Probing Electronic and Optical Properties of Complex Chemical and Material Systems
Large, complex chemical and material systems are extremely difficult to calculate with current density functional (DFT) based quantum calculation tools, due to their computational cost and due to their sensitivity to the choice of exchange correlation functionals. While classical methods can treat large material systems, they fail to account for quantum effects. In the first part of this thesis, I utilize the density functional tight-binding methodology to explore in detail the optical and excitation energy transfer properties of large plasmonic nanoantenna systems. For nanoantennas with large interparticle distances, we analyze the extremely long-ranged nature of electronic couplings in plasmonic systems. Additionally, for nanoantennas with subnanometer interparticle spacings, we observe a dramatic change in the nature of electronic couplings which reduces the energy transfer efficiency. Consequently, both these results have important ramifications for predicting and analyzing energy transfer in plasmonic systems. Our calculations show that classical models, which ignore quantum effects, are inadequate for accurately characterizing excitation energy transfer in plasmonic systems. Overall, these findings provide a real-time, quantum-mechanical perspective for understanding EET mechanisms and guide the enhancement of plasmonic properties in energy harvesting and transport systems. In the next part of the thesis, I present a detailed analysis of numerous DFT functionals for calculating polarizabilities of conjugated chain molecules and the chemical and radiation stability of ionic liquids. Specifically, we find that enhanced accuracy can be obtained with range-separated functionals by allowing the system to relax to lower-energy broken-symmetry solutions. In addition, our calculations also show that the ωB97XD range-separated functional is the most internally consistent method for calculating chemical and radiation stabilities of ionic liquids. Ultimately, this thesis emphasizes the importance of including quantum effects and range-separated functionals for accurately calculating the electronic properties of large material and complex chemical systems.