Charge transfer excitations and constrained density functional theory

Making use of the initial and final Fukui functions approximated by the charge density of the initial (occupied) orbital and the final (unoccupied in the ground state) orbital to identify the charge transfer excitations in a time-dependent density functional theory calculation, and the acceptance and donation regions, we analyze the performance of constrained density functional theory to predict the excitation energies by considering several alternatives to fix the amount of charge transferred. It is shown that charge transfer excitations energies may be accurately determined through this approach when one fixes the final charge in the acceptance region to a value that complements the net charge already present to minus one.