Towards the ‘Gold Standard’ in Bootstrap Embedding

We present a scalable implementation of Coupled Cluster Singles, Doubles, and Perturbative Triples [CCSD(T)] density matrix evaluation, integrated within the Bootstrap Embedding (BE2) framework and accelerated through the use of orbital permutation symmetry. Our approach achieves substantial reduction in computational cost compared to canonical all-electron CCSD(T) calculations, while retaining high accuracy in relative energies. Benchmark calculations on annulenes and representative isomerisation reactions show that BE2-CCSD(T) achieves accurate energetics with near-linear scaling in system size, enabling routine application to systems that are otherwise intractable with conventional post-Hartree–Fock methods. Additionally, we demonstrate that the BE2-CCSD(T) scheme with Intrinsic Atomic Orbital and Projected Atomic Orbital (IAO+PAO) partitioning not only recovers similar relative energies compared to Lowdin partitioning but also requires fewer fragment orbitals. This work lays the groundwork for extending fragment-based correlated wavefunction methods to broader classes of molecular systems.