A REFINED POTENTIAL-ENERGY SURFACE FOR THE ELECTRONIC GROUND-STATE OFTHE WATER MOLECULE

We report here an optimization of the parameters in an analytical repr esentation of the potential energy function for the electronic ground state of the water molecule on the basis of experimental data. The cal culations are carried out with the MORBID (Morse Oscillator Rigid Bend er Internal Dynamics) computer program (P. Jensen, J. Mol. Spectrosc. 128, 478-501, 1988; J. Chem. Sec. Faraday Trans. 2 84, 1315-1340, 1988 ; in ''Methods in Computational Molecular Physics,'' S., Wilson and G. H. F. Diercksen, Eds., Plenum Press, New York, 1992). In the least-sq uares fitting, we adjusted 28 parameters (and constrained one paramete r to its ab initio value) to fit a total of 2383 rotation-vibration en ergy spacings involving rotational spacings with J less than or equal to 10 in 120 vibrational states of the 10 isotopomers (H2O)-O-16, (D2O )-O-16, (T2O)-O-16, (HDO)-O-16, (HTO)-O-16, (H2O)-O-17, (HDO)-O-17, (H 2O)-O-18, (D2O)-O-18, and (HDO)-O-18. The root-mean-square deviation o f this fitting was 0.36 cm(-1). The potential energy function obtained in the present work represents an improvement of the function determi ned previously(P. Jensen, J. Mol. Spectr osc. 133, 438-460, 1989) on t he basis of input data involving J less than or equal to 2 for six iso topomers of water. In the new fitting reported here, we obtain the equ ilibrium bond length of the water molecule as r(12)(e) = 0.957848(16) Angstrom and the equilibrium bond angle as alpha(e) = 104.5424(46)degr ees (one standard error in units of the last digit given in parenthese s). We consider this to be the most accurate equilibrium geometry curr ently available for water. (C) 1994 Academic Press, Inc.