FragPT2: Multi-Fragment Wavefunction Embedding with Perturbative Interactions

TL;DR

FragPT2 introduces a multi-fragment embedding approach combining multi-reference methods and perturbation theory to accurately model complex interactions in large molecular systems, including challenging covalent bond cuts.

Contribution

It presents a novel multi-fragment embedding framework that incorporates inter-fragment correlations via perturbation theory, enabling detailed analysis of interaction types.

Findings

Effective for challenging test systems like N2 dimers and aromatic dimers.

Capable of handling fragments cut through covalent bonds.

Provides detailed classification of inter-fragment interactions.

Abstract

Embedding techniques allow the efficient description of correlations within localized fragments of large molecular systems, while accounting for their environment at a lower level of theory. We introduce FragPT2: a novel embedding framework that addresses multiple interacting active fragments. Fragments are assigned separate active spaces, constructed by localizing canonical molecular orbitals. Each fragment is then solved with a multi-reference method, self-consistently embedded in the mean field from other fragments. Finally, inter-fragment correlations are reintroduced through multi-reference perturbation theory. Our framework provides an exhaustive classification of inter-fragment interaction terms, offering a tool to analyze the relative importance of various processes such as dispersion, charge transfer, and spin exchange. We benchmark FragPT2 on challenging test systems, including \ce{N_2} dimers, multiple aromatic dimers, and butadiene. We demonstrate that our method can be succesful even for fragments defined by cutting through a covalent bond.