The behavior of dense colloidal fluids near surfaces can now be probed in great detail with experimental techniques like video and confocal microscopy. In fact we are approaching a point where quantitative comparisons of experiments with particle-level theory, such as classical density functional theory (DFT), are appropriate. In a forward sense, we may use a known surface potential to predict a particle density distribution function from DFT; in an inverse sense, we may use an experimentally measured particle density distribution function to predict the underlying surface potential from DFT. In this dissertation, we tested the ability of the closure-based DFT to perform forward and inverse calculations on potential models commonly employed for colloidal particles and surface under different surface topographies. To reduce sources of uncertainty in this initial study, Monte Carlo simulation results played the role of experimental data. The accuracy of the predictions depended on the bulk particle density, potential well depth and the choice of DFT closure relationships. For a reasonable range of choices of the density, temperature, potential parameters, and surface features, the inversion procedure yielded particle-surface potentials to an accuracy on the order of 0.1 kBT. Our results demonstrated that DFT is a valuable numerical tool for microcopy experiments to image three-dimensional surface energetic landscape accurately and rapidly. B

In order to understand the structural and electronic properties of titanium oxide nanoparticles of different sizes, the FIRE algorithm combining the simulated annealing method is employed to find the structures of TinO2n (n=1¡Ð6) nanoparticles with the global minimum potential energy. To deeply understand electronic properties, the relaxation structures of TinO2n (n=1¡Ð6) nanoparticle from previous method will be recalculated by density functional theory (DFT) method. The Fukui function, Frontier Molecular Orbital and density of state of TinO2n (n=1¡Ð6) nanoparticles are discussed for understanding the size effect of TiO2 nanoparticles on chemical reactivity. The adsorption and dissociation energy mechanism of the HN3 molecule and its fragments are also discussed and are compared with the mechanism about HN3 on the anatase surface.

The objective of this thesis is to study the crystal structures and electronic properties of solids at high pressure using state-of-the-art electronic structure computational methods. The thesis is divided into two main sections. The first part is to examine the performance and reliability of several current density functionals in the description of the electronic structures of small band gap materials and strongly correlated systems. The second part is to compare and evaluate two recently proposed first-principles methods for the prediction of stable structures of solids at high pressure. To accomplish the first goal, first-principle electronic structure calculations employing density functional theory (DFT) and several “correlation corrected” functionals calculations were used to investigate the properties of solid AlH3 and EuO at high pressure. The primary reason to study AlH3 is to resolve a discrepancy between previously predicted superconductivity behavior at 110 GPa but was not observed in experimental resistance measurements. The key to resolve the discrepancy is an accurate calculation of the valence and conduction band energies. The results shows that the Fermi surface is modified by the “improved” functionals over the previous calculations using “standard” gradient corrected functional. These changes in the Fermi surface topology removed the possibility of nesting of the electronic bands, therefore, solid AlH3 above 100 GPa is a poor metal instead of a superconductor. In the second system, we have studied EuO with highly localized electrons in the Eu 4f orbitals. A particular interest in this compound is the report of an anomalous isostructural phase transition with a significant volume reduction at 35-40 GPa and the relationship with the electronic state of Eu at high pressure. Using the Hubbard on-site repulsion model (LDA+U), we successfully predicted the insulator  metal transition of EuO at 12 GPa and the trend in the Mössbauer isomer shifts. However, the isostructural transition was not reproduced. The U on-site repulsion to localized Eu 4f orbtials helped to ameliorate some deficiencies of the PBE functional and improved the agreement with experimental observations but not all the properties were correctly reproduced. The second objective of this investigation is to predict energetically stable crystalline structures at high pressure. The reliability and relative efficiency of two recently proposed structure prediction methods, viz, Particle Swarm Optimization (PSO) and the Genetic Algorithm (GA) were critically examined. We applied the techniques to two separate systems. The first system is solid CS2. The motivation is that this compound was recently found to be a superconductor with a critical temperature of 6 K from 60 – 120 GPa. However, no crystalline structure was found by experiment in this pressure range. Our calculations suggest the energetic favorable structures contain segregated regions of carbon and sulfur atoms. The sulfur atoms adopt a planar closed pack arrangement forming 2D square or hexagonal networks and the carbon atoms tend to form hexagonal rings. A global minimum crystalline structure with structural features observed in the amorphous structure was found and shown to be superconductive. In the second case, we studied the possibility on the existence of Xe-halides (XeHn (H=Cl, Br and I, n = 1, 2 and 4)) compounds below 60 GPa. We reported the stability, crystal and electronic structures, vibrational and optical spectra of a number of stoichiometric crystalline polymorphs. We found that only XeCl and XeCl2 form thermodynamically stable compounds at pressure exceeding 60 GPa. A stable cubic fcc structure of XeBr2 was found to be a superconductor with critical temperature of 1.4 K. From these studies, we found both merits and shortcomings with the two structural prediction approaches. In the end, we proposed a hybrid approach to assure the same stable structure is predicted from both computational strategies.

