A transition occurring only in 'single-valent' systems from strongly correlated (U > W) to weakly correlated (U < W) electrons as a result of a change of bandwidth W. W reflects the strength of the interatomic interactions in a periodic array of like atoms and U is a measure of the intra-atomic interactions, i.e. the electrostatic energy involved in the creation of polar states by transferring an electron from one atom to the next, as shown schematically by the disproportionation reaction: \[\text{M}^{n+}+\text{M}^{n+}\rightarrow \text{M}^{(n+1)+}+\text{M}^{(n- 1)+}\] A U > W produces a magnetic semiconductor; a U < W gives rise to a metal (normally nonmagnetic). Note also that U and W can be altered by changes in interatomic distances, brought about through temperature or pressure variations or by introducing an alloying element. Synonymous with Mott–Hubbard transition.