View source for ELPH NBANDS SUM

{{elph_release}}
{{DISPLAYTITLE:ELPH_NBANDS_SUM}}
{{TAGDEF|ELPH_NBANDS_SUM|[integer array]|{{TAG|ELPH_NBANDS}}}}

Description: Number of intermediate states to include in the computation of the phonon-induced electron self-energy.

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The computation of the self-energy is achieved via a sum over intermediate states <math>|\Psi_{m \mathbf{k} + \mathbf{q}}\rangle</math>.
{{TAG|ELPH_NBANDS_SUM}} specifies the maximum number of bands, <math>N_{\text{b}}</math>, such that <math>m</math> runs from <math>1 \ldots N_{\text{b}}</math>.

Multiple values can be specified for {{TAG|ELPH_NBANDS_SUM}}, in which case the self-energy is computed once for each value.
The results are reported in separate groups inside the {{FILE|vaspout.h5}} file:
<pre>
/results/electron_phonon/electrons/self_energy_1
/results/electron_phonon/electrons/self_energy_2
/results/electron_phonon/electrons/self_energy_3
...
</pre>

This tag is useful for studying the convergence of the self-energy with respect to the number of intermediate states.
At a certain point, including more bands in the summation over states should no longer change the result.
{{NB|mind|When computing the renormalization of the electronic bandstructure, a large number of intermediate states may be necessary to reach convergence. If the self-energy still changes noticeably around {{TAGO|ELPH_NBANDS_SUM|{{TAG|ELPH_NBANDS}}}}, then you may have to increase {{TAG|ELPH_NBANDS}}.}}