Phys. Chem. Chem. Phys., 1999, 1 (23), 5291 - 5299
DOI: 10.1039/a905926a 

The set of valley-ridge inflection points on the 
potential energy surface of water

Michael Hirsch , Wolfgang Quapp,  Dietmar Heidrich

The MP2/6-31G** potential energy surface (PES) of the water molecule is used
as a model to locate curves of valley ridge inflection (VRI) points.
Crossing points between VRI curves and approximations of the reaction path
allow the identification of a symmetric bifurcation of the reaction path.
The VRI points are calculated with the help of Branin's method. The recently
proposed method of following the reduced gradient is used to calculate
reaction path approximations of "inversion'' and dissociation together with
their possible bifurcations. The results gained by a model PES 
(W. Quapp, M. Hirsch and D. Heidrich, Theor. Chem. Acc., 1998, 100, 285) 
are now confirmed by the ab initio surface of the water molecule. 
We discuss the chemical and physical meaning of VRI curves in terms of the 
fourfold cluster problem of highly excited rotational states; 
the core area of our surface is of sufficient quality for this.