Relaxed active space: fixing tailored-CC with high order coupled cluster. I

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Single-reference coupled cluster (SR-CC) methods struggle with multi-reference (MR) problems like bond breaking.
• Higher-order clusters (CCSDT, CCSDTQ) improve accuracy but are computationally expensive.
• Active space partitioning is necessary for large systems but can lead to inaccuracies.

Purpose of the Study:

• To develop an automated scheme for partitioning orbital spaces in coupled cluster calculations.
• To address the shortcomings of existing tailored-CC methods, specifically the neglect of inter-space coupling.
• To enable accurate and cost-effective treatment of strongly multi-reference systems.

Main Methods:

• Partitioning the orbital space into an active and external space based on statistical criteria.
• Treating the extended active space with singles, doubles, triples, and quadruples (CCSDTQ) and the external space with singles and doubles (CCSD).
• Automated determination of the extended space to eliminate user judgment and improve accuracy.

Main Results:

• The automated scheme successfully treats MR problems with SR-CC methods.
• Achieved experimental accuracy for dissociation energies of HF and F2 at a fraction of full CCSDT cost.
• The method overcomes nonparallelism error (NPE) inherent in previous tailored-CC approaches.

Conclusions:

• The developed automated extended space partitioning method offers a computationally efficient route to accurate MR calculations.
• This approach is suitable for strongly multi-reference systems and can be extended to higher-order clusters.
• It significantly reduces the computational burden compared to full high-order coupled cluster calculations.