Ab initio molecular dynamics (AlMD) and classical molecular dynamics (MD) simulations of the room temperature ionic liquid (RTIL), 1,3-dimethylimidazolium chloride, were performed in order to study its intermolecular structure and dynamics. Differences in the spatial distribution of chloride ions around the cation between AlMD and MD data are explained as due to the formation of a hydrogen bond between the acidic hydrogen on the imidazolium ring and the chloride ion. A strong interaction between the TE electron clouds of neighboring imidazolium rings enables the ring planes to be aligned nearly parallel to each other. The cation-anion hydrogen bond present in the melt is observed as a red shift in the C-H stretching frequency. (c) 2005 Elsevier B.V. All rights reserved.