Benchmark enthalpies of formation and binding energies of proton-bound pairs between HCN and HCN, NH3, H2O, and HF

The relative thermochemical properties of cluster ions (solvation enthalpie s, entropies, and free energies) can be obtained from experimental techniqu es such as high pressure mass spectrometry and selected-ion flow tube mass spectrometry. Theory can play an important role in these studies by providi ng both accurate binding energies of the smaller members of the cluster fam ilies and insight into the structure and bonding in the cluster ions. This study assesses the performance of a variety of levels of ab initio and dens ity functional theories for predicting the structures and energies of one f amily of cluster ions, the proton-bound dimers between HCN and HCN, NH3, H2 O, and HF. The theoretical procedures were assessed based on their performa nce relative to high-level treatments such as QCISD(T) correlation, the 6-3 11 + G(2df,p) basis set, and G2 energy calculations. The results of the ass essment indicate that MP2/6-31G(d) optimized geometries are sufficient for the calculation of binding energies and heats of formation with advanced me thods such as G2. Further increases in basis set size and electron correlat ion improve the geometries of the dimers, but these geometric changes have little impact on the final high-level energy calculations. The heats of for mation and binding energies of the clusters are best described by G2 theory , but modified versions of G2 such as G2(MP2) and G2(MP2, SVP) also provide reliable values. Calculated binding energies of these four proton- bound d imers are compared to available experimental values from the literature, an d the effect of basis set superposition error is examined. (C) 1999 America n Institute of Physics. [S0021-9606(99)01916-9].