Properties of Potential Energy Surfaces

The mathematical definition of the Born-Oppenheimer approximation implies following adiabatic surfaces. However, software algorithms using this approximation do not necessarily do so. The approximation does not reflect physical reality when the molecule undergoes nonradiative transitions or two 

FIGURE 20.2 Adiabatic paths for bond dissociation in two different electronic states and the diabatic path. 

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In this approach properties of potential energy surfaces are investigated from the point of view of all possible monomolecular transformations of the given reactants. A plausible suggestion concerning the mechanism of the reaction under study is usually made on the basis of reaction barriers or activation energies. Moreover, in some studies, partition functions are evaluated and rate constants are obtained within the framework of the absolute rate theory. 

The vast majority of quantum chemical studies focus on equilibrium properties. However, a detailed understanding of chemical reactions requires a description of their chemical dynamics, which in turn requires information about the change in potential energy as bonds are broken or formed. Even though modem electronic structure theory can provide near-spectroscopic accuracy for small molecular systems near their equilibrium geometries, the general description of potential energy surfaces away from equilibrium remains very much a frontier area of research. 

Hopkinson was hired by York to teach theoretical organic chemistry (the Woodward-Hoffmann rules were then a hot topic) and to carry out experimental chemistry. Despite the limited computing capacity at York at the time, he managed to complete some work on the electrophilic addition to alkenes. He is probably best known, however, for his work on proton affinities, destabilized carbocations,234 organosilicon compounds, silyl anions and cations, and more recently, on the calculation of potential energy surfaces and thermodynamic properties. He has had a particularly fruitful collaboration with Diethard Bohme.235... 

The concept of potential-energy surface (or just potentials) is of major importance in spectroscopy and the theoretical study of molecular collisions. It is also essential for the understanding of the macroscopic properties of matter (e.g., thermophysical properties and kinetic rate constants) in terms of structural and dynamical parameters (e.g., molecular geometries and collision cross sections). Its role in the interpretation of recent work in plasmas, lasers, and air pollution, directly or otherwise related to the energy crisis, makes it of even greater value. 

From time to time the broad front of advance in any field is pierced by significantly greater and more important developments in some subareas. Recent developments in laser technology, mass spectrometry, and molecular beam studies have made that the case for the properties and reactions of excited states of simple atoms and molecules. This volume of the Advances in Chemical Physics is, therefore, devoted to a collection of contributions that are relevant to aspects of the physics and chemistry of excited species. The articles cover topics as diverse as theoretical estimation of potential energy, surface properties, and upper atmosphere chemistry, but all are tied together by the common denominator of the need to understand the properties of the excited states of molecules. It is hoped that this and succeeding volumes will supplement the rather broadly scattered literature, and provide an introduction for both the interested student and the working scientist. 

On the theoretical side the H20-He systems has a sufficiently small number of electrons to be tackled by the most sophisticated quantum-chemical techniques, and in the last two decades several calculations by various methods of electronic structure theory have been attempted [77-80]. More recently, new sophisticated calculations appeared in the literature they exploited combined symmetry - adapted perturbation theory SAPT and CCSD(T), purely ab initio SAPT [81,82], and valence bond methods [83]. A thorough comparison of the topology, the properties of the stationary points, and the anisotropy of potential energy surfaces obtained with coupled cluster, Moller-Plesset, and valence bond methods has been recently presented [83]. 

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