  Beyond the dielectric continuum description initiated by Marcus theory, the
nowadays standard theoretical approach to study electron transfer (ET)
reactions in solution or at interfaces is to use classical force field or ab
initio Molecular Dynamics simulations. We propose here an alternative method
based on liquid-state theory, namely molecular density functional theory, which
is numerically much more efficient than simulations while still retaining the
molecular nature of the solvent. We begin by reformulating molecular ET theory
in a density functional language and show how to compute the various
observables characterizing ET reactions from an ensemble of density functional
minimizations. In particular, we define in that formulation the relevant order
parameter of the reaction, the so-called vertical energy gap, and determine the
Marcus free energy curves of both reactant and product states along that
coordinate. Important thermodynamic quantities such as the reaction free energy
and the reorganization free energies follow. We assess the validity of the
method by studying the model Cl$^0\rightarrow$ Cl$^+$ and Cl$^0\rightarrow$
Cl$^-$ ET reactions in bulk water for which molecular dynamics results are
available. The anionic case is found to violate the standard Marcus theory.
Finally, we take advantage of the computational efficiency of the method to
study the influence of confinement on the ET, by investigating the evolution of
the reorganization free energy of the Cl$^0\rightarrow$ Cl$^+$ reaction when
the atom approaches an atomistically resolved wall.
