[QE-users] Momentum matrix element discontinuity at Gamma point (possible reference help?)

[L Y]

Dear QE users,

I have calculated the momentum matrix elements between valence and
conduction bands along symmetry lines in MgO. Everything seems OK, except
the fact that there is a noticeable discontinuity of the matrix element at
the Gamma point. I did extensive debugging, but could find anything wrong
(the band energies are fine and continuous without discontinuity). I
suspect I need to do something special for the momentum matrix element, but
I cannot figure out the location of the bug. Does anyone have a possible
suggestion?

The PP/src/compute_ppsi.f90 routine is crucial for this calculation, but I
dunno which formula is used there. A reference to a paper would be
greatly appreciated!

Best regards,
Lun Yue
Louisiana State University
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