Localization of the filled and virtual orbitals in the nucleotide bases

For the four nucleotide bases cytosine, uracil, adenine and guanine both Boys (B) and Edminston-Ruedenberg (ER) localization procedures of the ab initio canonical orbitals have been performed. The results obtained for both σ-π separation and by treating all electrons together show a very good localization for all electrons (one-center lone-pairs and two-center localized orbitals even for π-electrons) and a rather good localization for the virtuals applying both B and ER criteria. The results of the two methods are essentially identical. These results suggest that the application of localized orbitals will open new possibilities for the calculation of correlation in extended systems.