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: $${\text{M}}^{n+}+{\text{M}}^{n+}\to {\text{M}}^{(n+1)+}+{\text{M}}^{(n-1)+}$$ 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.