The cyclic cluster model at Hartree-Fock level

We present the implementation of the cyclic cluster model (CCM) formalism at the Hartree-Fock (HF) level. In contrast to other periodic models, the CCM is a Γ-point approach. Integration is carried out in real space within a finite interaction area determined by the size and the shape of the cluster that corresponds to a supercell of the solid, surface, or polymer. Particular care has to be taken for the proper treatment of three- and four-center integrals that involve basis functions located at the boundaries of the Wigner-Seitz supercell, which defines the interaction region. The similarity between the CCM formalism and molecular approaches allows in principle the application of sophisticated post HF methods to solid-state problems with only moderate modification of the molecular code. We show for selected model systems, that with our approach, the electronic structure and energetics obtained by the conventional supercell model is fully reproduced.