We have suppressed the explicit spin variables so far. The spin structure of the BSE solutions depends on the spin-orbit coupling. If the ground state is a spin singlet state and spin-orbit coupling is small compared to the electron-hole coupling, the single particles can be classified as spin-up state or spin-down state. It can be shown that there are two distinct types of solutions to the BSE: spin-singlet and spin-triplet excitations. This allows in turn, to solve the BSE separately for the spin type of interest, with