NENCI-2020: A Large Benchmark Database of Non-Equilibrium Non-Covalent Interactions with an Emphasis on Close Contacts

In this work, we present NENCI-2020: a benchmark database of non-equilibrium non-covalent interaction energies for a large and diverse set of intermolecular complexes. Designed with an emphasis on close contacts, NENCI-2020 simultaneously samples both intermolecular distances (spanning 0.7-1.1x the equilibrium separation) and intermolecular angles for ~150 molecular dimers, yielding ~8,000 benchmark intermolecular interaction energies computed at the CCSD(T)/CBS level. Using SAPT, we demonstrate that NENCI-2020 contains a diverse array of binding motifs, making this database well-suited for testing and developing next-generation force fields, density functional theory (DFT) approximations, quantum chemical (QC) methods, and machine-learning based approaches. This is followed by a critical assessment of ~75 DFT and QC methods, in which we find that most of these approaches can describe equilibrium and near-equilibrium intermolecular interaction energies to within chemical accuracy (1 kcal/mol). However, we find that nearly all methods suffer from a rapid and systematic increase in error as the intermolecular distance becomes small, thereby suggesting that more work will be needed to describe intermolecular potential energy surfaces with uniform accuracy.