Geometries, vibrational frequencies and stability of Hg₂(OH)₂ as well as Hg₂F₂ and Hg₂Cl₂ have been investigated by means of ab initio HF, MP2 and density functional theory methods with LanL2DZ, SDD basis sets. The 5s, 5p semi-core of Hg has significant contribution to Hg—Hg bonding. HF method is not suitable for the [Hg—Hg]²⁺ system due to the strong correlation effects. The non-local exchange correction of Becke functional gives too long Hg—Hg distances. Local spin density functional theory and MP2 may reproduce the spectroscopic constants reasonably. Relativistic effects reduce the Hg—Hg bond lengths by about 24 pm and stabilize the Hg—Hg bond significantly by about 20%. Although Hg₂L₂ (L=OH, F, Cl) in disproportionation reaction Hg₂L₂→HgL+Hg is stable in the gas phase, relativistic effects reduce the tendency. The stability of Hg₂(OH)₂ is comparable to that of mercurous halides.

Key words: mercury(I)ion, ab initio, density functional theory, relativistic effect