Sensitivity analyses have shown that the gravitational collision efficiency influences post hypothetical core disruptive accident aerosol behavior in liquid-metal fast breeder reactor (LMFBR) containment in important ways. Our research was directed toward improving expressions for this quantity. Following the work of atmospheric sciences, dynamical equations for two particle motions were developed. A computer program GCEFF was constructed, options for using a variety of drag forces were provided, and the dynamical equations were solved by using Gear’s method. Results were compared with the previous work of atmospheric sciences, and explicit results for several cases of interest in the LMFBR studies were provided. It was concluded that the particle density plays an important role in determining the collisional efficiency, and the present results were substantially different from the results provided by the model currently being used in the aerosol behavior codes. Finally, for the collisional efficiency, a computer program that can be conveniently used in the CRAB computer program (U.S. Nuclear Regulatory Commission, Battelle-Columbus, under development) or some other similar program was described.