The fluid-beta and beta-delta coexistence lines for a state of the art intermolecular nitrogen potential [R. D. Etters et nl., Phys. Rev. B 33, 8615 (1986)] are located by absolute free energy calculations using Monte Carlo simulations. We have found that the Etters potential provides an accurate description of the fluid-beta coexistence line, but significantly overestimates the pressures of the beta-delta coexistence line. This suggest that for a more accurate description of the delta phase the Etters potential needs to be improved. The present results constitute, to our knowledge, the first numerical calculation of full coexistence lines of a realistic molecular solid.