The work presented in this dissertation focuses on the [FeFe]-hydrogenase active site as inspiration for the design and synthesis of complexes capable of the electrocatalytic generation of molecular hydrogen from protons and electrons. The majority of work discussed uses gas-phase photoelectron spectroscopy (PES) and density functional theory (DFT) to probe and analyze the bonding and electron distribution in potential catalysts. These two techniques are also used to explore the nature of cyanide as a ligand, due to its presence and unknown role in these enzymes. This dissertation begins with the study of (η⁵-C₅H₅)Fe(CO)₂X (FpX) and (η⁵- C₅Me₅)Fe(CO)₂X (Fp*X) complexes where X = H⁻, Cl⁻, and CN⁻ to assess and compare their π-accepting abilities, which is contradicted in the literature. The shifts in ionization energies measured by PES provide a measure of the relative bonding effects. The results indicate cyanide is, overall, a weak π-acceptor, and the σ- and π-donor interactions are important to understanding the chemistry. The molecule [(μ-ortho-C₆H₄S₂)][Fe(CO)₃]₂ was examined, in part due to the delocalized π-orbitals of the C₆H₄S₂ ligand, which could facilitate the redox chemistry necessary for catalysis. Computations show that upon ionization, the complex adopts a semi-bridging carbonyl; termed “rotated structure”. The reorganization energy of this geometry change was determined, which may provide understanding of how the active site in the enzyme enables electron transfer to achieve this catalysis. Next complexes of the form (μ-SCH₂XCH₂S)[Fe(CO)₃]₂, where X=CH₂, O, NH, ᵗBuN, MeN, were explored in order to provide insight to the unknown atom at the central bridging position of the alkyl chain in the [FeFe]-hydrogenase enzyme. The likelihood of a rotated cationic structure is also shown, with reorganization energy values similar to that seen for [(μ-ortho-C₆H₄S₂)][Fe(CO)₃]₂. The final chapter explores the replacement of selenium for sulfur in (μ- X(CH₂)₃X)[Fe(CO)₃]₂ and (μ-X(CH₂)₂CH(CH₃)X)[Fe(CO)₃]₂, where X is either sulfur or selenium. The PES data show destabilization of the selenium complex ionizations compared to the sulfur complexes and a lower reorganization energy was calculated. The computed HOMO-LUMO gap energy for the selenium-based complex is roughly 0.17 eV smaller than for the sulfur analogs, which may indicate a lower reduction potential is needed.

The properties of a new family of carbon structures based on stacked cyclic C6 rings and intercalated cyclic C6 structures: (C6)n and (C6)nMen-1 have been studied theoretically using ab initio DFT (Density Functional Theory). Calculations of the structural, electronic, and vibrational properties of a range of these molecules have been carried out using DFT techniques with the best correspondence to experimental results. The chemical and structural stability of structures based on stacks of cyclic C6 has also been estimated for pure carbon molecules (C6)n and for metal-organic sandwich molecules intercalated with Fe and Ru atoms. These have (C6)nFen-1 and (C6)n Run-1 compositions, respectively These structures are predicted to show a variety of new electronic, vibrational and magnetic properties. Ultra-small diameter tubular molecules are also found to have unique rotational electron states and high atomic orbital pi-sigma hybridization giving rise to a high density of electron states. All phonons in these structures have collinear wave vectors leading to an ultrahigh density of phonon states in dominant modes suggesting that some of these structures may exhibit superconductivity. These properties, as well as a predicted high electron mobility, make these structures promising as components in nanoelectronics. Experiments using femto-second laser pulses for the irradiation of organic liquids suggest that such structures may appear under certain conditions. In particular, a new type of iron carbide has been found in these experiments.

