Atomic and molecular interactions of aromatic compounds in carbon dioxide are studied with ab initio and molecular dynamics techniques. Ab initio calculations are used to determine the nature of the CO2-CO2, CO2-benzene, and benzene-benzene interactions. We select an explicit all-atom force field without partial atomic charges to describe the intermolecular and intramolecular pair interactions of CO2 with benzene and toluene. Molecular dynamics simulations are used to calculate diffusion coefficients for benzene and toluene at infinite dilution in CO2 along isotherms at 313.15, 323.15, and 333.15 K, in the density range from 1.35 rho(c) to 2.10 rho(c) (rho(c) = critical CO2 density). Diffusion coefficients are also calculated with a model based on perturbation theory of simple liquids. The calculated diffusion coefficients agree fairly well with the experimental results of Suarez et al. (Chem. Eng. Sci. 1993, 48, 2419).