Four-site four-electron model for Bi(III)-Bi(V) mixed valence complex

In most quantum models for the four-site four-electron problem the lowest singlet state has two short and two long bonds in the absence of lattice polarization and is called the resonating valence bond (VB) state. It is shown here that if the lattice polarization is large, the ground state is a ‘negative U’ state with valence disproportionation (for example BaBiO₃ with Bi(V) and Bi(III) sites). Furthermore the model shows that the coupling between the pairs on different sites is provided via the VB state.