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 wat er many-body model TCPE have been applied to the study of four water t etramers corresponding to various molecular arrangements. For the cycl ic tetramer (where each molecule is simultaneously donor and acceptor of hydrogen bonds, HBs), cooperative effects have been shown from ab i nitio computations to be stabilizing and to represent a contribution i n the binding energy of 9 kcal mol(-1), while for the tetramer where o nly two molecules are simultaneously donor and acceptor of HE, such ef fects are stabilizing by only 1.5 kcal mol(-1). At last, for the tetra mer where no molecule is simultaneously donor and acceptor of HBs, coo perative effects are smoothly destabilizing. TCPE predictions have bee n shown to be in good agreement with these ab initio estimates, both i n terms of binding energy and cooperative effect contribution, which e xhibits the accuracy of this potential. (C) Academie des sciences/Else vier, Paris.