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

This work reports refinements of the energetic ordering of the known low-energy structures of sulphate-water clusters SO42-(H2O)(n) (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 Moller-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.