Reduction of the CI dimension based on the use of local orbitals: Application to conjugated systems and excited states

A description of some excited states in conjugated systems based on the use of local orbitals was recently reported. A reduced active space has been used to describe the n -> pi* excitation in polyenals and the pi ->pi* excitation in polyenes. In the present work, we report the strategy for reducing the space of determinants generated in configuration interaction (0) calculations on the same type of systems. The approach is based on the fact that the rest of the inactive and virtual orbitals remain local. This opens the possibility of freezing those local orbitals distant from the region where the overlap between these orbitals and the active ones is small. The results show that it is possible to reach vertical excitation energies with no more than 0.1 eV error, dealing with only 5% of the total CI space. We focus the discussion principally on CAS-SDCI calculations that would be impracticable if the present reduction were not applied, such as those describing the interaction between a sodium atom and the C,0 fullerene. This new strategy is completely general and could be considered as an extremely useful tool to handle large systems with high-level correlated methods. (c) 2005 Wiley Periodicals, Inc.