An irreducible four-point vertex composed of the most-crossed diagrams involved in Bethe-Salpeter equation for an averaged two-photon propagator in a system of metal nanoparticles was calculated directly in coordinate representation without the traditional introduction of an EM-energy diffusion coefficient. These diagrams describe the interference of amplitudes corresponding to two possible ways for a photon to pass the closed loop on its trajectory. Localization is interpreted as a bound state generation of two virtual photons passing the loop clockwise and counterclockwise. This bound state is described by a pole of the irreducible vertex. The influence of this interference correction on light scattering and effective absorption by a random system of particles is investigated.