Electronic structure theory: Approximations to the exchange-correlation energy and models for electron transport

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Electronic structure theory provides the basis for our understanding of much of chemistry, biochemistry, material science, condensed matter etc. It also provides the tools for computational modeling in the various areas mentioned. The proposed research aims at advancing several fields of electronic structure theory. One focus will be on density functional theory (DFT), where we address the exchange-correlation energy among electrons. This is the crucial component of Kohn-Sham DFT and it accounts for the complicated interactions between electrons. There has been enormous progress in this area which made Kohn-Sham DFT the most used tool in chemistry and the most cited topic in the history of the physical sciences. However, there remain numerous systems and properties for which the existing approximations are not reliable. Building on and continuing our previous work, our research program aims at deepening our understanding of the exchange-correlation energy and at improving upon existing exchange-correlation functionals. This will provide better computational tools, further simplifying the design of new materials and compounds in all areas of science and technology. Furthermore, we intend to continue our work in the area of molecular electronics where molecules are connected to contacts and the current that passes through the molecule is measured as a function of the applied voltage. In the past we developed simple models for molecular electronic devices that enabled us to predict new phenomena some of which have been verified experimentally. In the proposed research we want to improve and extend our theories. For example, we want to develop simple Hückel-based as well as sophisticated DFT-based models for coherent molecular photovoltaic cells. In these devices, the electron energy is increased upon traversing the molecule through the absorption of a photon. The entire mechanism of harvesting the photon energy is coherent, meaning that the process can be described by a single wave function that is a stationary solution of an eigenvalue equation. This coherent mechanism distinguishes molecular photovoltaic cells from conventional photovoltaic cells. Molecular photovoltaic cells might turn out to be more efficient to convert the energy of sunlight into electrical energy and with our research we explore the feasibility of such devices and provide guidance for their realization.