Empirical valence bond

The Empirical Valence Bond (EVB) approach is an approximation that allows you to calculate reaction free-energies in condensed-phase. It was first developed by Arieh Warshel.[1] And was inspired by the way Marcus theory uses potential surfaces to calculate the probability of electron transfer.

EVB coupling parameter H₁₂

Where most methods for reaction free-energy calculations require at least some part of the modeled system to be treated using quantum mechanics, EVB uses a calibrated Hamiltonian to approximate the potential energy surface of a reaction. For a simple 1 step reaction that typically means that a reaction is modeled using 2 states. These states are valence bond descriptions of the reactants and products of the reaction. The function that gives the ground energy then becomes:

${\displaystyle E_{g}={\frac {1}{2}}(H_{11}+H_{22}-{\sqrt {(H_{11}-H_{22})^{2}+4H_{12}^{2}}})}$

Where H₁₁ and H₂₂ are the valence bond descriptions of the reactant and product state respectively. And H₁₂ is the coupling parameter. The H₁₁ and H₂₂ potentials are usually modeled using force field descriptions U_reactants and U_products . H₁₂ is a bit trickier as it needs to be parameterized using a reference reaction. This reference reaction can be experimental, typically from a reaction in water or other solvents. Alternatively quantum chemical calculations can be used for calibration.