Toward a wavefunction-based treatment of strong electron correlation in extended systems by means of incremental methods

Incremental expansions have been used in the past to make electron correlation energies of large molecules and solids accessible to a treatment with wavefunction-based quantum-chemical methods. For this purpose, the Hartree-Fock space of occupied orbitals is partitioned into groups of localized orbitals; these groups are then correlated separately, one at a time, and their correlation energies are added up to a first estimate of the total correlation energy; subsequently, this estimate is systematically improved by correlating pairs, triples, etc., of the localized groups simultaneously. This works well when dynamic correlation effects are predominant and correlation effects rapidly decrease with the distance of the localization centers. For strong static correlation, Hartree-Fock is no longer a good starting point, localization is less well effective, and the convergence of the incremental scheme deteriorates. Here, two ways of improvement are considered for the latter case: optimization of the localized orbitals at a higher level than Hartree-Fock and use of incremental expansions from more than one reference determinant. These improvements are illustrated for H-10 rings and H-12 sheets up to dissociation and for the transition between the intra-atomic and interatomic bonding regimes in Be-6 rings.