Empirical valence bond

In theoretical chemistry, the Empirical Valence Bond (EVB) approach is an approximation for calculating free-energies of a chemical reaction in condensed-phase. It was first developed by Israeli chemist 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:

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

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.