Determination of Barrier Heights for Proton Exchange in Small Water, Ammonia, and Hydrogen Fluoride Clusters with G4(MP2)-Type, MPn, and SCS-MPn Procedures-A Caveat.

Calculation of accurate water-water interaction energies is of fundamental importance in computational modeling of many biological and chemical phenomena. We have obtained benchmark barrier heights for proton-exchange reactions and complexation energies in water clusters (H2O)n (n = 1-6) by means of the high-level W1-F12 procedure. We find that lower-cost composite procedures (e.g., G4(MP2) and G4(MP2)-6X), as well as MP2 and SCS-MP2, exhibit surprisingly poor performance for the barrier heights of reactions involving multiple proton exchanges. Moreover, the performance significantly deteriorates with increasing size of the clusters. Similar observations apply to complexation energies in water clusters, and to barrier heights for proton exchange in ammonia and hydrogen fluoride clusters. We propose a modified version of G4(MP2)-6X (denoted G4(MP2)-6X+) that includes sp- and d-diffuse functions in the CCSD(T) term, which gives excellent proton-exchange barrier heights at a computational cost only slightly greater than that of standard G4(MP2). G4(MP2)-6X+ also leads to a substantial improvement over G4(MP2) and G4(MP2)-6X for the calculation of electron affinities.