D. E. Ellis

The Discrete Variational Method in Density Functional Theory and its Applications to Large Molecules and Solid-State Systems

ABSTRACT The Discrete Variational method for molecules and clusters (DVM), in the framework of Density Functional theory, is described in detail. The numerical grids utilized, basis functions and potential are discussed, as well as spin-polarization for magnetic systems, total energy and dynamics. The relativistic version of the DV method is also described. Applications to large molecules range from porphyrins, a transition metal complex of thiophene, the circular molecule “ferric wheel” containing ten Fe atoms and other transition metal complexes investigated by fragments. Examples of relativistic calculations are given for 5d-metal complexes. Calculations for solids, represented by embedded clusters as large as 65–75 atoms, include transition metals, perovskites, silicates and rare-earth borocarbides. Properties investigated and analysed are structural, optical, hyperfine, magnetic and superconducting. The electronic structure and chemical bonds are also studied by Mulliken populations and charges, bond order, density of states and spin density maps; results are related to experimentally observed characteristics.

Effect of magnetism on superconductivity in rare-earth compounds RENi2B2C

Abstract Spin-polarized first-principles density-functional electronic structure calculations were performed for 73-atoms embedded clusters representing antiferromagnetic RENi 2 B 2 C (RE = Pr, Nd, Sm, Gd, Ho, Tm). A substantial difference in the extent of the exchange polarization of the conduction electrons between early and late rare-earth compounds is revealed due to the reduced coupling between RE 4f and 5d electrons with increasing Z. This result is relevant to observed phenomena of coexistence of superconductivity and magnetism in these compounds.

Toward structure-function relations — A hybrid quantum/classical approach

Abstract The calculation of electronic structure of materials by first principles quantum methods is now essentially routine, thanks to development of Density Functional approaches. However, the electronic structure depends in detail upon the nuclear positions, which are not generally known in advance. The prediction and verification of atomic positions— the geometrical configuration— of a complex molecule like a protein, or of a chemically reactive surface, or of a doped solid interface, for example, begins to yield to atomistic simulations based upon classical or semiclassical force-fields. A significant remaining problem is to couple together quantum and classical methodologies on several length- and time-scales which would be capable of carrying information forward into the continuum regime of materials modelling. We describe a particular approach to the so-called multiscale modelling problem spanning the range 1–1000 _and 1–10 7 femtosec and its intended applications to predicting structure-function relationships required for materials analysis and design. Principles are illustrated by three examples: an isolated protein, a defective oxide surface, and a bioceramic analog.

Ligand Substitution Effect on Optical Properties in Conducting Tetraazaporphyrines

Self-consistent Density Functional calculations have been performed on a variety of planar conjugated Ni-centered macrocycles with a basic tetraazaporphyrinic core and dithiolene groups (PZ) or fused-benzo groups (PC). Theoretical energy diagrams, charge and spin distributions and densities of states have been obtained in order to understand the electronic structure modifications due to peripheral ligand substitution. The substituents role in altering electronic properties and charge distribution of the porphyrazine macrocycles has been used to interpret the observed variations in optical absorption profiles. In the Q-band ({approximately} 680 nm) region, a single peak is seen for high symmetry (D{sub 4h}) macrocycles and a double peak for lower symmetry (D{sub 2h} and C{sub 2v}) systems. Calculated intensities and band splittings are compared in detail with qualitative molecular orbital models and experiment in the visible and UV regions. Predictions are made for the infrared absorption and semiconducting band gap.

Density functional study of fcc iron and iron particles in copper

The first‐principles spin‐polarized discrete variational method in the framework of density functional theory was employed to investigate the electronic and magnetic structure of fcc (γ) Fe and of γ‐Fe particles in copper, represented by 62‐atom embedded clusters of cubic geometry. The influence of Al substitutional impurities in γ‐Fe and in the Fe particle in Cu was also investigated. Magnetic moments and hyperfine fields were obtained.

Spin and magnetism of rare-earth intermetallic nickel borocarbides: RENi2B2C

Abstract The spin and magnetism in quaternary intermetalic borocarbides RENi2B2C, REPr, Nd, Sm, Gd, Ho, Tm are determined by first-principles density functional theory, using the embedded cluster formalism. Spin polarization of the lattice by the RE moments is examined in detail, and related to observed magnetic ordering phenomena. The observed superconductivity and regions of coexistence with antiferromagnetism are discussed in terms of the lattice polarization, magnitude of moments, and differences in interatomic distances.