Calculating intermolecular potentials with SIMPER: the water–nitrogen and water–oxygen interactions, dispersion energy coefficients, and preliminary results for larger molecules

Abstract

Recent theoretical studies of the van der Waals complexes formed between water and the two main components of air, nitrogen and oxygen, are reviewed. Combined with previous work on the water–argon complex, the results allow non-ideal thermodynamic properties of water–air mixtures to be calculated. The intermolecular potential energy surfaces for these complexes have been calculated using a combination of supermolecule methods and perturbation theory, as described in a previous review. Theoretical techniques introduced since this previous work include the extension of the intermolecular perturbation theory to open-shell electronic species, as required for the water–oxygen complex, and new and more accurate calculations of dispersion energy coefficients. Spin-unrestricted, time-dependent coupled cluster theory has been used for calculating dispersion energy coefficients for the water–oxygen complex, and this method is described and compared with other accurate methods, and a possible alternative method is suggested for future work. The results obtained for the water–nitrogen complex are highly satisfactory. With little more computational effort than is required to produce a second-order Moller–Plesset supermolecule potential energy surface, the intermolecular potential is calculated to an accuracy which appears to be comparable to coupled-cluster calculations with perturbative triple excitations. For the water–oxygen complex, the different theoretical methods produce potential energy surfaces with larger discrepancies than for water–nitrogen, although they predict the main features of the potential energy surface better than calculations in the literature. Although few experimental measurements of water–oxygen virial coefficients are available, the agreement of theoretical predictions with these is reasonable, and the agreement with the better characterized water–air virial coefficients is very good. The review concludes with a forward look to work on larger molecules. Increasing the size of the interacting molecules creates a number of practical problems. Some problems, including the steep scaling of computation time with system size, are common to all methods. The use in the current work of a damped multipole expansion about the molecular centres also causes problems when larger molecules are considered. The review therefore considers methods that can be used to reduce the unfavourable size scaling, to reduce the size of the basis set, and to use damped atomic multipole expansions, which are centred on the nuclei of the interacting molecules.

Acknowledgements

The authors gratefully acknowledge funding from EPSRC, the Leverhulme Trust, and the Royal Society of Chemistry, and valuable discussions with A. H. Harvey, A. J. Stone and K. Szalewicz.