Perturbative universal state-selective correction for state-specific multi-reference coupled cluster methods.

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• The universal state-selective (USS) multireference coupled-cluster (MRCC) formalism enhances accuracy in electronic structure calculations.
• The full USS(2) approach, while accurate, faces challenges with high computational and storage costs.

Purpose of the Study:

• To introduce a computationally efficient variant of the USS(2) method, termed USS(pt).
• To reduce the numerical overhead and memory requirements of the USS(2) approach.
• To maintain the accuracy of the USS(2) method for describing quasidegenerate electronic states.

Main Methods:

• Development of a perturbative variant (USS(pt)) of the USS(2) multireference coupled-cluster formalism.
• On-the-fly calculation of triple and quadruple projections to eliminate memory bottlenecks.
• Application to benchmark systems to assess accuracy and efficiency.

Main Results:

• The USS(pt) method significantly reduces numerical overhead compared to the full USS(2) approach.
• Accuracy comparable to the USS(2) method is preserved.
• The USS(pt) approach effectively alleviates issues with orbital rotation invariance in MRCC theories.
• Efficient description of quasidegenerate electronic states is demonstrated.

Conclusions:

• The USS(pt) method offers a practical and accurate solution for electronic structure calculations involving complex systems.
• This development expands the applicability of advanced MRCC methods by mitigating computational demands.
• The USS(pt) approach provides a robust tool for studying quasidegenerate electronic states and improves theoretical consistency.