Potential Energy Surfaces and Intermolecular Forces

For diatomic reagents, the potential energy curves can often be obtained through inversion of experimental data. If sufficient spectroscopic data are available, an RKR inversion - o can be performed with very high accuracy. More recently, Bernstein, Zewail, and co-workers inverted femtosecond temporal spectra to obtain diatomic potentials of high accuracy. 

The intramolecular and intermolecular solvent forces can be obtained in several ways. For rare gas solvents, it is most commonly the case that Lennard— Jones potentials are used to represent the intermolecular interactions (although more sophisticated potential energy functions are available from atom—atom scattering data). Molecular solvents, such as water, require more complicated models. The construction of these models is an active field and well beyond the scope of this review. Many of these models aim to reproduce bulk properties of the solvent and are quite successful in doing that (although some properties. 

Intermolecular solvent-reagent forces are usually represented through Lennard-Jones interactions between the component atoms. For dipolar or ionic systems, partial or full charges can be placed on individual atoms to represent Coulomb interactions. (More complex methods exist for dealing with charged systems, such as Ewald summation see ref. 4 for more details.) 

In calculating the forces, one must be cautious if the parameters defining the potential depend on the configuration of the system. This parameter dependence leads to extra terms in the forces. Similar expressions arise in the feathering of long range forces to zero as the distances involved approach the size of the simulation unit cell. 

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