Toward the "Gold Standard" in Bootstrap Embedding.

Area of Science:

• Quantum chemistry
• Computational physics
• Theoretical chemistry

Background:

• Coupled Cluster Singles, Doubles, and Perturbative Triples (CCSD(T)) is a highly accurate quantum chemistry method.
• Conventional CCSD(T) calculations are computationally expensive, limiting their application to small systems.
• Fragment-based methods offer a potential solution for scaling correlated wave function calculations.

Purpose of the Study:

• To implement and validate a computationally efficient CCSD(T) density matrix evaluation within the Bootstrap Embedding (BE2) framework.
• To accelerate CCSD(T) calculations using orbital permutation symmetry.
• To enable accurate and routine application of CCSD(T) to larger molecular systems.

Main Methods:

• Implementation of CCSD(T) density matrix evaluation integrated with the Bootstrap Embedding (BE2) framework.
• Acceleration of calculations using orbital permutation symmetry.
• Benchmarking against canonical all-electron CCSD(T) calculations and using Intrinsic Atomic Orbital and Projected Atomic Orbital (IAO + PAO) partitioning.

Main Results:

• The BE2-CCSD(T) approach significantly reduces computational cost compared to canonical CCSD(T).
• Accurate relative energies were obtained, with benchmark calculations on annulene systems showing near-linear scaling with system size.
• The method demonstrates high accuracy for isomerizations when using IAO + PAO partitioning.

Conclusions:

• The BE2-CCSD(T) method provides a computationally feasible and accurate alternative to conventional CCSD(T) for large systems.
• This work enables the routine application of high-accuracy correlated wave function methods to previously intractable molecular systems.
• The developed framework lays the foundation for extending fragment-based correlated methods in quantum chemistry.