Complexes containing a transition metal atom with a 3d^4 - 3d^7 electron configuration typically have two low-lying, high spin (HS) and low spin (LS) states. The adiabatic energy difference between these states, known as the spin-crossover energy, is small enough to pose a challenge even for electronic structure methods that are well known for their accuracy and reliability. In this work we analyze the quality of electronic structure approximations for spin-crossover energies of iron complexes with four different ligands by comparing energies from self-consistent and post-self-consistent calculations for methods based on the random phase approximation and the Fermi-L\"{o}wdin self-interaction correction. Considering that Hartree-Fock densities were found by Song et al. J. Chem. Theory Comput. 14,2304 (2018) to eliminate the density error to a large extent, and that the Hartree-Fock method and the Perdew-Zunger-type self-interaction correction share some physics, we compare the densities obtained with these methods to learn about their resemblance. We find that evaluating non-empirical exchange-correlation energy functionals on the corresponding self-interaction-corrected densities can mitigate the strong density errors and improves the accuracy of the adiabatic energy differences between HS and LS states. / Physics

<p>Understanding the chemical and physical properties of molecules and materials at a fundamental level often requires quantum-mechanical models for these substance's electronic structure. This type of many body quantum mechanics calculation is computationally demanding, hindering its application to substances with more than a few hundreds atoms. The supreme goal of many researches in quantum chemistry-and the topic of this dissertation-is to develop more efficient computational algorithms for electronic structure calculations. In particular, this dissertation develops two new numerical integration techniques for computing molecular and atomic properties within conventional Kohn-Sham-Density Functional Theory (KS-DFT) of molecular electronic structure. </p> <p>The first of these grid-based techniques is based on the transformed sparse grid construction. In this construction, a sparse grid is generated in the unit cube and then mapped to real space according to the pro-molecular density using the conditional distribution transformation. The transformed sparse grid was implemented in program deMon2k, where it is used as the numerical integrator for the exchange-correlation energy and potential in the KS-DFT procedure. We tested our grid by computing ground state energies, equilibrium geometries, and atomization energies. The accuracy on these test calculations shows that our grid is more efficient than some previous integration methods: our grids use fewer points to obtain the same accuracy. The transformed sparse grids were also tested for integrating, interpolating and differentiating in different dimensions (n = 1, 2, 3, 6).</p> <p> The second technique is a grid-based method for computing atomic properties within QTAIM. It was also implemented in deMon2k. The performance of the method was tested by computing QTAIM atomic energies, charges, dipole moments, and quadrupole moments. For medium accuracy, our method is the fastest one we know of.</p> / Thesis / Doctor of Philosophy (PhD)

This dissertation covers the study of the thermoelectric properties of GeTe and SnTe. The goal of this research is to develop high-performance lead-free thermoelectric materials that can replace PbTe-based systems so that thermoelectric technology could be bring into real application. During the study, extensive investigations on the electrical and thermal transport behaviors were conducted both experimentally and theoretically. In Chapter 1 ~ 3, the origin of thermoelectricity, modelling and characterization methods are discussed in detail. In Chapter 4, study on the thermoelectric properties of Bi, Zn and In co-doped GeTe was presented. Initial doping with Bi enhanced the performance by tuning the electronic properties and bringing down the thermal conductivity. Subsequent Zn doping permitted to maintain the high power factor by increasing carrier mobility and reducing carrier concentration. Subsequent In doping boosted the density of state effective mass. A peak zT value of 2.06 and an average zT value of 1.30 have been achieved in (Ge0.97Zn0.02In0.01Te)0.97(Bi2Te3)0.03. In Chapter 5, we thoroughly investigated the transport properties of SnTe-Sb2Te3 alloying system, provided useful insight of the mechanism of the enhanced Seebeck coefficient. To also overcome the poor carrier mobility, Pb compensation was performed which effectively optimized the carrier mobility. Meanwhile, Pb compensation broke the charge balance, allowing Sb to precipitate out of the structure. These second-phase particles provided additional source of phonon scattering, effectively suppressing the lattice thermal conductivity. As a result, a peak zT of 1.1 at 778K and an average zT of 0.56 from 300K to 778K was achieved in (Sn0.98Ge0.05Te)0.91 (Sb2Pb0.5Te)0.09, which is one of the best SnTe-based thermoelectric systems. / Thesis / Master of Science (MSc) / Thermoelectric materials can generate energy from temperature gradient, making them potential solutions for the escalating energy crisis. The state-of-the-art thermoelectric material is PbTe which shows outstanding performance and high stability. However, the toxicity of Pb element limits its practical application. It is the purpose of this work to develop high-performance GeTe and SnTe-based thermoelectrics to reduce the usage of PbTe. Combining theoretical calculations and experimental characterizations, detailed investigation on the transport properties, crystal structure and microstructure were performed on both GeTe and SnTe. Relations between their thermoelectric properties and their composition, synthesis method and microstructure were revealed. This work paves the path for the development of environmentally friendly and high-performance thermoelectric systems.

In the search for nano-scale, highly customizable materials for next-generation electronic devices, two-dimensional (2D) materials have generated much interest. 2D materials have complex, layer-dependent optical and electronic properties of which many aspects remain yet to be explored and fully understood. The aim of this thesis was to investigate and explain optoelectronic properties of several 2D materials systems towards device design. This was accomplished using predictive physical modelling at the density functional theory level (DFT) as well as many-body theory (GW+BSE). The optical transitions of bulk ReS2 and ReSe2 were studied using DFT in comparison with experiment. We found that the orbital composition of the band edges determined the sign of the pressure coefficient of the optical gap. Our results provide a step towards understanding the perceived layer-independence of the optical properties of ReS2 and ReSe2. The exciton landscape of MoS2 monolayer was explored in detail using many-body theory (GW+BSE). We found dark excitons very close to bright excitons and even lower in energy. Our results help reverse the common assumption that the lowest-energy exciton in MoS2 is bright. The ideal band offset between recently predicted monolayers of the CaP3 family was predicted using GW theory. We observed chemical trends in the band offsets and explained their origin. Our results serve as indicators for heterojunction design with these novel materials. The effective mass of a test set of eighteen semiconductors including several 2D materials was calculated using DFT with semi-local and non-local hybrid exchange-functionals and compared for accuracy with respect to experimental data. Our analysis details the effect of the nonlocal exchange potential on the accuracy of the effective mass. Our results give guidelines for high-throughput calculations of the effective mass for different material classes, including 2D materials. / Thesis / Doctor of Philosophy (PhD)

No description available.

No description available.

No description available.

No description available.

No description available.

No description available.