The EVB Calibration Procedure

The approach presented above is referred to as the empirical valence bond (EVB) method (Ref. 6). This approach exploits the simple physical picture of the VB model which allows for a convenient representation of the diagonal matrix elements by classical force fields and convenient incorporation of realistic solvent models in the solute Hamiltonian. A key point about the EVB method is its unique calibration using well-defined experimental information. That is, after evaluating the free-energy surface with the initial parameter a , we can use conveniently the fact that the free energy of the proton transfer reaction is given by 

Now we can adjust a until the calculated and observed AGPr coincide. This calibrated surface can then be used with confidence for studying the reaction in different solvents (and different environments), since a remains unchanged and only Ais recalculated. In this way, the error associated with the evaluation of does not affect the calculations of the relative effects of different solvents. 

In general, when one deals with a more complicated reaction, for which it is hard to obtain gas phase estimates of a °, it is convenient to use solution experiments from aqueous solutions to obtain the first estimate of a°. This is done by using 

The EVB approach described in this chapter provides a convenient way for estimating the energetics of chemical reactions in various solvents. However, the approximation involved in eq. (2.21) cannot be justified without detailed studies by more rigorous models. Such models will be described in Chapter 3. 

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