Monte Carlo simulations of small sulfuric acid-water clusters

Effective atom-atom potentials are developed for binary water-sulfuric acid clusters and used 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 for {dollar}Hsb2SOsb4-Hsb2O{dollar} consist of Lennard-Jones short-range and Coulombic long range terms and assume rigid {dollar}SOsbsp{lcub}4{rcub}{lcub}-2{rcub}{dollar} and {dollar}Hsb2O{dollar} structures with two unconstrained {dollar}Hsp+{dollar} ions free to bond with either species. Potential parameters are determined in part from small cluster quantum chemistry studies using GAMESS and in part from tests of the potentials using the revised central force (RSL2) potentials for {dollar}Hsb2O-Hsb2O{dollar} and a statistical mechanical formalism developed for binary cluster size distributions, {dollar}Nsb{lcub}km,m{rcub}.{dollar} In the potential tests, fixed composition free energy differences, {dollar}Csb{lcub}km,m{rcub},{dollar} for (km, m) clusters containing {dollar}m Hsb2SOsb4{dollar} molecules and {dollar}km Hsb2O{dollar} molecules are plotted versus {dollar}msp{lcub}-1/3{rcub}{dollar} and the slope and the intercept are used to extract model dependent bulk binary solution surface tension, {dollar}sigma(k,T),{dollar} and partial vapor pressures at 298 K. Potential parameters are adjusted to obtain approximate agreement with experiment for {dollar}k=1{dollar} and {dollar}k=4{dollar} (57% and 84% weight {dollar}Hsb2SOsb4,{dollar} respectively). Results of the free energy differences for clusters containing up to 10 acid molecules and 40 water molecules are presented together with cluster binding energies, snapshots of cluster structure and a discussion of simulation methods and binary cluster properties when {dollar}mle5.{dollar} The long range goals of this work have been to test the free energy difference procedure for studying binary clusters and to apply the model potentials to small binary clusters at low temperatures characteristic of stratospheric sulfuric-acid.