Implementation of the locally renormalized CCSD(T) approaches for arbitrary reference function.

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Coupled-cluster (CC) methods are essential for accurate electronic structure calculations.
• Existing methods like completely renormalized CCSD(T) (CR-CCSD(T)) have limitations in certain scenarios.
• The numerator-denominator-connected (NDC) expansion provides a foundation for new CC approaches.

Purpose of the Study:

• To formulate and implement new variants of locally-renormalized coupled-cluster (LR-CCSD(T)) methods.
• To enable these methods for arbitrary reference states using automated code generation.
• To assess the accuracy and efficiency of LR-CCSD(T) for describing molecular potential-energy surfaces and properties.

Main Methods:

• Development of locally-renormalized coupled-cluster (LR-CCSD(T)) approximations based on the NDC expansion.
• Implementation using the TENSOR CONTRACTION ENGINE for automatic, efficient parallel code generation.
• Application to challenging molecular systems (F2, N2, CN, O3) to evaluate potential-energy surfaces and equilibrium properties.

Main Results:

• The new LR-CCSD(T) approaches provide highly accurate descriptions of potential-energy surfaces, even for bond breaking.
• These methods maintain the favorable N(7) computational scaling of the standard CCSD(T) approach.
• Accurate predictions of equilibrium properties like bond lengths, angles, and harmonic frequencies were achieved for ozone.

Conclusions:

• LR-CCSD(T) methods offer a powerful and accurate tool for quantum chemistry, particularly for describing challenging electronic structures.
• The use of local denominators ensures additive separability in the noninteracting limit.
• These methods represent a significant advancement in computational chemistry for predicting molecular behavior.