The use of this monomer basis set is advantageous for large molecular systems as it allows for considerable savings in the overall computational cost. It should be noted that the KS-like orbitals forming Slater determinants of the charge-localized states Φ 1 and Φ 2 , are in general not orthogonal, which leads to non-orthogonality of these states. The overlaps for the latter are given by ]