in order to gain insight into the factors responsible for the different behavior of H2O and H2S with respect to hydrate formation, we have studied theoretically the (H2O)(21) and (H2O)(20) center dot H2S cluster systems. For each cluster, two isomers resulting from insertion of an H2O or H2S molecule into a dodecahedral (H2O)(20) Cage were examined. The calculations were performed using the density functional and RIMP2 methods. (c) 2006 Elsevier B.V. All rights reserved.