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

Vikas Varshney vv0210 at gmail.com
Mon Jun 27 15:02:44 CEST 2011

Dear Pablo, Stefano

Thank you very much for the clarification.

Best Regards,
Vikas

On Sun, Jun 26, 2011 at 9:15 AM, Stefano de Gironcoli <degironc at sissa.it> wrote:
> 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 the second part of the sentence is obsolete and referred to the case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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>
>
>
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