  We discuss how to construct a tight binding model Hamiltonan for the simplest
possible solid, composed of hydrogen-like atoms. A single orbital per atom is
not sufficient because the on-site electron-electron repulsion mixes in higher
energy orbitals. The essential physics is captured by a dynamic Hubbard model
with one electronic orbital and an auxiliary spin degree of freedom per site.
We point out that this physics can lead to a substantial shift in the position
and width of electronic energy bands relative to what is predicted by
conventional band structure calculations.
