Ab initio calculations have been carried out to study the water clusters of undecamer [(H2O)(11)] and dodecamer [(H2O)(12)]. At the level of Moller-Plesset second-order perturbation theory using TZ2P++ basis set, the lowest-energy conformer of the undecamer has the skeletal structure of Prism56 that a cyclic pentamer and a cyclic hexamer are fused into a prism shape with 16 hydrogen bonds (HBs). In this case, there are quite a number of nearly isoenergetic conformers with different hydrogen orientations. Among these, more stable conformers tend to have dangling H atoms separated (i.e., less clustered). The lowest energy conformer of the undecamer is different in hydrogen orientation from any previously suggested structure (including the ones obtained from various minimization algorithms). A second lowest energy skeletal structure is of Prism56(B) that a cyclic pentamer and an open-book hexamer are fused into a prism shape with 17 HBs. The most stable dodecamer is a fused cubic or tetragonal prism skeletal structure (Prism444) with 20 HBs. The lowest energy structure among these skeletal conformers has HB orientations with opposite helicities between adjacent tetragonal rings. The second lowest-energy skeletal structure is the hexagonal prism structure (Prism66) with 18 HBs fused by two cyclic water hexamers. The OH stretching vibrational spectra and electronic properties of several low energy conformers of (H2O)(11,12) are also reported.