A classical force field for the room temperature molten salt 1-ethyl-3-methylimidazolium tetrachloroaluminate has been developed and successfully tested against experimental data (neutron diffraction, diffusion constants) by molecular dynamics computer simulation corresponding to a temperature of 298 K. The force field parameters for the cation have been derived from the AMBER description for the protonated amino acid histidine, whereas the AlCl4- parameters have been achieved by parametrization of intramolecular terms with van der Waals parameters taken from the literature. All atomic partial charges have been obtained from ab initio calculations using the RESP methodology.