[Pw_forum] electron-phonon at q=0

[Burak Himmetoglu]

Dear QE users,

Referring to example07 about electron-phonon cooupling, the calculation
ends up finding nonzero line-widths (gamma) at q=0 (printed in elph.
0.000000. 0.000000. 0.000000 file).
I find this rather odd, since the electron-phonon matrix elements "g"
should vanish at q=0. This can be seen from the explicit form of the
interaction term in Fourier expansion,
which contains a dot product of q with the phonon polarization vector.

Could anyone explain the reason for the nonzero values? Could it be related
to the fact that at q=0 acoustic sum rules not enforced in the calculation
of the line-width?

Regards,

Burak Himmetoglu

Post-Doctoral Associate
Materials Department
University of California at Santa Barbara
CA 93106
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