We analyze the results of molecular-dynamics simulations of the interfacial (Kapitza) resistance of representative grain boundaries in Si. Simulations of the interactions of phonon wave packets with the Si grain boundaries show that the scattering process depends strongly on both the branch and wavelength of the incident phonons. This approach has the potential for providing detailed spectral information to mesoscale simulations of thermal transport in interfacial systems. (c) 2005 Springer Science + Business Media, Inc.