The topographies of two potential energy surfaces are examined in the vicinity of their intersection. A brief account of the basic theory is given and the possible surface types are discussed explicitly. Two main patterns are found. One of these (‘‘peaked’’) has the character of a tilted double cone in that the lower (upper) surface decreases (increases) in all directions from the intersection which is a point where an infinite number, in fact, all orthogonal trajectories emanate. The other pattern (‘‘sloped’’) results when both surfaces are monotonically sloped and touch each other along the slope, with most orthogonal trajectories bypassing the intersection. When the latter pattern prevails, the intersection can lie on a steepest descent line which originates at a transition state and hence may qualify as a reaction path model. An intermediate pattern, involving a horizontal slope on both surfaces, is also possible. The topographical patterns also differ markedly with respect to the bunching of the steepest descent lines. In general, the latter tend to veer away from the intersection on the lower surface favoring bifurcations, but are funneled towards the intersection on the upper surface, making the vicinity of the intersection a region favoring radiationless transitions. The various cases are classified and illustrated through quantitative graphs of contours and orthogonal trajectories.

An electronic structure-based construction of diabatic states from adiabatic states is formulated that is applicable when individual diabatic states contain several dominant con®gurations. It is accomplished by maximizing the electronic uniformity of the diabatic states with respect to their dominant con®gurations throughout the entire nuclear coordinate region. The con®gurations are generated from unambiguously de®ned diabatization-adapted molecular orbitals. The orthogonal transformation from adiabatic to diabatic states is deduced by an intrinsic analysis of the adiabatic CI coecients, without calculating matrix elements of additional, derivative or non-derivative operators. The practicality of the method is demonstrated by applying it to the conical intersection region of the 1 1 e 1 and 2 1 e 1 states of ozone.

The cross section through the ground~state potential energy surface of ozone which contains the open minimum, the ring minimum, and the ring-opening reaction path, including the ringopening transition state, is determined through full-valence-space muiticonfiguration selfconsistent-field calculations. It is shown that, at a point on the ridge separating the openstructure basin from the ring-structure basin in C 2V symmetry, very close to the transition state, the ground-state surface connects with the lowest excited state surface of the same symmetry (IA 1)' This point is part of an intersection seam between these two IA I surfaces in C s symmetry. It is furthermore found that the upper state has its eqUilibrium structure very close to the transition state of the lower state. The quantitative data of all critical points are calculated. In addition, a ground-state potential energy surface cross section describing the detachment of an oxygen atom is determined. For several other states, C zv constrained and bond-length-optimized energy curves E(¢) are also reported.