These flags default both to .TRUE.. If LADDER=.FALSE., the ladder diagrams (i.e. the exchange terms related to or the screened exchange) are not included. If LHARTREE=.FALSE., the Hartree diagrams or bubble diagrams are not included. The following table summarizes all possible combinations:
LHARTREE | LADDER | |
.TRUE. | .TRUE. | full BSE |
.FALSE. | .TRUE. | only excitonic effects (ladders) |
.TRUE. | .FALSE. | random phase approximation (rings = bubbles only) |
.FALSE. | .FALSE. | independent particle |
The last combination can be useful for sanity checks: the results must be identical to the results obtained using LOPTICS = .TRUE in the preceding calculations. If this is not the case, it usually implies that the one-electron energies have been updated in the WAVECAR file, or that the WAVEDER file is not properly set up. The end of section 6.74 explains how to recalculate the WAVEDER file from an existing WAVECAR file.