Figure 12: Graphical representation of the single-photon Bloch equation (66) (c.f. Figure 3). The last diagram represents the “folded” term, that is, the last term of the equation. This equation can be compared with the Bethe-Salpeter equation, valid only in the single-reference case, where there is no folded contribution. The order-by-order expansion of this equation is illustrated in Figure 11 [6, Figure  8.12].