On the selection of domains and orbital pairs in local correlation treatments

The accuracy of local correlation calculations can be controlled by the size of the virtual orbital subspaces (domains) used in the denition of the conguration spaces. Various strategies to dene these domains are described and compared. The domains can be chosen on the basis of the localized occupied orbitals, as originally proposed by Pulay and co-workers. Extension of these domains leads to rapid convergence of the correlation energy towards the full canonical result. Alternatively, in local coupled cluster calculations energy criteria can be employed using the amplitudes of extended LMP2 calculations. Furthermore, orbital pairs can be classied according to distance, connectivity, or energy criteria and depending on the importance be treated at dierent computational levels. The accuracy and eciency of the various approaches is compared