The term strong correlation refers to behavior of electrons in solids that is not well-described (often not even in a qualitatively correct manner) by simple one-electron theories such as the local-density approximation (LDA) of density-functional theory or Hartree–Fock theory. For instance, the seemingly simple material NiO has a partially filled 3 d band (the Ni atom has 8 of 10 possible 3 d -electrons) and therefore would be expected to be a good conductor. However, strong Coulomb repulsion (a correlation effect) between d electrons makes NiO instead a wide- band gap insulator. Thus, strongly correlated materials have electronic structures that are neither simply free-electron-like nor completely ionic, but a mixture of both.