The 21st century has seen theoretical computational chemistry reach such a great level of accuracy and computational speed that enormous chemical insights can be gained from routine quantum chemical calculations. In this thesis I discuss a number of applications where computational chemistry has played a role in understanding experimental work. One such work investigated the modes of degradation of piperidinium functionalized polyethylene in basic solution, where density functional theory showed that the local polyethylene backbone morphology plays a critical role in determining the primary mode of degradation. In another work I cover our investigation of the enantioselective hydrocyanation of conjugated alkenes via an electrocatalytic reaction, where density functional theory and transition state theory are used to rationalize the preferential formation of the R-enantiomer. In addition, I discuss insights modern theoretical chemistry tools give into the fundamental nature of ion-pi interactions. I then discuss the creation and use of benchmark databases to train/test approximations in computational chemistry. The first of these databases is NENCI-2021, comprised of non-equilibrium non-covalent interactions. The next such database is QM7-X, a large database of quantum-mechanical properties containing ~4.2 million molecular conformations. Finally, I discuss new state-of-the-art density functional approximations built to both fit benchmark data and satisfy known exact physical constraints.