To promote a single electron from the valence band to the conduction band, all that is required is to replace one of the one-electron orbitals in the Slater determinant part of the trial wavefunction that describes a state in the valence band with one that describes a state in the conduction band. i.e.
where the i 
ground state orbital has been replaced in the excited
state Slater determinant by the 
excited state orbital.