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The potential variation term in eph matrix http://epwforum.uk/viewtopic.php?f=7&t=706 
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Author:  Menglin [ Mon Sep 17, 2018 6:55 am ] 
Post subject:  The potential variation term in eph matrix 
Dear developers and users, According to "Giustino et al., Phys. Rev. B, 76, 165108 (2007)", the electronphonon matrix element is defined as: <mk+qpartial(qv)Vnk>, where the variation (perturbation) of the potential term is derived from the DFPT results implemented in Phonon code. 1. How is this variation term defined or what is the exact form of this variation? Because I thought any qpoint or phonon mode is irrelevant to the potential in position space, but they can be linked with normal modes in a phonon calculation. 2. I prefer to believe the potential term is expressed as: partial_V/partial_Q(qv), where Q is the normal coordinate. Am I right? 3. Is there any information about the normal coordinate in the code? Thank you! Best, Menglin 
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