An analytical fit to an accurate ab initio (¹A₁) potential surface of H₂O

The accurate ab initio MBPT quartic force field of Bartlett, Shavitt and Purvis has been fit to an analytical function using a method developed by Sorbie and Murrell (SM). An analysis of this surface indicates that it describes most properties of the H₂O molecule very accurately, including an exact fit to the MBPT force field, and very close to the correct energy difference between linear and equilibrium H₂O. The surface also reproduces the correct diatomic potentials in all dissociative regions, but some aspects of it in the "near asymptotic" O(¹D) + H₂ region are not quantitatively described. For example, the potential seems to be too attractive at long range for O + H₂ encounters, although it does have the correct minimum energy path geometry and correctly exhibits no barrier to O atom insertion. Comparisons of this surface with one previously developed by SM indicates generally good agreement between the two, especially after some of the SM parameters were corrected, using a numerical differentiation algorithm to evaluate them. A surface developed by Schinke and Lester (SL) is more realistic than outs in the O(¹D) + H₂ regions, but less quantitative in its description of the H₂O molecule. Overall, the present fit appears to be both realistic and quantitative for energy displacements up to 3-4; eV from H₂O equilibrium, and should therefore be useful for spectroscopic and collision dynamics studies involving H₂O.