Cyclic Cluster Model (CCM): In the CCM periodic boundary conditons are applied to large molecular clusters. In this way translationally equivalent atoms are guaranteed to have the same environment and becom indistinguishable. Boundary effects from atoms at the cluster borders are removed in the CCM.
The CCM can treat polymers, surfaces and solids. It has been shown that calculated properties converge rapidly with increasing cluster size, so that already medium-sized models describe bulk properties reasonably well. For ionic systems long-range Coulomb interactions are taken into account via the Ewald summation technique.
The CCM is related to the more commonly used supercell model (SCM) with restriction to k=0 except that the interaction are around each atom is not determined by an arbitrary cutoff threshold, but is defined by the Wigner-Seitz cell constructed by the translation vectors of the cyclic cluster. The CCM can be easily implemented in molecular codes at semiemipirical level, but it is also possible to modify an existing molecular density- functional formalism.
a) Application to problems in solid-state chemistry, surface chemistry, and heterogeneous catalysis. These studies are performed with semiempirical, density-functional and Hartree-Fock methods with the program packages CRYSTAL03, VASP, and MSINDO using the CCM and SCM. Recent activities of the group comprise
- Effect of anion substitution on electronic properties, e.g. changes of optical transitions in the system ZnS/ZnSe
- Point defect formation in solids, e.g. oxygen defects in Li2O, Li2B4O7
- Effect of anion distribution in ternary systems, e.g. TaON and Zr2ON2
- Surface structure of thin films, e.g. rutile on tungsten
- Oxygen defect diffusion on surfaces, e.g. in the catalytically active TiO2 rutile(110) surface
- Adsorption of small molecules and metal clusters on perfect and defective surfaces, e.g. NO/NiO, Pd2/TiO2, CO/SnO2
- Molecular-Dynamics simulation of spectroscopic features of adsorbed molecules, e.g. H2O/rutile and oxalic acid/anatase.
- Surface reactions of adsorbed molecules on surfaces, e.g. the DeNOx process on mixed V2O5/TiO2 catalysts o Electric field gradients for the identification of lattice sites, e.g. in LixTiS2.
- Diffusion of small cations, e.g. Li at the interface region of nanocrystalline Li2O:B2O3 composites.
b ) Development of the semiempirical SCF-MO method MSINDO
- Extension to elements of the fourth row transition elements Y to Cd.
- Implementation of the CCM
- Born-Oppenheimer molecular dynamics technique
- Inclusion of higher multipole integrals
- Parallelisation via SCALAPACK and BLACS