AN AB-INITIO DERIVED TORSIONAL POTENTIAL-ENERGY SURFACE FOR THE CYCLIC WATER TETRAMER

A four-dimensional intermolecular torsional potential energy surface w as calculated for the cyclic homodromic water tetramer at the level of second-order Moller-Plesset perturbation theory, using a large basis set (82 basis functions per monomer). The four torsional angles omega( i) (i=1-4), which describe the positions of the ''free'' O-H bonds rel ative to the planar hydrogen-bonded O ... O ... O ... O framework were varied, while all other internal coordinates were held fixed. Ab init io calculations were performed at 307 points in the asymmetric unit of the torsional space. The points were fitted using the same seven-para meter analytical potential function as previously employed for the tor sional surface of the water trimer [Burgi ed al., J. Chem. Phys. 103; 1077 (1995)]. Fits were performed for four interaction energy ranges s panning 1100, 1500, 3500 and 11 500 cm(-1) relative to the global mini mum; respectively. A number of important stationary points were studie d in greater detail and geometry optimized using larger basis sets at the MP2 level. Torsional interconversion pathways are mapped in terms of isopotential surface representations and contour plots. This accura te and rapidly computable potential can be employed to calculate the c oupled anharmonic torsional level energies, wave functions and associa ted molecular properties, presented in the following paper [D. Sabo et al., J. Chem. Phys. 109, 5404 (1998)]. (C) 1998 American Institute of Physics. [S0021-9606(98)30236-6].