Pergamon J. Aemsol Sci. Vol. 29. Suppl. 1. pp. S537-S538. 1998 8 1998 Published by Else&r Science Ltd. All tights resetwd Rimed in Great Britain 0021-8502/98 $19.OO+O.M) Al3 INITIO STUDY OF SMALL SULPHURIC ACID - WATER CLUSTERS HANNA ARSTILA, KARI LAASONEN’, ARI LAAKSONENt University College London Department of Physics and Astronomy Gower Street, London WClE 6BT, U.K. + Department of Chemistry, University of Oulu, PO. Box 333,90571 Oulu, Finland + Dept. of Applied Physics University of Kuopio P.O. Box 1627 70211 Kuopio, Finland KEYWORDS HzS04, HaO, molecular clusters, ab initio, nucleation Sulphuric acid tends to form hydrates, small cluster containing a few water molecules. Hydrate formation affects the predicted nucleation rate as the vapour pressures are lowered compared to unhydrated vapour. In classical nucleation theories the formation energies and the size distribu- tion of hydrates are predicted assuming that hydrates can be described as liquid droplets having properties of bulk liquid. To obtain a better understanding of the structures and formation ener- gies of the smallest clusters, we have performed ab initio density functional calculations of the mono- di- and trihydrates. We have used Car-Parrinello molecular dynamics method based on density functional theory (Car and Parrinello (1985)). The effect of nuclei plus their core electrons to valence electrons are described using pseudopotentials. The valence electronic wavefunctions are expressed with plane waves, and we are using periodic boundary conditions. The regularly used local density approximation has turned out not be accurate enough to describe hydrogen bonding, and thus we are using gradient corrected local density approximation, which seems to work very well (Arstila et al (1998)). Figure 1: Optimised geometries of mono- (a), di- (b) and trihydrate (c). The hydrogen bonds between sulphuric acid molecule or hydrogen sulphate ion and water are generally strong. The binding energies are much larger and the oxygen-oxygen distances are s537