  We evaluate the interaction potential between a hydrogen and an antihydrogen
using the second-order perturbation theory within the framework of the
four-body system in a separable two-body basis. We find that the H-Hbar
interaction potential possesses the peculiar features of a shallow local
minimum located around interatomic separations of r ~ 6 a.u. and a barrier
rising at r~5 a.u. Additional theoretical and experimental investigations on
the nature of these peculiar features will be of great interest.
