Alkaline earth metal halide radical dimers, (MX)(2) (M = Be, Mg, Ca and X = F, Cl) have not yet been observed experimentally and their existence could be doubted since MX is known to disproportionate in the bulk. We study the (MX)2 species using DFT and MP2 methods and show that they are either thermodynamically or kinetically stable with respect to various reactions including disproportionation. Energetics, geometries, and vibrational frequencies of sing let and triplet equilibrium structures and some transition states are calculated. The ground states of (MX)(2) are all singlet except (CaF)(2) which is triplet and, with the possible exception of(CaF)(2), the ground states have a rhombic structure in which the X atoms are shared equally with each M atom. The ionization potentials of the (MX)(2) species are significantly lower than those of the corresponding MX species suggesting that their chemistry may be significantly different. (C) 2001 American Institute of Physics.