NENCI-2021 Part I: A Large Benchmark Database of Non-Equilibrium Non-Covalent Interactions Emphasizing Close Intermolecular Contacts

In this work, we present NENCI-2021, a benchmark database of approximately $8,000$ non-equilibrium non-covalent interaction energies for a large and diverse selection of intermolecular complexes of biological and chemical relevance. To meet the growing demand for large and high-quality quantum mechanical data in the chemical sciences, NENCI-2021 starts with the $101$ molecular dimers in the widely used S66 and S101 databases, and extends the scope of these works by: (i) including $40$ cation- and anion-$\pi$ complexes, a fundamentally important class of non-covalent interactions (NCIs) that are found throughout nature and pose a substantial challenge to theory, and (ii) systematically sampling all $141$ intermolecular potential energy surfaces (PES) by simultaneously varying the intermolecular distance and intermolecular angle in each dimer. Designed with an emphasis on close contacts, the complexes in NENCI-2021 were generated by sampling seven intermolecular distances along each PES (ranging from $0.7\times\mathrm{-}1.1\times$ the equilibrium separation) as well as nine intermolecular angles per distance (five for each ion-$\pi$ complex), yielding an extensive database of $7,763$ benchmark intermolecular interaction energies ($E_{\rm int}$) obtained at the CCSD(T)/CBS level of theory. In addition, a wide range of intermolecular atom-pair distances are also present in NENCI-2021, where close intermolecular contacts involving atoms that are located within the so-called van der Waals envelope are prevalent -- these interactions in particular pose an enormous challenge for molecular modeling and are observed in many important chemical and biological systems. A detailed SAPT-based energy decomposition analysis also confirms the diverse and comprehensive nature of the intermolecular binding motifs present in NENCI-2021.