Research Interests:

Calculations of the optical properties of rare earth compounds

The optical properties of rare earth compounds are of fundamental as well as practical importance. For example, alkali-doped cerium sesquisulfides, Ce_2-xA_3xS₃ (A = alkali metal), are inorganic pigments of red to orange hue and are currently at the stage of industrial production. Band structure calculations within the local density approximation (LDA) provide qualitative insight into the electronic structure of these compounds and pointed to the importance of f-electrons in the optical transitions. However, pure LDA calculations are unable to provide quantitative predictions of optical gaps. The present work aims at a clearer understanding of the absorption mechanism and at quantitative predictions of the absorption energies in this class of compounds.

Density functional molecular dynamics simulations of hydroxyapatite nucleation on silica based glass surfaces

Materials based on silica (SiO₂) glass are used for prosthetic bones and dental implants because they initiate and promote the deposition of bone-like substances dissolved in a fluid, on their surfaces. Bone is a composite material that mainly consists of hydroxy apatite, Ca₅(PO₄)₃OH. However, the atomistic structure of the interfaces and the detailed physical and chemical factors which lead to the nucleation of hydroxy apatite from the fluid at room temperature are essentially unclear. The attention is focused on the deposition of hydrogen phosphate, HPO₄^2-, the predominant phosphate ion at pH 7.3, the pH value of the human blood plasma. One goal of this project is to determine reaction pathways for the nucleation of HPO₄^2- on a silica glass surface using density functional molecular dynamics simulations. The knowledge about the chemistry of the nucleation would be of help for bio-engineers designing materials that promote the formation of bone on their surfaces. More ...

Structural and electronic properties of haloform-intercalated fullerenes

Solid C₆₀ is insulating but can be made conducting and even superconducting upon intercalation of alkali atoms (e.g. K, Rb). between the fullerene molecules. The alkali atoms transfer their valence electrons into the conduction bands of the C₆₀ subsystem (electron doping), and the materials become metallic. A noticeable enhancement of the superconducting critical temperature T_c, of C₆₀ was expected when electrons are removed (hole doping) because of the larger density of states of the valence bands of the host system. A further increase of the critical temperature is assumed when C₆₀ is intercalated with haloform molecules CHX₃ (X = Cl, Br). Such an enhancement of T_c upon intercalation of molecules into a superconductor has been already observed earlier.
This work is focused on the normal properties of haloform-intercalated C₆₀. All-electron density-functional calculations on several crystal structures of bulk haloform-intercalated C₆₀ are used to predict their electronic structures and to examine the effect of introduced charge carriers (doping with electrons as well as with holes). Employed are methods that rely on the generalized gradient approximation and the spin-unrestricted Kohn-Sham approach. Particular attention is paid to the interaction of the haloform molecules with the electronic structure of the C₆₀ subsystem. More ...

Past project: Development of the density functional program DMol³

Extension of the DMol³ program to perform first principles molecular dynamics (MD) simulations on the Born-Oppenheimer potential energy surface. The program allows MD simulations within the microcanonical ensemble (constant number of atoms, constant volume and constant energy) as well as simulations within the canonical ensemble (constant number of atoms, constant volume and constant temperature). Temperature control during the isothermal MD simulations is achieved by the implementation of explicit reversible extended-system methods like the Nosé-Hoover (NH), Nosé-Hoover chain (NHC) and the generalized Gaussian moment thermostats (GGMT). Furthermore structural constraints can be imposed to keep bond distances, bond angles and dihedral angles at particular values during MD simulations. For that purpose the SHAKE and RATTLE algorithms are implemented. More ...

Publications:

1. R. Windiks and J. Sauer, "Sodium doped sodium sodalite: magnetic coupling between F centers and hyperfine interactions with framework atoms", Phys. Chem. Chem. Phys., 1(18), 4505 (1999).
2. R. Windiks and J. Sauer, "Electronic structure and magnetic coupling in sodium electro sodalite. All-electron density functional calculations", J. Chem. Phys., 113(13), 5466 (2000).
3. A. Bill, R. Windiks, B. Delley and V. Z. Kresin, "High-temperature superconductivity in intercalated molecular C₆₀/CHX₃ (X = Cl, Br, I)", Int. J. Mod. Phys. B, 16, 1533 (2002).
4. R. Windiks, A. Bill, B. Delley and V. Z. Kresin, "Crystal structures and electronic properties of haloform-intercalated C₆₀ materials", Phys. Rev. B 66, 195418 (2002).
5. R. Windiks, A. Bill, B. Delley and V. Z. Kresin, "Crystal structures and electronic properties of haloform-intercalated C₆₀ materials", Virt. Nanosc. Sci. Techn. 6 (2002).
6. R. Windiks and B. Delley, "Massive thermostatting in density functional molecular dynamics simulations", J. Chem. Phys. , 119(5), 2481 (2003).
7. J. Sauer and R. Windiks, in Host-Guest-Systems Based on Nanoporous Crystals, edited by F. Laeri, F. Schüth, U. Simon, and M. Wark (Wiley-VCH, Heidelberg, 2003).
8. P. R. Willmott, R. Herger, B. D. Patterson and R. Windiks, "Experimental and theoretical study of the strong dependence of the microstructural properties of Sr_xBa_1-xNb₂O₆ thin films as a function of their composition", Phys. Rev. B 71, 144114 (2005).

Education

• Dr. rer. nat., Humboldt University Berlin, Department of Chemistry, Quantum Chemistry Research Group.
  Thesis: "Paramagnetic Na₄³⁺ guest clusters embedded in sodalite hosts: Density functional calculations of hyperfine interactions and magnetic couplings" (english, pdf , 1845KB).

• Diploma, Humboldt University Berlin, Department of Chemistry, Quantum Chemistry Research Group.
  Thesis: "Eignung von Dichte-Funktional-Methoden zur Beschreibung von Wasserstoff-Brückenbindungen in kleinen Molekülen" (german, pdf, 534KB).

• Curriculum Vitae (pdf, 2.5KB).

Links:

• Materials Design
• Paul Scherrer Institute (PSI)
• Condensed Matter Theory @ PSI
• Linux on the Dell Centrino Notebook Latitude D600