# [Pw_forum] Questions about the DFPT review paper

Stefano Baroni baroni at sissa.it
Sat Oct 27 10:31:51 CEST 2012

On Oct 25, 2012, at 2:38 AM, Caloma Trumica wrote:

> Dear all,
>
> I do not understand why the two highlighted places in the attached DFPT review paper are correct.
>
> I repeat my questions here:
>
> 1. Why the contributions from valence bands cancel each other?

Because the mn and nm terms have the same magnitude and opposite sign. This is obvious when the orbitals are real. It requires some thinking when they are not. (in the absence of a magnetic field they can always be chosen as real).

> 2. Why the left side of Eq.25 has a null eigenvalue?

because \epsilon_v is (by definition) an eigenvalue of H_{SCF}

SB

---
Stefano Baroni -  http://stefano.baroni.me, stefanobaroni (skype)
on leave of absence from SISSA, Trieste, presently at the Department of Materials, EPF Lausanne (untill March 2013)

La morale est une logique de l'action comme la logique est une morale de la pensée - Jean Piaget

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