Ab initio incremental correlation treatment with non-orthogonal localized orbitals

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.