Light-harvesting systems and plasmonic materials continue to garner significant attention due to their importance in both energy conversion and detection technologies. However, several challenges exist in improving their performance: these electronic multifunctional systems are coupled, many-body systems with complex electronic interactions with their surrounding environments. All of these processes occur at different time and length scales, and span an immense multi-scale space (i.e., chemical interactions within their environment, all in tandem with external light and electric fields). To this end, we have utilized a new computational approach based on density functional tight binding (DFTB) theory to directly probe and calculate these complex systems. This implementation allows us to calculate the electronic and structural properties of large systems (~10,000 atoms), whereas conventional DFT approaches are typically limited to only hundreds of atoms. Using this approach, I will highlight our work on photo-initiated dynamics in large plasmonic nanoantennas. In particular, these time-domain, quantum-mechanical studies provide an approach to probe real-time electron dynamics mechanisms to understand and tailor these complex material systems for realistic applications.