  Ultracold systems offer an unprecedented level of control of interactions
between atoms. An important challenge is to achieve a similar level of control
of the interactions between photons. Towards this goal, we propose a
realization of a novel Lennard-Jones-like potential between photons coupled to
the Rydberg states via electromagnetically induced transparency (EIT). This
potential is achieved by tuning Rydberg states to a F{\"o}rster resonance with
other Rydberg states. We consider few-body problems in 1D and 2D geometries and
show the existence of self-bound clusters ("molecules") of photons. We
demonstrate that for a few-body problem, the multi-body interactions have a
significant impact on the geometry of the molecular ground state. This leads to
phenomena without counterparts in conventional systems: For example, three
photons in 2D preferentially arrange themselves in a line-configuration rather
than in an equilateral-triangle configuration. Our result opens a new avenue
for studies of many-body phenomena with strongly interacting photons.
