[Pw_forum] k-mesh vs q-mesh

[A. Jain]

Hi All,

I have a query regarding selecting the size of q-grids to use when 
trying to obtain phonon dispersion. Obviously there is a difference 
between k-mesh and q-mesh as they are used to calculate different 
properties, as referenced in this thread from 2012: 
http://qe-forge.org/pipermail/pw_forum/2012-January/097425.html.

However my question is, how do we know the quality of a q-mesh that we 
have selected? Is that simply from the phonon dispersion and asking 
matdyn to calculated frequencies at a phonon point not in the grid then 
checking to see how well it fits, or is there another way where complete 
phonon frequencies do not have to be calculated to determine the quality 
of a q-mesh?

Also adding onto that, when selecting a q-mesh how does one go about it? 
Is it ok to simply use the same grid size as that of the k-mesh (if not, 
how come?) and can the size of the k-mesh used give any clues as to what 
size q-mesh to use?

Best,

Apoorv Jain
PhD Student
Department of Chemical Engineering and Biotechnology
University of Cambridge