An "ultimate" coupled cluster method based entirely on T2.

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Coupled cluster methods are essential for accurate electronic structure calculations.
• Existing methods struggle with diverse electron correlation types.
• Higher-rank excitations (T2m) improve accuracy but increase computational cost.

Purpose of the Study:

• Develop a computationally efficient "ultimate" coupled cluster method based on T2 operators.
• Emulate the accuracy of methods including higher-rank excitations.
• Systematically account for higher-order electron correlations.

Main Methods:

• Introduced a hierarchy of augmented T2 methods.
• Utilized factorization theorem of many-body perturbation theory (MBPT).
• Applied expectation-value coupled cluster theory.

Main Results:

• Methods are correct through a specified order in MBPT.
• Hierarchy reduces to standard Coupled-Dipole- গ্রহণযোগ্য (CCD) for dynamic correlations.
• Improved accuracy for systems with non-dynamic and static correlations.

Conclusions:

• The proposed augmented T2 methods provide a computationally efficient alternative.
• Cheapest methods emulate O(N10) CCDQ accuracy with O(N6) cost.
• Effective for diverse electron correlation regimes in molecules.