Recent our applications of the ab initio density functional calculations to the structures and energies of octahedral [M(H₂O)₆]²⁺ and [MX₆]^4- complexes (M=Mg, Ca, Cr, Mn, Fe, Co, Ni, Cu, Zn, and Cd; X = Cl and Br) are reviewed. The criterion for formations  of  the structures of double salt crystals, 2MCl₂ · MCl₂ · 12H₂O, which constituted of two kinds of isolated polyhedra such as tachyhydrite, 2[Mg(H₂O)₆] · [CaCl₆], are discussed on the basis of the non-empirical formation energies of the polyhedra.