Abstract:
The local incremental expansion of the correlation energy of extended systems is applied to non-orthogonal localized orbitals and compared to the standard approach, which uses orthogonal Foster-Boys orbitals. Several methods of how to generate suitable non-orthogonal orbitals for the investigated covalent systems, bulk silicon and beryllium rings, are discussed. For the non-orthogonal orbitals the correlation energy contributions from increments involving more than one correlated orbital decay faster with the distance between these orbitals than for standard Foster-Boys orbitals. Also, the transferability of the individual energy increments from one cluster to another cluster is better in case of the non-orthogonal orbitals. Yet, the convergence of the incremental series with the order of the increments, i.e. the number of correlated bonds involved, is somewhat slower compared to the orthogonal Foster-Boys orbitals.