  Solid mercury in the rhombohedral structure is unbound within the
self-consistent field (Hartree-Fock) approximation. The metallic binding is
entirely due to electronic correlations. We determine the cohesive energy of
solid mercury within an ab-initio many-body expansion for the correlation part.
Electronic correlations in the $5d$ shell contribute about half to the cohesive
energy. Relativistic effects are found to be very important. Very good
agreement with the experimental value is obtained.
