= Electronic structure
<Electronic structure> describes the quantum states and spatial distribution of a system's <electrons>, including their energies, occupancies and correlations. It determines much of the <chemical bonds>, optical response and reactivity of <atoms>, <molecules> and solids.
An <electron configuration> is one compact approximate description, but it is not the full many-electron state. In molecular calculations, <molecular orbitals> and <electron density> reveal where charge is distributed; a many-electron <wave function> also encodes correlations. In a <crystal>, <energy bands> express how allowed electronic energies depend on <wavevector>. Approximations such as <Hartree-Fock theory> and <density functional theory> address different representations of the same underlying quantum problem.
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