Rung-singlet charge-ordering in alpha'-NaV2O5

We show that the proposed zig–zag charge–ordering model for alpha’–NaV2O5 is incompatible with a single transition that induces both charge–ordering and spin gap. We introduce a two–band model for alpha’–NaV2O5 within which the simultaneous charge–spin order transition is driven principally by electron-phonon interactions. The spin gap is due to the formation of local dimers, thus explaining the weak magnetic effect on this transition.