# [Pw_forum] Clarification of a confusion related to q vector specifications in ph.x

Stefano de Gironcoli degironc at sissa.it
Sun Jun 26 15:15:14 CEST 2011

```Dear Vikas Varshney ,

the second part of the sentence is obsolete and referred to the case
in which the single point calculation was done combining a nscf call
to pw.x and one to ph.x... the two call ought obviously to be
consistent.

As stated somewhat above in the manual page you linked
"NOTE: the alternative procedure in which a band-structure calculation
with calculation='phonon was performed as an intermediate step is no
longer implemented since version 4.1. It is also no longer needed to
specify lnscf=.true. for q\$ \ne\$ 0 . "

so just specify the q point you desire and the code wil run the
nscf+dfpt calculation in sequence.

stefano

Quoting Vikas Varshney <vv0210 at gmail.com>:

> Dear QE usesrs,
>
> I wanted to have a clarification on the following sentence from the
> phonon calculations part of the manual: "After the namelist you must
> specify the q-vector of the phonon mode. This must be the same
> q-vector given in the input of pw.x.". The link for the following text
> is given below.
> http://www.quantum-espresso.org/user_guide/node31.html
>
> In ph.x, we specify three values for q-vector (0,0,0) for gamma point,
> for example.
>
> However, in pw.x, the k points are often specified as
>
> K_POINTS automatic
> 4 4 2 0 0 0.
>
> I wanted to understand if these are related. If not, what does the
> above sentence mean. I am not sure but it might be a silly question to
> ask but I am trying to clear it in my head.
>
> Any insights will be highly appreciated.
>
> Best Regards,
>
> Vikas Varshney
> Computational Materials Scientist,
> Wright Patterson Air Force Base,  Dayton OH
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