THE DETERMINATION OF AN ACCURATE ISOTOPE DEPENDENT POTENTIAL-ENERGY SURFACE FOR WATER FROM EXTENSIVE AB-INITIO CALCULATIONS AND EXPERIMENTAL-DATA

We report on the determination of a high quality ab initio potential e nergy surface (PES) and dipole moment function for water. This PES is empirically adjusted to improve the agreement between the computed lin e positions and those from the HITRAN 92 data base with J less than or equal to 5 for (H2O)-O-16. The changes in the PES are small, nonethel ess including an estimate of core (oxygen 1s) electron correlation gre atly improves the agreement with the experiment. Using this adjusted P ES, we can match 30 092 of the 30 117 transitions in the HITRAN 96 dat a base for (H2O)-O-16 with theoretical lines. The 10, 25, 50, 75, and 90 percentiles of the difference between the calculated and tabulated line positions are -0.11, -0.04, -0.01, 0.02, and 0.07 cm(-1). Nonadia batic effects are not explicitly included. About 3% of the tabulated l ine positions appear to be incorrect. Similar agreement using this adj usted PES is obtained for the O-17 and O-18 isotopes. For (HDO)-O-16, the agreement is not as good, with a root-mean-square error of 0.25 cm (-1) for lines with J less than or equal to 5. This error is reduced t o 0.02 cm(-1) by including a small asymmetric correction to the PES, w hich is parameterized by simultaneously fitting to (HDO)-O-16 and (D2O )-O-16 data. Scaling this correction by mass factors yields good resul ts for T2O and HTO. The intensities summed over vibrational bands are usually in good agreement between the calculations and the tabulated r esults, but individual line strengths can differ greatly. A high-tempe rature list consisting of 307 721 352 lines is generated for (H2O)-O-1 6 using our PES and dipole moment function. (C) 1997 American Institut e of Physics.