The speaker says the Kohn–Hohenberg theorem allows the ground state of a quantum-mechanical system to be treated through its charge density rather than an exponentially large space.

“Cone and Hohenberg realized that turns out you don't need to deal with an exponential space,”

“all you have to do is think about the charge density,”

“at least for the ground state of quantum mechanical systems. So it's actually the theorem is pretty simple, so even I think an undergraduate in physics can understand,”