Reliable Structures and Electronic Energies of Small Water Clusters Using Density Functional and Local Correlation Coupled Cluster Model Chemistries

In this paper we have assessed the ability of the domain-based local pair natural orbital (DLPNO)-CCSD(T) method to match the explicitly correlated CCSD(T) relative energies of (H₂O)_<i>n</i>=3-7 isomers along with the impact of the level of theory utilized to optimize the water cluster geometries. The benchmark structures were optimized using a 2-body:Many-body procedure in which all of the 1- and 2-body contributions are computed using CCSD(T) while all of the higher-order many-body interactions are computed using MP2 (denoted CCSD(T):MP2). Benchmark relative energies were computed for these CCSD(T):MP2 optimized geometries with explicitly correlated CCSD(T)-F12b single point energies (SPEs) using the cc-pVQZ-F12 and cc-pV5Z-F12 basis sets augmented with diffuse functions on the O atoms. The benchmark structures and energies were used to gauge the performance of less demanding computational protocols. For example, DLPNO-CCSD(T) computations on the 31 benchmark structures with the analogous family of correlation consistent basis sets (cc-pVNZ for H and aug-cc-pVNZ for O, or simply haNZ where N = D-6) were used to estimate relative energies at the complete basis set (CBS) limit via three-point extrapolations. When compared to the CCSD(T)-F12 benchmark data, the mean absolute differences (MADs) were ≤ 0.13 kcal/mol when triple-ζ and larger basis sets were employed. Using these DLPNO-CCSD(T) results, we demonstrate that 2 less-demanding geometry optimization procedures, specifically the ωB97X-D density functional theory (DFT) method paired with the 6-31++G(d,p) basis set and the density-fitted MP2 method paired with the haTZ basis set, give structures that yield nearly identical relative energies (MADs of only 0.07 and 0.02 kcal/mol, respectively, when comparing DLPNO-CCSD(T)/ha6Z data). In addition, we show how the presence or absence of diffuse functions in the basis sets used for DLPNO-CCSD(T) SPEs impact the quality of the relative energies. The protocol that combines ωB97X-D/6-31++G(d,p) optimized structures with DLPNO-CCSD(T) SPEs using triple-ζ or higher Dunning basis sets that include augmentation with diffuse functions on the oxygen atoms provides a fast and accurate method for determining the relative electronic energies of (H₂O)_<i>n</i>=3-7 water cluster isomers.