  We present a two-state empirical valence bond (EVB) potential describing
interactions between sulphuric acid and water molecules and designed to model
proton transfer between them within a classical dynamical framework. The
potential has been developed in order to study the properties of molecular
clusters of these species, which are thought to be relevant to atmospheric
aerosol nucleation. The particle swarm optimisation method has been used to fit
the parameters of the EVB model to density functional theory (DFT)
calculations. Features of the parametrised model and DFT data are compared and
found to be in satisfactory agreement. In particular, it is found that a single
sulphuric acid molecule will donate a proton when clustered with four water
molecules at 300 K and that this threshold is temperature dependent.
