A Monte Carlo simulation study is performed on the ionic liquid 1-n-butyl-3-methylimidazolium hexafluorophosphate. A modified united atom forcefield is developed for the ionic liquid. Volumetric properties such as density, isothermal compressibility and volume expansivity are computed at temperatures ranging from 298 to 343 K. Henry's constants and infinite dilution enthalpy and entropy of absorption are computed for CO2 in the ionic liquid. In addition, local structural information is obtained from radial distribution function analysis. Comparison is made between calculated properties and available experimental data. The performance of the new forcefield is assessed by comparison with two other forcefields developed for this compound. (C) 2004 Elsevier B.V. All rights reserved.