A refined set of potential parameters for I-alkyl-3-methylimidazolium based room temperature ionic liquids with anions such as acetate, dicyanamide, and thiocyanate has been obtained. Site charges of ions were derived through the density-derived electrostatic and charge partitioning (DDEC/c3) method utilizing periodic density functional theory calculations of these liquids. Intermolecular structure and dynamics, in particular, the collective quantities predicted by the refined force field, match experimental results quantitatively.