Importance and Reliability of Small Basis Set CCSD(T) Corrections to MP2 Binding and Relative Energies of Water Clusters

MP2 describes hydrogen-bonded systems well, yet a higher-order electron correlation correction in the form of a CCSD(T) calculation is usually necessary to achieve benchmark quality energies. We evaluated the importance and reliability of small basis set CCSD(T) corrections to MP2(δCCSD(T)) both on the binding (ΔE) and relative (ΔΔE) MP2 energies for a large number of systems including four water dimer stationary points and 57 other clusters up to undecamers, (H2O)11. By comparing the MP2 energies with CCSD(T) and the explicitly correlated MP2-F12 energies with variants of CCSD(T)-F12 using different basis sets, we were able to establish that the correction to the binding energy (ΔE) is sensitive to basis set size, especially for small double-ζ basis sets. On the other hand, the basis set sensitivity of the correction to the relative energy (ΔΔE) within each cluster size is very small. While the δCCSD(T) correction to the binding energy might vary in magnitude with basis set size, its effect on relative energy MP2 (and hence the stability of isomers) is remarkably consistent. Therefore, we recommend the inclusion of this correction to obtain the relative stability of closely spaced isomers using a double-ζ basis set with polarization and diffuse functions such as aug-cc- pVDZ.