Basis set and correlation effects in the calculation of accurate gas phase dimerization energies of two M to give M (M = S,Se)

The dimerization energies of two M to give M (M=S,Se) were calculated. They depend strongly on the size of the basis set and the correlation 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, MP3, MP4(SDQ), CCSD, CCSD(T)]. The introduction of a second set of polarising 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 achieved by CCSD(T), compound methods or hybrid HF/DFT calculations employing flexible basis sets e.g., CCSD(T)/cc‐pV5Z, CBS‐Q or B3PW91/6‐311+G(3df)] and revealed an average dimerization energy of 261 (199) kJ/mol for sulfur (selenium), Δ_rH²⁹⁸ (2S → S) is 257 kJ/mol. In the selenium system the dependence on basis set and correlated method was less pronounced. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 218–226, 2000