The partitioning of the electron density into subsystems that is provided by sDFT can also be used to address known shortcomings of existing approximations to the xc functional, most importantly the self-interaction error that leads to an over-delocalization of electron densities. First, this error can be reduced by enforcing a particular distribution of the electrons, i.e., the assignment of the electrons to localized subsystems avoids spurious delocalization. It was shown that this effect leads to an improved description of liquid water and of hydroxyl radicals in aqueous solution (see also ). Similarly, it allows for the treatment of open-shell subsystems to arbitrary local-spin patterns (see and to obtain quasi-diabatic states (see Section 6.3). Second, if one allows for fractional electron numbers in the subsystems , the intermolecular self-interaction error can be specifically addressed by enforcing the correct fractional-charge behavior of the subsystem energies. To this end, our (CRJ's) lab developed a scheme in which the subsystem energies at fractional electron numbers are calculated as linear interpolation between those obtained at integer electron numbers , i.e.,