Notebook 3 exposes students in a step-by-step fashion to the equations and practical aspects of a DFT code applied to the PIB system considered in Notebook 1. A detailed theoretical discussion is beyond the scope of this work but, in short, the Hohenberg-Kohn theorems state that (1) there exists a one-toone correspondence between the ground-state electron density and the external potential, which in turn specifies a Hamiltonian, and (2) a variational principle exists for the density, justifying the Thomas-Fermi 1,2 approach and modifications thereof. Equationally, the theorems state that instead of directly evaluating the electronic Schrödinger equation