Application of the Active Space Self-Interaction-Correction Method to Molecular Systems

Within the context of the active space of the self-interaction- correction (SIC) optimized effective potential (OEP) method (J. Chem. Phys. 2001, 114, 639-651), the effect of the inclusion of the SIC at the level of only use the HOMO orbital is analyzed for a set of small molecules and for a model of an interstitial region surrounded by posi- tively charged groups in a polypeptide; the model is representative of a class of regions occurring in proteins. It is shown, for the molecular systems treated in this work, that the inclusion of the HOMO orbital, within the SIC-OEP, induces a remarkable change on the eigenvalue spectrum. For the interstitial state model, the improvement is system- atic as one increase the active space from one to ten orbitals; also, the influence on the local behavior of the interstitial virtual state closest to the Fermi level is important and enhances its regional character. As this method reduces the computational effort to introduce the SIC it seems promising to deal with self-interaction-corrections in systems