The two-particle coherent-potential approximation is used to calculate Frenkel-exciton group-velocity correlation functions for substitutionally disordered crystals. Most of the results can be described by a relaxation-time approximation, provided that k-dependent and complex relaxation times are allowed; however, some evidence for long-time tails, associated with the finite frequency range of the scattering potentials, is found. The probable accuracy of the approximation and its relationships with localization and kinetic theory are discussed, as is its relevance to experimental systems (triplet excitons in isotopic mixed naphthalene and anthracene crystals).Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/24699/1/0000118.pd
