Double-valued potential energy surface for H2O derived from accurate ab initio data and including long-range interactions

In a recent work we have been able to model the long-range interactions within the H2O molecule. Using these long-range energy terms, a complete potential energy surface has been obtained by fitting high-quality ab initio energies to a double-valued functional form in order to describe the crossing between the two lowest-potential-energy surfaces. The two diabatic surfaces are represented using the double many-body expansion model, and the crossing term is represented using a three-body energy function. To warrant a coherent and accurate description for all the dissociation channels we have refitted the potential energy functions for the H-2((3)Sigma(u)(+)), OH((2)Pi), and OH((2)Sigma) diatomics. To represent the three-body extended Hartree-Fock nonelectrostatic energy terms, V-1, V-2, and V-12, we have chosen a polynomial on the symmetric coordinates times a range factor in a total of 148 coefficients. Although we have not used spectroscopic data in the fitting procedure, vibrational calculations, performed in this new surface using the DVR3D program suite, show a reasonable agreement with experimental data. We have also done a preliminary quasiclassical trajectory study (300 K). Our rate constant for the reaction O(D-1)+H-2((1)Sigma(g)(+))-->OH((2)Pi)+H(S-2), k(300 K)=(0.999+/-0.024)x10(-10) cm(3) molecule(-1) s(-1), is very close to the most recent recommended value. This kinetic result reinforces the importance of the inclusion of the long-range forces when building potential energy surfaces. (C) 2003 American Institute of Physics.