Potential energy surface based

Song L, Gao J (2008) On the construction of diabatic and adiabatic potential energy surfaces based on ab initio valence bond theory. J Phys Chem A ASAP... 

Rice et al. [99] developed a global potential energy surface based on the Mowrey et al. [103] results and performed extensive classical trajectory calculations to study the dynamics of the CH2NN02 dissociation reactions. They calculated rates for reactions (III) and (IV) with classical barriers of 35 and 37 kcal/mol, respectively. They found that N-N bond fission dominates at low energy but that HONO elimination is competitive. Chakraborty and Lin [104] predict the opposite on the basis of their ab initio barriers and RRKM theory calculations. The two dissociations channels are closely competitive and it is not clear that ab initio methods are sufficiently reliable to distinguish between two reactions that have such similar energy requirements. Also, the Zhao et al. results [33] are not in accord with the theoretical predictions. 

Fig. 12.20. Representation of tiie 1,3-butadiene potential energy surface based on experimental observations. The broken arrows show die path for return to the ground state reactant and odier products. Lifetimes are given in femtoseconds. The dashed lines indicate a conical intersection for partitioning between products. Adapted from Chem. Phys. Lett.. 342, 91 (2001).

Raff used a six-body potential energy surface based on the results of semi-empirical and ab initio calculations and the thermodynamic data for reactants and products, but not adjusted to fit kinetic data for the reaction. Cross-sections for reactions (53) and (54) and for the corresponding reactions (55) and (56) of T ... 

Sufficiently accurate complete potential energy surfaces based on half-empirical or ab initio methods are now available only for the simplest gas phase reactions such as the collinear three-... 

The depiction in Figure 12.55 is a very simple model of a photodissociation. As noted in Chapter 11, states of the same symmetry may not cross on a potential energy surface based on just one nuclear coordinate (the reaction coordinate). When more than one set of nuclear coordinates is included, however, then states of the same symmetry may cross. " Thus, photodisso-... 

These studies demonstrate the utility of classical trajectory calculations in probing the kinetics and dynanGiics of elementary gas phase reactions. When accurate potential energy surfaces based on ah initio quantum chemistry calculations are used, quantitative agreement with experimental results can be obtained. However, these studies required lengthy trajectory calculations because of the complicated nature of the potential energy functions used. 

Perhaps the defining characteristic of current classical tra-jectoiy simulations of molecular collisions is the use of potential energy surfaces based on ab initio quantum mechanical results. Although the earliest applications were made with some reliance on ab initio theory, only in the past decade have technology and theory reached the point where ab initio-hased potentials can be used almost routinely. The use of empirical formulations still has its place, but mainly in studies of relatively large systems where it is not feasible to do the quantum mechanical calculations and, of course, model PESs remain useful for studies of general, fundamental dynamics problems. 

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