Grote-Hynes theory

Kramers' theory of barrier crossing under the influence of friction is based on a simple Langevin equation with Markovian friction and random forces coupled to reaction coordinate motion Kramers (1940) [1] . The random forces and friction represent the ‘bath’, i.e. every degree of freedom except for the explicitly modeled ‘reaction coordinate.’ In the Kramers model, the response by the bath to a change in the reaction coordinate is both immediate and immediately forgetful. Real solvents require time to reshape the solute cavity, to rearrange the hydrogen bond network, and to reorient polar solvent molecules. These solvent relaxation processes can take many picoseconds to complete. By comparison, chemical bonds can be broken and made in just fractions of a picosecond because typical reactions only require a few atoms to move by a few Angstroms. Clearly, Markovian friction models cannot accurately describe solvent dynamics during a chemical reaction.