PNNL

Abstract

Effective atom-atom potentials are developed for binary sulfuric acid - water clusters and applied in a Bennett Metropolis Monte Carlo calculation to determine free energy differences for small neighboring sized clusters of fixed composition at 298 K. The atom-atom pair potentials consist of Lennard-Jones short-range and Coulombic long range terms and assume rigid $SO_{4}^{2\delta -}$ with two unconstrained $H^{\delta +}$ and rigid $H_{2}O$ molecules interacting via revised central force ({\sc RSL2}) potentials. The potential parameters are determined from both {\it ab initio} studies and tests of the potential using the statistical mechanical formalism for binary cluster size distributions. In the potential tests fixed composition free energy differences, delta f, for $[H_{2}O]_{km}[H_{2}SO_{4}]_{m}$ clusters are plotted versus $(km+m)^{-1/3}$ and the resulting slope and intercept (in the large cluster regime) are used to extract model dependent binary liquid surface tension and partial vapor pressures at $298K$. The potential parameters are adjusted to obtain approximate agreement with experimental surface tension and partial vapor pressures for $k=1$ and $4$ (57\% and 84\% weight percent $H_{2}SO_{4}$, respectively). The free energy differences for $m\leq 15$ are presented, together with internal cluster energy contributions, snapshots of cluster structure and evidence for onset of the large cluster regime near $m=5$. The long range goals have been to test the free energy difference procedure for studying binary cluster properties and to develop model potentials appropriate for the simulation of small binary clusters at low temperatures characteristic of stratospheric sulfuric acid - water aerosols.