Communication: The performance of non-iterative coupled cluster quadruples models

Area of Science:

• Quantum chemistry
• Computational physics
• Theoretical chemistry

Background:

• Coupled cluster (CC) theory is a powerful method for describing electron correlation in molecules.
• Accurate calculations of electron correlation are crucial for predicting molecular properties.
• Higher-order excitations (triples and quadruples) significantly impact correlation energy.

Purpose of the Study:

• To evaluate the numerical performance of various non-iterative coupled cluster (CC) quadruples models.
• To identify CC models capable of achieving high accuracy comparable to the full CC singles, doubles, triples, and quadruples (CCSDTQ) model.
• To compare the efficacy of different approaches for incorporating quadruple excitation effects.

Main Methods:

• Numerical comparison of several non-iterative CC quadruples models.
• Assessment of models that correct CC singles and doubles (CCSD) energy for higher excitations.
• Evaluation of models that add corrections for quadruple excitations to the CC singles, doubles, and triples (CCSDT) energy.

Main Results:

• Non-iterative CC models correcting for combined triple and quadruple excitations fail to reach CCSDTQ accuracy.
• Models correcting the CCSDT energy for isolated quadruple excitations demonstrate superior performance.
• The CCSDT(Q-3) and CCSDT(Q-4) models, from the Lagrangian-based CCSDT(Q-n) series, outperform other methods using [Q] and (Q) corrections.

Conclusions:

• Achieving CCSDTQ-level accuracy requires careful treatment of quadruple excitations.
• Lagrangian-based perturbation methods, specifically CCSDT(Q-3) and CCSDT(Q-4), offer a promising route to accurate CC quadruples calculations.
• Simpler correction schemes applied to lower-level CC models are insufficient for high-accuracy correlation energy recovery.