This thesis presents the results of torque magnetometry and resistivity measurements of the electronic structure of novel quantum materials, specifically using the techniques of quantum oscillations and angle-dependent magnetoresistance oscillations. Measurements of the Fermi surfaces of these materials, alongside comparisons to the electronic structure predicted by density functional theory calculations, can elucidate much about the novel physical properties they exhibit and the competing interactions which govern their phase diagrams. The first system studied is the Iron-based superconductor FeSe_1-xS_x, an isoelectronically doped version of a system of much current interest, FeSe. Doping up to x = 0.2 is found to suppress the structural transition in this system entirely, with superconductivity continually present at low temperatures. Shubnikov-de Haas measurements across this range find a small quasi-two dimensional Fermi surface that increases in size and warping continuously with doping, with orbital dependent effective masses that do not change significantly within the orthorhombic phase. The second material studied is the antiferromagnetic intermetallic CeZn₁₁ which, featuring an unpaired 4f electron, is considered a possible candidate for heavy fermion behaviour. De Haas-van Alphen oscillations are seen once the antiferromagnetic phase is suppressed, and comparable frequencies of oscillation are measured in the non-magnetic analogue LaZn11, although with relatively smaller effective masses. GGA+U calculations, once magnetic breakdown is considered, match well the measured frequencies, confirming CeZn₁₁ to be a localised moment system with the 4f electron well below the Fermi level. The final material studied is the transition metal dichalcogenide IrTe₂, which undergoes dimerisation upon cooling into a number of possible charge modulated structures. Low temperature de Haas-van Alphen measurements find multiple domains of a quasi-two dimensional Fermi surface, no longer perpendicular to the lattice planes. Angular-dependent magnetoresistance oscillations observe a similarly tilted quasi-one dimensional Fermi surface, again with many domains present. Together these measurements confirm the unusual dimensionality of the dimerised Fermi surface of IrTe₂.

No description available.

Harmonic vibrational frequencies of several small organic molecules which were used to validate the scaled quantum mechanical (SQM) force field procedure of Pulay et al. were calculated using six popular density functional (DFT) methods and compared with experimental results. The combination of Becke's exchange with either Lee-Yang-Parr (BLYP) or Perdew's correlation functional (BP86) reproduces the observed frequencies satisfactorily with deviations similar to those of the Hartree-Fock SQM methods. Three hybrid DFT methods are found to yield frequencies which were generally higher than the observed fundamental frequencies. When the calculated frequencies were compared with 'experimental' harmonic frequencies however, Becke's three-parameter hybrid method with Lee-Yang-Parr correlation functional is found to be slightly more accurate, especially for C-H stretching modes. The results indicate that BLYP calculation is a very promising approach for understanding the observed spectral features.

The chemistry of fullerenes, which are a class of carbon allotropes that can be prepared by vaporization of graphite in an electric arc in a low pressure atmosphere,1 has become a topic of much experimental and theoretical study over the past 25 years.  Herein we present a series of theoretical studies related to recently discovered or studied endohedral metallofullerenes (EMF) and a theory as to the selective stability of certain isomers of EMFs. Computational treatments of the anions of C80 and C94 are presented and compared in an effort to gain an understanding and predictive model for which isomers of each cage size EMF will be most stable.  A model is proposed in which the pentagons of fullerene anions are seen as charge localization centers that repel one another, making the pyracyclene bonding motif much more unstable for fullerene anions than for fullerene neutral cages. Computational treatments are also presented for two newly discovered EMFs, Y2C2@C92 and Gd2@C79N.  Y2C2@C92 is reported to exhibit a previously undiscovered mode of internal cluster rotation, while Gd2@C79N is shown to have unusual stability for an azofullerene with a large spin quantum number (15/2). Finally, computational techniques are employed to predict the thermodynamic feasibility of a chemical reaction replacing one metal atom in a trimetallic-nitride template (TNT) endohedral metallofullerene with different metal atom.  At least two of these are predicted to be thermodynamically practical. / Ph. D.

We investigate the effect of the substitution of Ni, Ti and Co in MnBi using first principles calculations based on density functional theory (DFT) within the generalized gradient approximation (GGA). We also performed total energy calculations to compare different structures to determine the ground state structures and investigate their magnetic properties. Our calculation shows that the substitution of Ni, Co and Ti lowers the total magnetization of MnBi. We also found that the stable structure of Ni and Ti substitute is to replace Mn atoms in their regular site while the substitute Co is most stable when Co occupies the interstitial site of MnBi unit cell.