THE POTENTIAL-ENERGY SURFACE OF (H2O)-O-16

We report here a new determination of the H-2 O-16 potential energy su rface from experimental data. The calculations have been carried out b y means of the very accurate and highly efficient method proposed and applied to H-2 O-16 in a previous paper [Polyansky, Jensen, and Tennys on, J. Chem. Phys. 101, 7651 (1993)]. This previous work has been sign ificantly improved by inclusion of additional terms in the analytical expression used to represent the potential energy surface. Previously, 1600 rotation-vibration term values for H-2 O-16 were fitted with a s tandard deviation of 0.36 cm(-1). With the extended model of the prese nt work, this standard deviation could be improved to 0.25 cm(-1). Wit h the extended model and the new fitted potential function we have cal culated a data set comprising 3200 term values, all of which can be co mpared with experimentally derived values. The standard deviation for this data set is 0.6 cm(-1). The data set contains rotationally excite d energy levels for all the 63 vibrational states which have been char acterized by high resolution spectroscopy. The potential energy functi on obtained in the present work improves drastically the agreement wit h experiment for the highly excited local mode stretching states above 20 000 cm(-1). For the vibrational band origins of these states, the highest of which is measured at 25 118 cm(-1), our previous fitted pot ential produced discrepancies of more than 100 cm(-1). These deviation s are reduced to less than 1 cm(-1) by the potential energy function o f the present work. We show that no significant improvement of the fit can be obtained by extending the analytical expression for the potent ial energy by further high-order terms. An analysis of the residuals s hows that at the level of accuracy achieved, the major contribution to the error originates in the neglect of nonadiabatic correction terms in the Born-Oppenheimer kinetic energy operator. We conclude that any further improvement of the potential energy surface requires that such correction terms be included in the Hamiltonian. With the present pot ential, reliable extrapolations towards higher rotational and vibratio nal energies can be carried out, and we expect that such calculations can be very helpful in the assignment of experimental spectra involvin g highly excited states. (C) 1996 American Institute of Physics.