The ability to tune electronic properties in photovoltaics and nanomaterials holds great promise for their incorporation in next-generation transistors and nanoscale devices. In particular, the use of predictive first-principles calculations plays a vital role in rationally guiding experimental efforts to optimize energy harvesting in nanoscale and mesoscale materials. In this seminar, I will highlight recent work on energy transfer mechanisms in 3 areas of computational materials science. First, I will demonstrate that the electronic and optical properties in photovoltaic molecules can be accurately predicted by constructing new exchange-correlation functionals for time-dependent density functional theory (DFT). Next, the use of large-scale DFT calculations is presented to understand optical detection mechanisms in chromophore-functionalized carbon nanotubes. Finally, I will present a new theoretical approach to understand electron confinement in heterostructure nanowires. The reduced dimensionality in these nanowires dramatically changes their electronic structure, leading to novel properties such as ballistic transport and conductance quantization.