An orbital-invariant internally contracted multireference coupled cluster approach

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Traditional multireference coupled cluster methods face challenges with computational cost and orbital invariance.
• The Jeziorski-Monkhorst ansatz, while powerful, has limitations in scaling and energy invariance.

Purpose of the Study:

• To formulate and implement an internally contracted multireference coupled cluster (ic-MRCC) approach.
• To address the computational scaling and orbital rotation invariance issues in quantum chemical calculations.
• To provide a more efficient and robust method for electronic structure studies.

Main Methods:

• Generalization of the single-reference coupled cluster ansatz using an exponential operator on a multiconfigurational wave function.
• Implementation of the internally contracted multireference coupled cluster singles and doubles (ic-MRCCSD) approximation.
• Utilizing truncated Baker-Campbell-Hausdorff expansions for computational approximations.

Main Results:

• The ic-MRCCSD method accurately predicts potential energy curves for benchmark reactions like Be + H(2) insertion and water dissociation.
• Approximations using truncated Baker-Campbell-Hausdorff expansions show rapid convergence to the full theory, with two commutators sufficient for high accuracy.
• The ic-MRCC method demonstrates invariance with respect to active orbital rotations, confirmed by formal analysis and numerical tests.

Conclusions:

• The developed ic-MRCC approach offers a significant improvement over existing methods by enhancing computational efficiency and ensuring orbital invariance.
• Approximated ic-MRCC theories maintain the desirable invariance properties of the full method.
• This work provides a valuable computational tool for accurate electronic structure calculations in complex chemical systems.