How a bar potential can force a spiral structure in the gas component is now well understood. This response is due to the dissipative character of the gas. Results of the computations of the response differ greatly with the numerical code adopted, either particle (Schwarz 1981), or hydrodynamic (e.g. Sanders and Huntley, 1976) and within a given code, with the spatial resolution used, since the artificial viscosity is thus varied (see e.g. van Albada et al 1981). According to observations, the gas component is mostly cloudy. Hence our aim is to compute the response to a bar potential of the ensemble of molecular clouds for which collision rate (and therefore dissipation rate) is relatively well-known. Using a particle code and explicitly treating the collisions between clouds, the viscosity parameter is more easily controlled and less artificial. Also, since the mass transfer between clouds is taken into account, we will obtain insight into the formation of large molecular clouds, which are the preferential sites of active star formation.