Of particular interest are the cyclic clusters (H2O)n; these have been predicted to exist for n = 3 to 60. With increasing cluster size the oxygen to oxygen distance is found to decrease which is attributed to so-called cooperative many-body interactions: due to a change in charge distribution the H-acceptor molecule becomes a better H-donor molecule with each expansion of the water assembly. Many isomeric forms seem to exist for the hexamer (H2O)6: from ring, book, bag, cage, to prism shape with nearly identical energy. Two cage-like isomers exist for heptamers (H2O)7, and octamers (H2O)8 are found either cyclic or in the shape of a cube.