The determination of an accurate isotope dependent potential energy surface for water from extensive ab initio calculations and experimental data

Abstract

We report on the determination of a high quality ab initio potential energy surface (PES) and dipole moment function for water. This PES is empirically adjusted to improve the agreement between the computed line positions and those from the HITRAN 92 data base with J⩽5 for H216O. The changes in the PES are small, nonetheless including an estimate of core (oxygen 1s) electron correlation greatly improves the agreement with the experiment. Using this adjusted PES, we can match 30 092 of the 30 117 transitions in the HITRAN 96 data base for H216O 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. Nonadiabatic effects are not explicitly included. About 3% of the tabulated line positions appear to be incorrect. Similar agreement using this adjusted PES is obtained for the 17O and 18O isotopes. For HD16O, the agreement is not as good, with a root-mean-square error of 0.25 cm−1 for lines with J⩽5. This error is reduced to 0.02 cm−1 by including a small asymmetric correction to the PES, which is parameterized by simultaneously fitting to HD16O and D216O data. Scaling this correction by mass factors yields good results for T2O and HTO. The intensities summed over vibrational bands are usually in good agreement between the calculations and the tabulated results, but individual line strengths can differ greatly. A high-temperature list consisting of 307 721 352 lines is generated for H216O using our PES and dipole moment function.