LADDER

LADDER = [logical]
Default: LADDER = .TRUE. 

Description: Controls whether the ladder diagrams are included in the BSE calculation.

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LADDER is used together with LHARTREE. If LADDER=.FALSE., the ladder diagrams (i.e. the exchange terms related to $W$ or the screened exchange) are not included. If HARTREE=.FALSE., the Hartree diagrams or bubble diagrams are not included. The following table summarizes all possible combinations: \begin{tabular}{lll} {\tt LHARTREE} & {\tt LADDER} & \\ {\tt .TRUE.} & {\tt .TRUE.} & full BSE \\ {\tt .FALSE.} & {\tt .TRUE.} & only excitonic effects (ladders) \\ {\tt .TRUE.} & {\tt .FALSE.} & random phase approximation (rings = bubbles only) \\ {\tt .FALSE.} & {\tt .FALSE.} & independent particle \\ \end{tabular}

The last combination can be useful for sanity checks: the results must be identical to the results obtained using [[{{{1}}}]] 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 BSE explains how to recalculate the WAVEDER file from an existing WAVECAR file.

Related Tags and Sections

LHARTREE, LOPTICS

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