# Volume IV — Electronic Structure

The previous volumes treated atoms as classical particles on a potential surface.
That potential comes from the electrons, and computing it is the business of
*electronic-structure theory*: solving the quantum many-electron problem, usually
by density-functional theory, to obtain energies, forces, and the band structure
that decides whether a material is a metal, a semiconductor, or an insulator.

These notebooks are **code-essential**: density-functional theory cannot be
reimplemented in a teaching notebook, and the real workflow — running Quantum
ESPRESSO or CP2K and analysing the output — is the point. So here we show the real
input decks and analyse the **real committed output** of the course calculations.

```{admonition} Source lectures
:class: note
This volume develops **Lecture 5** of *Molecular and Materials Modelling*
(FS 2023) — *band theory of crystals* — and **Lecture 6** — *molecular orbital
theory*. The remaining density-functional lectures land where their systems
live: Lecture 7 in Volume VI and Lecture 8 in Volume V. Each notebook repeats its specific lecture provenance in its
overview; the full lecture map is in `manifest.yml`.
```

```{tableofcontents}
```