Sub-kJ/mol Precision at Practical Cost for Molecular Solids through Local Coupled-Cluster Approaches

Generation of numerically converged coupled-cluster reference energies of molecular crystals remains highly non-routine. Yet, such data points are critically needed as computational methodologies strive to reach correlated wave-function accuracy in condensed-phase systems. Localized-orbital coupled-cluster approaches applied within the many-body expansion promise a scalable route to these reference results. Here, we report solutions to a technical challenge when using this methodology─at long intermolecular distances, the numerical noise due to local approximations can accumulate for some molecular crystals into a large-scale drift of the correlation energy, especially in the nonadditive three-body contributions. Within the local natural orbital (LNO) coupled-cluster method, we suggest well-balanced approaches to overcome this issue. At the benchmark level, we propose more accurate LNO-CCSD(T) settings that remove the artifacts at long distances and attain a numerical precision of 0.2 kJ/mol (maximum error) in the lattice energy, with a practical cost that is orders of magnitude lower than that of canonical CCSD(T). As an even more scalable solution, we propose a multi-level strategy that utilizes the established routine LNO-CCSD(T) precision tiers (Tight, vTight) but retains the numerical stability of the benchmark level by treating long-distance three-body systems with the random-phase approximation (RPA).