DMSO No! Pushing It to the CCSD(T) CBS(Q56) Limit

This work serves as a benchmark study on the structures and energetics of the anticlinal geometries of X-0-0-Y, X-O-S-Y, and X-S-S-Y (where X,Y = Hor Me) as well as the corresponding pyramidal isomers including X-010)-Y, X-S(O)-Y, X-O(S)-Y, and X-S(S)-Y (where X,Y = H or Me). Geometries were fully characterized with post-SCF ab initio and 22 density functional methods in conjunction with 14 different correlation consistent basis sets of double-, triple-, or quadruple-zeta quality. The relative energetics of these compounds containing oxygen and/or sulfur were determined at the CCSD(T) complete basis set (CBS) limit using the augmented standard, tight d, and core valence correlation consistent basis sets as large as a6Z and significant basis set effects are analyzed. The relative energies of oxygen/sulfur containing compounds exhibit exceptionally slow convergence to the CBS limit with canonical methods. Small basis set can give rise to a qualitatively incorrect AE.