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

We report here an optimization of the parameters in an analytical representation of the potential energy function for the electronic ground state of the water molecule on the basis of experimental data. The calculations are carried out with the MORBID (Morse Oscillator Rigid Bender 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-squares fitting, we adjusted 28 parameters (and constrained one parameter to its ab initio value) to fit a total of 2383 rotation-vibration energy 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, (H2O)-O-18, (D2O)-O-18, and (HDO)-O-18. The root-mean-square deviation of this fitting was 0.36 cm(-1). The potential energy function obtained in the present work represents an improvement of the function determined previously(P. Jensen, J. Mol. Spectr osc. 133, 438-460, 1989) on the basis of input data involving J less than or equal to 2 for six isotopomers of water. In the new fitting reported here, we obtain the equilibrium 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)degrees (one standard error in units of the last digit given in parentheses). We consider this to be the most accurate equilibrium geometry currently available for water. (C) 1994 Academic Press, Inc.