ELPH SELFEN BAND START KP

ELPH_SELFEN_BAND_START_KP = [integer]
Default: ELPH_SELFEN_BAND_START_KP = -1 

Description: Index of the first [math]\displaystyle{ k' }[/math] band included in the electron self-energy sum; -1 uses all bands, and -2 activates automatic determination based on energy conservation.

Mind: Available as of VASP 6.6.0

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The Fan-Migdal self-energy involves a sum over intermediate scattering states at [math]\displaystyle{ k' = k + q }[/math]:

[math]\displaystyle{ \Sigma^{\text{FM}}_{n\mathbf{k}}(T) = \frac{1}{N_q}\sum_{m\nu\mathbf{q}} |g_{mn\mathbf{k},\nu\mathbf{q}}|^2 \left[\frac{\cdots}{\varepsilon_{n\mathbf{k}} - \varepsilon_{m\mathbf{k}+\mathbf{q}} \pm \omega_{\nu\mathbf{q}} + i\delta}\right] }[/math]

ELPH_SELFEN_BAND_START_KP and ELPH_SELFEN_BAND_STOP_KP control the range of band indices [math]\displaystyle{ m }[/math] (at [math]\displaystyle{ k' }[/math]) included in this sum. This is distinct from ELPH_SELFEN_BAND_START and ELPH_SELFEN_BAND_STOP, which select the bands [math]\displaystyle{ n }[/math] at [math]\displaystyle{ k }[/math] for which the self-energy is computed.

The following values are accepted:

• -1 (default): include all bands from 1 to ELPH_NBANDS.
• -2: determine the range automatically from energy conservation. Only [math]\displaystyle{ k' }[/math] bands whose eigenvalues fall within ±ω_max of the target [math]\displaystyle{ k }[/math] bands are included, where ω_max is the maximum phonon energy. This can significantly reduce the cost when ELPH_NBANDS is large.
• Positive integer: use the specified band index as the lower bound.

Setting both ELPH_SELFEN_BAND_START_KP and ELPH_SELFEN_BAND_STOP_KP to -2 is recommended when a large number of bands is required for convergence but most of them lie far from the relevant energy window.

Related tags and articles

• ELPH_SELFEN_BAND_STOP_KP
• ELPH_SELFEN_BAND_START
• ELPH_NBANDS
• ELPH_RUN