Physical Sciences and Mathematics Commons^™

Theoretical Investigation Of Pressure-Induced Structural Transformations Of Ethylenediamine Bisborane, Rongfeng Guan

Electronic Thesis and Dissertation Repository

High-pressure experiments on hydrogen-rich compounds provide crucial data for the rational design of hydrogen storage materials. Ethylenediamine bisborane (EDAB), BH₃·NH₂CH₂CH₂NH₂·BH₃, is one of the prime candidates for this role due to its high hydrogen content (10 wt%) and good kinetic stability under ambient conditions. Previous studies of EDAB using in situ Fourier-transform infrared (IR) spectroscopy, Raman spectroscopy, and synchrotron X-ray diffraction (XRD) techniques suggested that EDAB undergoes two possible phase transitions in the pressure range of 0 to 17 GPa. However, the crystal structures of the two new …

Construction And Analysis Of Accurate Exchange-Correlation Potentials, Sviataslau V. Kohut

Electronic Thesis and Dissertation Repository

Practical Kohn–Sham density-functional calculations require approximations to the exchange-correlation energy functional, E_XC[ρ], or the exchange-correlation potential, v_XC(r), defined as the functional derivative of E_XC[ρ] with respect to the electron density, ρ. This thesis focuses on the following problems: (i) development of approximate exchange-correlation potentials by modelling the exchange-correlation charge distribution; (ii) accurate approximation of functional derivatives of orbital-dependent functionals; (iii) generation of exchange-correlation potentials from many-electron wavefunctions; (iv) analysis of accurate exchange-correlation potentials in atoms and molecules.

The advantage of modelling the exchange-correlation potential through the exchange-correlation …

Integration Of Orbital-Dependent Exchange-Correlation Potentials, Hanqing Zhao

Electronic Thesis and Dissertation Repository

In density-functional theory, one can approximate either the exchange-correlation energy functional or the corresponding Kohn--Sham effective potential, which is then converted into an energy functional by functional integration. A directly approximated potential may depend on the electron density explicitly or implicitly through Kohn--Sham orbitals. A potential that depends on the electron density explicitly can be converted into an energy functional by evaluating the Leeuwen--Baerends line integral along some path of electron densities. We extend this technique to orbital-dependent potentials by integrating them along the path of scaled orbitals. Using this method, we assign energy expressions to the Slater, Becke--Johnson and …

Computational Studies Of Compressed Diborane And Engineered Narrow-Gap Semiconductors, Amin Torabi

Electronic Thesis and Dissertation Repository

The research contained in this thesis is two-fold: understanding the behavior of diborane under pressure, and engineering wide-gap semiconductors in order to promote their optical eciency. Each of these themes are further explained below.

Diborane (B₂H₆), is a prototypical electron-deficient molecule and has received a great deal of attention in recent years due to its unique and peculiar structure, as well as its potential applications as a hydrogen-storage material. At high pressures, vibrational spectroscopy analysis have revealed several changes in the spectral profile that suggest occurrence of polymorphic transformations; however, the new crystal structures at high …

Approximation Of Exchange-Correlation Potentials For Orbital-Dependent Functionals, Alexei Kananenka

Electronic Thesis and Dissertation Repository

Density-functional theory (DFT) is the most widely used method of modern computational chemistry. All practical implementations of DFT rely on approximations to the unknown exchange-correlation functional. These approximations may be devised in terms of energy functionals or effective potentials. In this thesis, several approximations of the latter type are presented.

Given a set of canonical Kohn–Sham orbitals, orbital energies, and an external potential for a many-electron system, one can invert the Kohn–Sham equations in a single step to obtain the corresponding exchange-correlation potential, vXC(r). We show that for orbitals and orbital energies that are solutions of the Kohn–Sham equations with …