Here, K x is the repulsive exchange interaction originating from the bare Coulomb term v C , while the direct interaction K d contains the attractive, but screened, interaction W between electron and hole, causing the binding of the electron-hole pair. In Eq. ( ) it is assumed that the dynamic properties of W (ω) are negligible, and the computationally less demanding static approximation ω = 0 is employed. If off-diagonal blocks K in Eq. ( ) are small, the additional use of the Tamm-Dancoff Approximation (TDA) is convenient, in which the electron-hole amplitude is expressed only as resonant transitions from occupied v to unoccupied c states: