FragPT2: Multi-Fragment Wavefunction Embedding with Perturbative Interactions

• Published in: arXiv.org
• Main Authors: Koridon, Emiel, Sen, Souloke, Visscher, Lucas, Polla, Stefano
• Format: Paper
• Language: English
• Published: Ithaca Cornell University Library, arXiv.org 08.08.2024
• Subjects: Charge exchange; Charge transfer; Covalent bonds; Dimers; Embedding; Fragments; Molecular orbitals; Perturbation theory; Spin exchange; Wave functions

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.