Density functional theory (DFT) is a powerful technique used in theoretical chemistry (and physics) that unfortunately can be quite difficult to explain.
A short (yet complete) introduction to DFT is given by Klaus Capelle as a very nice, freely accessible, educational material.
https://arxiv.org/abs/cond-mat/0211443
It also happen to be one of my favorite teaching material!
Any noteworthy educational contents about theoretical chemistry? Please let us know!