Basis set and correlation effects in the calculation of accurate gas phasedimerization energies of two M-2(+) to give M-4(2+) (M = S, Se)

The dimerization energies of two M-2(+) to give M-4(2+) (M = S, Se) were ca lculated. They depend strongly on the size of the basis set and the correla tion method used (ranging from 217 to 522 kJ/mol, M = S) and, therefore, a systematic study of basis set and correlation effects was performed [MP2, M P3, MP4(SDQ), CCSD, CCSD(T)]. The introduction of a second set of polarisin g d-functions caused a significant reduction of the dimerization energies, but neither of the above limits is reached by the MPn (n = 2, 3, 4) theory even with the largest basis sets [cc-pVQZ]. However, convergence was achiev ed by CCSD(T), compound methods or hybrid HF/DFT calculations employing fle xible basis sets [e.g., CCSD(T)/cc-pV5Z, CBS-Q or B3PW91/6-311 + G(3df)] an d revealed an average dimerization energy of 261 (199) kJ/mol for sulfur (s elenium), Delta(r)H(298) (2S(2)(+) --> S-4(2+)) is 257 kJ/mol. In the selen ium system the dependence on basis set and correlated method was less prono unced. (C) 2000 John Wiley & Sons, Inc.