ELPH_NBANDS_SUM

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ELPH_NBANDS_SUM = [integer array]
Default: ELPH_NBANDS_SUM = ELPH_NBANDS

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

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

Multiple values can be specified for ELPH_NBANDS_SUM, in which case the self-energy is computed once for each value. The results are reported in separate groups inside the vaspout.h5 file:

/results/electron_phonon/electrons/self_energy_1
/results/electron_phonon/electrons/self_energy_2
/results/electron_phonon/electrons/self_energy_3
...

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.

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 ELPH_NBANDS_SUM = ELPH_NBANDS, then you may have to increase ELPH_NBANDS.