Accurate Calculation of Noncovalent Interactions Using PNO-LCCSD(T)-F12 in Molpro

Noncovalent interactions (NCIs) are fundamental to understanding biomolecular systems, material properties, and chemical reactivity. Accurately modeling these forces with commonly applied and less costly approximate quantum chemical (QC) methods such as dispersion-corrected DFT requires reliable theoretical benchmarks since accurate experimental data are rarely available. Currently, NCI benchmarks mainly focus on smaller molecules (typically < 50 atoms), as scaling issues and shortcomings of conventional correlated wavefunction theory (WFT) methods (e.g., MP2 and CCSD(T)) limit their applicability for larger systems with significant NCIs. CCSD(T) has long been the "gold standard" reference for NCIs, yet recent studies reveal its overbinding tendency in π-stacked complexes and other NCI systems with high polarizability. Discrepancies between (local) CCSD(T) and alternative approaches like FN-DMC further emphasize the need for in-depth investigations and improvements. The explicitly correlated local coupled cluster PNO-LCCSD(T)-F12 method implemented in Molpro, possibly combined for very large systems with the recently introduced region approach [<i>J. Phys. Chem. A</i> <b>2024,</b> <i>128,</i> 10936-10947], offers solutions by reducing basis set errors and scaling problems. This method bridges the gap between computational efficiency and high accuracy. This study reexamines key NCI systems previously evaluated using local CCSD(T) and FN-DMC with the PNO-LCCSD(T)-F12 approach employing Molpro's recently extended and user-friendly infrastructure for such calculations. By analyzing known limitations in detail and providing refined interaction energies, this work sets a new benchmark for reliable QC calculations of large, complex NCI systems.