The aim of this chapter is to present constricted variational density functional theory (CV-DFT), a DFT-based method for calculating excited-state energies. This method involves constructing from the ground-state orbitals, a new set of ‘occupied’ excited-state orbitals. Consequently, a constraint is applied to ensure that exactly one electron is fully transferred from the occupied to the virtual space. This constraint also prevents a collapse to a lower state. With this set of orbitals, one obtains an electron density for the excited-state and therewith the CV-DFT excitation energy. This excitation energy can now be variationally optimized. With our successful applications to systems differing in the type of excitation, namely, charge-transfer, charge-transfer in disguise, and Rydberg excitations, as well as in size, we demonstrate the strengths of the CV-DFT method. Therewith, CV-DFT provides a valid alternative to calculate excited-state properties, especially in cases where TD-DFT has difficulties. Finally, our studies have shown that the difficulties arising in the TD-DFT excited states are not necessarily stemming from the functional used, but from the application of these standard functionals in combination with the linear response theory.
Part of the book: Density Functional Calculations