John S Tse
I would like to confirm in a BSE calculation interband transitions between different k points are not computed, i.e. no indirect transitions. For example the indirect gap transition in Si is not computed.
John Kay Dewhurst
The BSE matrix has coupling between different k-points in order to allow for excitonic states. For the calculations of the the BSE dielectric function, only vertical transitions are allowed, i.e. using the dipole approximation.
Hope that helps.