An efficient algorithm to account for nonparabolicity in carrier-carrier scattering in an ensemble Monte Carlo simulator is proposed. The major difficulty of modeling intercarrier scattering in semiconductors with nonparabolic bands arises from the two particle momentum and energy conservation laws that have to be simultaneously satisfied. A numerically efficient method is thus proposed to compute the final states accounting for a nonparabolic density of states and included in an ensemble Monte Carlo simulator. The developed algorithm makes use of rejection procedures in order to determine the correct number of scattering events as well as the distribution of the final states. This algorithm is then applied to compute constant-energy contours in various semiconductors, including narrow gap, highly nonparabolic materials, such as InAs. The model is also used to investigate the electron dynamics in an InAs quantum well heterostructure. Our results show that nonparabolicity significantly alters the transient regime since it increases the number of carrier-carrier scattering events. Finally, we investigate the validity of the parabolic approximation in the modeling of a typical GaAs resonant phonon terahertz quantum cascade laser. It is shown that, although electron-electron scattering plays a crucial role in setting the device performance of the laser, the effect of nonparabolicity in this interaction alters only slightly the population inversion while the subband temperatures are increased. ACKNOWL