  We compute the interaction energies of a two-atom system placed in the middle
of a perfectly reflecting planar cavity, in the perturbative regime. Explicit
expressions are provided for the van der Waals potentials of two polarisable
atomic dipoles as well as for the electrostatic potential of two induced
dipoles. For the van der Waals potentials, several scenarios are considered,
namely, a pair of atoms in their ground states, a pair of atoms both excited,
and a pair of dissimilar atoms with one of them excited. In addition, the
corresponding phase-shift of the two-atom wavefunction is calculated in each
case. The effects of the two-dimensional confinement of the electromagnetic
field by the cavity are analyzed in each scenario.
