Potential energy surface characteristic points

Secondly, it is usual to calculate only a few points which are assumed to be characteristic with full optimization of geometry instead of the complete potential energy surface 48). For a pure thermodynamical view it is enough to know the minima of the educts and products, but kinetic assertions require the knowledge of the educts and the activated complex as a saddle point at the potential energy surface (see also part 3.1). 

A stable nuclear configuration on a potential energy surface is associated with a point for which there is zero slope in any direction and for which there is no direction in which the curvature is negative or zero. Such points are uniquely defined in any system of internal coordinates but we shall see that some other characteristic features of a surface are dependent on the choice of coordinate. 

Fig. 9.4. Left-hand side Representation of an inelastic potential energy surface of the form (6.35) with e = 0.2. The heavy arrows represent two characteristic trajectories starting at the respective FC points. Right-hand side The corresponding final vibrational state distributions.

The accurate description of a chemical reaction requires detailed knowledge of its potential energy surface (PES). In principle Bom-Oppenheimer PESs can be theoretically obtained using a grid of PES points, but in practice it is not possible because of the drastic increase in computer resources, even for small systems. The alternative way is to find stationary points (minima, maxima and saddle points) and to estimate characteristics of the PES along the reaction path. 

A first step, before to compute rotatory strength, is to determine the molecular structure. This point involves geometry optimization by proper method with a proper quality basis set (with polarization functions), followed by a confirmation of the stationary point characteristics on the potential energy surface (PES), i.e. 

In 2013, Li et al. [15] used the ROHF-UCCSD(T)-F12 method with the aug-cc-pVTZ basis set and a frozen-core (FC) approximation to study the characteristics of the three X - H20 (X = F, Cl, Br) complexes, but no other stationary points in the potential surface were reported. Since the reaction rate constants and their temperature dependence are sensitive to the accuracy of the potential energy surface, we wiU, in the present paper, adopt high-level ab initio coupled-cluster methods along with correlation-consistent cc-pV5Z-PP basis sets to study all stationary points on the potential surface for the Br -I- H2O HBr -I- OH reaction. 

At this point, it might be useful to remember the timescales of the pro- timescales cesses during a catalytic reaction. The electronic processes of the potential energy surface of the reaction have characteristic times of 10s, while the vibrational motions of the atoms are in the order of 10 s. The timescales of the bond formation and breaking of the catalytic processes are reported to be in the order of 10" to 10 s (van Santen and Neurock,... 

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