The McLone-Power retarded potentials for coupled molecule pairs are applied to study guest-host interactions affecting the spectra of mixed crystals. The results are compared with those earlier found with static interaction. In the deep trap limit a new feature is that over a certain range of crystal size the excitation energy of the guest molecule diverges linearly with sample dimensions, before being cut off exponentially at extreme range. Numerical values indicate that an experimental test is feasible. In the shallow trap limit of trap depth small compared with host exciton bandwidth, retardation changes mixed crystal properties through its influence on the levels of the parent pure host crystal, calculable in the first non-vanishing order of perturbation theory. Numerical values in this limit are calculated for host anthracene. The effect of retardation is shown to be small. The passage through increasing trap depths to the deep trap limit cannot be made in a manner maintaining close parallelism with the static interaction method. The new features are described.