Refined energetic ordering for sulphate–water (n = 3–6) clusters using high-level electronic structure calculations

Abstract

This work reports refinements of the energetic ordering of the known low-energy structures of sulphate–water clusters (n = 3–6) using high-level electronic structure methods. Coupled cluster singles and doubles with perturbative triples (CCSD(T)) is used in combination with an estimate of basis set effects up to the complete basis set limit using second-order Møller–Plesset theory. Harmonic zero-point energy (ZPE), included at the B3LYP/6-311 + + G(3df,3pd) level, was found to have a significant effect on the energetic ordering. In fact, we show that the energetic ordering is a result of a delicate balance between the electronic and vibrational energies. Limitations of the ZPE calculations, both due to electronic structure errors, and use of the harmonic approximation, probably constitute the largest remaining errors. Due to the often small energy differences between cluster isomers, and the significant role of ZPE, deuteration can alter the relative energies of low-lying structures, and, when it is applied in conjunction with calculated harmonic ZPEs, even alters the global minimum for n = 5. Experiments on deuterated clusters, as well as more sophisticated vibrational calculations, may therefore be quite interesting.

Keywords:

• Sulphate-water clusters
• microsolvation
• benchmarks
• electronic and vibrational effects

Acknowledgements

This work was supported in part by the U.S. Department of Energy under Contract No. DE-AC02-05CH11231, and by supercomputing resources provided by NERSC and in EMSL under the ‘emsl43893’ User proposal. This work was partly supported by the US Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences. Pacific Northwest National Laboratory (PNNL) is a multiprogram national laboratory operated for DOE by Battelle. It is a pleasure to dedicate this paper to Peter Taylor, despite its remaining limitations in accuracy! We thank Narbe Mardirossian for carefully checking the data section.