convergence problems enhance the method's reliability.
• Orbital choice significantly impacts SS-MRCC accuracy, favoring localized orbitals for improved performance.