The dimerization energies of 2M 2 + to give M 4 2+ (M ) S, Se) were calculated as input into thermodynamic Born-Fajans-Haber cycle calculations to determine the relative stabilities of salts of these mono-and dications in the solid state. Computed dimerization energies showed a strong dependence on the basis set and correlated method utilized. Coupled cluster calculations, compound methods or hybrid HF/DFT methods employing large basis sets [CCSD(T)/cc-pV5Z, CBS-Q or B3PW91/6-311+G(3df)//B3PW91/6-311+G*] had to be used and showed an average dimerization energy of 258 (199) kJ/mol for sulfur (selenium). Square planar M 4 2+ (M ) S, Se) was fully optimized (B3LYP, B3PW91), and the calculated vibrational spectra of M 4 2+ were then compared to averaged experimental data to derive scaling factors. The structure, bonding, and energetics of seven starting geometries of the M 4 2+ (M ) S, Se) dication were computed (B3PW91), as well as AIM and NBO analyses of these species. The global minimum of the examined sulfur (selenium) species is the planar, 6π-aromatic D 4h symmetric square, which is 76 (106) and 155 (115) kJ/mol more stable than a D 2h symmetric π*-π*-bonded rectangular (S 2 + ) 2 [(Se 2 + ) 2 ] dimer and a classical, σ-bonded, butterfly-shaped isomer, respectively. This supports the thesis that the observed geometries of the homopolyatomic cations of groups 16 and 17 and related species maximize positive charge delocalization, resulting in thermodynamically stable np π -np π (n g 3) and π*-π* bonds. The formation of chain-like (Te 4 2+ ) n , polymeric Te 8 4+ , and square planar Te 4 2+ is accounted for semiquantitatively. The published, experimental enthalpy of formation of gasous S 4 + (1131 kJ/mol) was computationally shown to be due to a fragmentation of S 6 to give S 4 + and S 2 , confirming earlier photoionization studies. An enthalpy of formation of 972 kJ/mol was then established for the gaseous S 4 + cation, 159 kJ/mol lower than the erroneously assigned published experimental value.