Free molecules MX, MX(2), M(2)X(2), and M2X2(s) in the solid state (M = Zn, Cd, Hg; X = F, Cl, Br, I) are studied by using the relativistic density-functional method. The crystalline environment has been simulated by a cut-off type Madelung potential of point charges at the lattice sites. Energies, geometries, force constants, vibrational frequencies, and dipole moments have been determined. The calculated molecular properties are either in good agreement with available experimental data, they suggest their reinterpretation, or they are approximate predictions of so far unknown values, All M(2)X(2) molecules, especially the Zn(2)X(2) ones, are predicted to be stable against disproportionation in the gas phase, but the equilibrium is shifted toward MX(2) (especially for M = Zn and Cd) by condensation of the metal. The ligands and the crystal field are found to have a significant influence on the properties of the compounds. The calculated enthalpies of solid M(2)X(2)(s) reveal that they are unstable against decomposition into MX(2)(s) + M(s) for M = Zn and Cd. The conclusions concerning the influence of differential aggregation energies drawn by Kaupp and von Schnering from pseudopotential calculations of the fluorides and chlorides are corroborated and extended. Relativity influences tile energies and properties of Cd and especially of Hg compounds significantly in a complex manner, due to relativity-ionicity-cross effects.