The exact exchange-correlation kernel is a functional derivative of the exact time-dependent exchange-correlation (XC) potential with respect to the time-dependent density, evaluated at the ground-state density. As the XC potential is not known, the exact kernel is also unavailable. Therefore, it must be modeled either using many-body perturbation theory or by satisfying the exact constraints for various prototype systems such as the paradigm uniform electron gas (UEG). The random phase approximation (RPA) neglects the kernel, therefore, fails to provide the accurate ground- and excited-state properties for various systems from a simple uniform electron gas to more complex periodic ones. There are numerous corrections to RPA available, including kernel-corrected RPA, often called the beyond-RPA (bRPA) methods. In this work, we employed various bRPA methods for a diverse set of systems together with RPA. At first, we applied RPA based methods to study the phase stability of the cesium halides. Cesium halides phase stability is one of the stringent tests for a density functional approximation to assess its accuracy for dispersion interaction. Experimentally, CsF prefers the rocksalt (B1) phase, while the other halides CsCl, CsBr, and CsI prefer the cesium chloride (B2) phase. Without dispersion interaction, PBE and PBE0 predict all halides to prefer the B1 phase. However, all RPA based methods predict the experimental observations. The bRPA methods usually improve the quantitative prediction over RPA for the ground-state equilibrium properties of cesium halides. Next, we explored binary intermetallic alloys, where we showed that RPA successfully predicts the accurate formation energies of weakly bonded alloys. However, a kernel corrected RPA is needed when dealing with strongly bonded alloys with partially filled d-band metals. We utilized the renormalized ALDA (rALDA) and rAPBE kernel as bRPA methods. Exact constraints and appropriate norms such as the uniform electron gas are very useful to construct various approximations for the exchange-correlation potentials in the ground-state, and the exchange-correlation kernel in the linear-response theory within the TDDFT. These mathematical formulations not only guide us to formulate more robust nonempirical methods, but they also have more predictive power. We showed the importance of these constraints by calculating plasmon dispersion of the uniform electron gas using the non-local, energy-optimized (NEO) kernel using only a few constraints. More predictive power comes with more constraint satisfaction. As a result, we developed a new wavevector- and frequency-dependent exchange-correlation kernel that satisfies all the constraints that it should satisfy with a real frequency. It gives accurate ground-state correlation energy and describes the charge density wave in low-density UEG. It also predicts an accurate plasmon dispersion with a finite lifetime at wavevectors less than the critical one, where the plasmon dispersion meets the electron-hole continuum. / Physics

This work investigates the correlation of structural and photovoltaic properties of dyes used in dye-sensitized solar cells. Experimental methods, including ultraviolet-visible spectroscopy, fluorescence spectroscopy, cyclic voltammetry and electrochemical impedance spectroscopy are employed to study optical and electrochemical properties of dye molecules. Computational methods, including density functional theory and time-dependent density functional theory, are used to validate and predict the optical and electronic properties of dye molecules, in their isolated state and once embedded into a working electrode device environment that comprises a dye...TiO2 interface. The results chapters begin with the presentation of a series of quinodimethene dyes that are experimentally validated for their photovoltaic application, and associated computational studies reveal that an inner structural factor - a phenyl ring rotation occurring during the optical excitation process - leads to the competitive photovoltaic device performance of these dyes. Carbazole-based dyes are then systematically studied by computation, especially considering charge transfer paths and binding modes of these dyes on a titania surface. The theoretical models for the basic building block of this chemical family of dyes, known as MK-44, successfully support and explain structural discoveries from X-ray diffraction and reflectometry that impact of their function. A benzothiadiazole-based dye, RK-1, is then systematically studied by both experimental and computational methods, and the results show that the π-bridge composed of thiophene, benzothiadiazole and benzene rings leads to excellent charge separation; and the rotation of these rings during the optical excitation process may well be consistent with the fluorescence spectrum. Finally, the well-known ruthenium-based dyes are theoretically studied to determine the properties of different ligands connected to the metal core of the complex. Conformations with different NCS ligands are calculated in terms of energy and explain well the corresponding results from X-ray diffraction. Acid-base properties of carboxyl groups connected to pyridine ligands in N3 and N749 are theoretically calculated based on thermodynamics and density functional theory. Implicit and explicit models are both adopted to predict these acid dissociative constant values, which are generally in a good agreement with the reported experimental data. The thesis concludes with conclusions and a future outlook.