  A general approach to calculate the diabatic surfaces for electron-transfer
reactions is presented, based on first-principles molecular dynamics of the
active centers and their surrounding medium. The excitation energy
corresponding to the transfer of an electron at any given ionic configuration
(the Marcus energy gap) is accurately assessed within ground-state
density-functional theory via a novel penalty functional for
oxidation-reduction reactions that appropriately acts on the electronic degrees
of freedom alone. The self-interaction error intrinsic to common
exchange-correlation functionals is also corrected by the same penalty
functional. The diabatic free-energy surfaces are then constructed from
umbrella sampling on large ensembles of configurations. As a paradigmatic case
study, the self-exchange reaction between ferrous and ferric ions in water is
studied in detail.
