Quantum Dynamics
Chemistry exists at the edge between quantum and classical dynamics. Classical dynamics works for large things: the sorts of things you can see with your eye, and even all the way down to larger molecules. But small atoms, such as hydrogen, are light enough that quantum mechanics can have a large effect on their dynamics. This can play a role in everything from combustion reactions to hydrogen transfer in biological molecules. In addition, energy or charge transfer problems, such as photosynthesis or molecular electronics, often require including some quantum treatment of the dynamics.