Optical Response Of Finite And Extended Systems

We developed a method for the calculation of the optical response of infinite systems within time dependent density functional theory. The use of a linear combination of local orbitals and the explicit evolution of the system in time are the main characteristics of our method. It was implemented within SIESTA and is designed to treat large systems with reasonable accuracy and computational ease. We tested the method by calculating the optical response of C60, which is a very well studied system, and we proceeded by calculating the optical response of hydrogen-passivated Si and Ge nanoclusters which exhibit many interesting properties and have many promising applications. Our method is non-perturbative and in principle we can get the non-linear optical response. Along these lines we developed a second method by which we can calculate the non-linear response of the system to an external time dependent field. As an application we calculated the non-linear response of C60 to a step function external electric field. In addition, we studied the effect of the coupling of the forward- and backward going electron hole pairs on the static local field factor of jellium with and without a gap, using the Bethe-Salpeter equation. We only considered diagrams where the electron hole pair interact, via a statically screened Coulomb interaction, and we summed these diagrams to infinite order. First we coupled forward- and backward going electron-hole pairs and compared these results with corresponding results obtained when coupling is omitted, and concluded that the coupling of both kinds of electron-hole pairs is relevant for the most accurate calculation using the Bethe- Salpeter equation.U of I OnlyAuthor has not yet granted open access permissio