The speaker contrasts three-dimensional charge density with the exponentially large Hilbert spaces and wave functions used in quantum mechanics.

“Yeah, which is amazing because charge density is just this three-dimensional object, whereas the Hilbert space and wave functions are exponential. Um so that was a really interesting observation and then Cohen published another paper, I think with his post-doc uh Sham, Cohen-Sham uh wave functions kind of had a practical way to solving s”