Cohesive properties of small mercury clusters are analyzed for electronic ground and excited states on the basis of the diatomics-in-molecules method. The theory takes into account ab initio diatomic potential energy curves and configuration mixing of the ground and excited (1)Sigma(g) states. Increment of binding energies due to s-p mixing is thus predicted in the ground state. Optimized structures of the clusters in the lowest excited state exhibit peculiar D-4h and C-4v symmetries with significantly small nearest-neighbor distance. As the cluster size increases, the bottom of the excited energy level is lowered rapidly. The calculated potential energy surfaces exhibit strong dependence of excitation energy gap on interatomic separation. (c) 2006 Elsevier B.V. All rights reserved.