ELPH_USERECIP
ELPH_USERECIP = logical
Default: ELPH_USERECIP = .FALSE.
Description: Computes electron-phonon matrix elements in reciprocal space instead of real space.
| Mind: Available as of VASP 6.5.0 |
The electron-phonon matrix element
- [math]\displaystyle{ \langle \psi_{n\mathbf{k}} | \Delta V_{\mathbf{q}} | \psi_{m\mathbf{k}'} \rangle }[/math]
can be evaluated either in real space (the default) or in reciprocal space as a contraction over plane-wave coefficients. ELPH_USERECIP = .TRUE. selects the reciprocal-space path.
Automatic activation for meta-GGAs. When a meta-GGA functional is used, VASP sets ELPH_USERECIP = .TRUE. automatically. This is required because the kinetic-energy density contribution to the electron-phonon potential is non-local in real space and can only be evaluated in reciprocal space.
Band structure renormalization. The reciprocal-space path can also be beneficial for calculations where the number of bra k-points ([math]\displaystyle{ n_\mathbf{k} }[/math]) is much smaller than the number of ket k-points ([math]\displaystyle{ n_{\mathbf{k}'} }[/math]), such as band structure renormalization along high-symmetry lines. In this case the real-space path would require an FFT for each bra state, whereas the reciprocal-space path works directly with the stored plane-wave coefficients and avoids these transforms.