A classical reactive potential for molecular clusters of sulphuric acid and water

Abstract

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.

Free energy of formation of clusters of sulphuric acid and water molecules determined by guided disassembly

“…The comparison between our results and those of Henschel et al [13] is very reasonable and this study will pave the way for further investigations based on improved force fields [36] and a wider variety of molecular species [30].…”

“…It is likely that atmospheric particle nucleation proceeds with the participation of additional molecular species, so we do not expect to find that our model reproduces such events. Furthermore, it is quite possible that the microscopic interactions used in this study are in need of improvement: an extended model that permits proton transfer has recently been developed and could be used to rectify some of its deficiencies [36]. A similar situation was encountered in the earlier demonstration of the cluster disassembly procedure for a single species, argon [23].…”

“…The force field employed is simplified, and in particular does not allow proton transfers, but more elaborate models have recently been developed and could be employed in further studies [36]. We give particular attention to efficiencies available through a suitable performance of the manipulation.…”

“…The extracted thermodynamic information can be used as the basis of a kinetic theory of binary nucleation [15,38,39] but our main objective is to compare the results with a recent thermodynamic analysis based on optimised cluster configurations and harmonic thermal fluctuations obtained from quantum chemistry [13]. We expect differences in outcomes since the force field we use is a classical representation of quantum mechanical interactions and might lack detail such as a description of molecular dissociation (though this can be remedied [36]), but equally, the harmonic approach might not capture the correct entropic contributions to the cluster free energy since it is fundamentally based on a solid-like conception of each structure. We compare the approaches by computing the equilibrium populations of clusters.…”

“…We expect differences in outcomes since the force field we use is a classical representation of quantum mechanical interactions and might lack detail such as a description of molecular dissociation (though this can be remedied [36]), but equally, the harmonic approach might not capture the correct entropic contributions to the cluster free energy since it is fundamentally based on a solid-like conception of each structure. We compare the approaches by computing the equilibrium populations of clusters.…”