[Pw_forum] elphon.f90

[Sridhar Sadasivam]

Dear espresso users and developers,

I have a couple of clarifications regarding the implementation of
electron-phonon coupling in elphon.f90. I am not sure if I am understanding
things correctly, so I would really appreciate some help.

1. After the call to elphon.f90, I believe el_ph_mat (ibnd,jbnd,k,disp)
contains the electron-phonon matrix element for coupling between an
electron of wavevector k in band 'ibnd' to an electron of wavevector k+q in
band 'jbnd' due to the displacement pattern given by disp. The displacement
patterns are given by the perturbations stored in the data-file.q.xml
files. Is my understanding correct?

2. I would like to obtain the matrix elements for coupling corresponding to
a particular phonon eigenmode, say u_{q,nu}, where nu is the mode number.
To do this, I think I would just need to do a linear combination of the
coupling matrix elements for the various displacement patterns (contained
in el_ph_mat) where the coefficients of the linear combination are the same
coefficients needed to express the phonon eigenvector in terms of the
displacement patterns stored in data-file.q.xml file. Is this correct?

Thanks in advance.
Sridhar
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