Cooperative effects in water tetramers. Comparison between the empirical many-body model TCPE and ab initio calculations

Ab initio calculations at the MP2/6-311+G(2d,2p) and the empirical water many-body model TCPE have been applied to the study of four water tetramers corresponding to various molecular arrangements. For the cyclic tetramer (where each molecule is simultaneously donor and acceptor of hydrogen bonds, HBs), cooperative effects have been shown from ab initio computations to be stabilizing and to represent a contribution in the binding energy of 9 kcal mol(-1), while for the tetramer where only two molecules are simultaneously donor and acceptor of HE, such effects are stabilizing by only 1.5 kcal mol(-1). At last, for the tetramer where no molecule is simultaneously donor and acceptor of HBs, cooperative effects are smoothly destabilizing. TCPE predictions have been shown to be in good agreement with these ab initio estimates, both in terms of binding energy and cooperative effect contribution, which exhibits the accuracy of this potential. (C) Academie des sciences/Elsevier, Paris.