[Pw_forum] question on q2r.f90

[Stefano de Gironcoli]

in q2r one declares a regular grid and then provide a number of matrices 
that should correspond to that grid.
This is what is checked in that part of the code.
If this is the error you get, some of your q-points does not correspond 
to the grid you declared.

Stefano de Gironcoli - SISSA and DEMOCRITOS

PS: Please provide your affiliation

Guoying Gao wrote:
> Dear everyone,
>       When I read the routine q2r.f90 given in the pwtools directory,
> I find the following lines:
>   %%% %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
>         lq = .true.
>         do ipol=1,3
>            xq = 0.0
>            do icar=1,3
>               xq = xq + at(icar,ipol) * q(icar,nq) * nr(ipol)
>            end do
>            lq = lq .and. (abs(nint(xq) - xq) .lt. eps)
>            iq = nint(xq)
>            !
>            m(ipol)= mod(iq,nr(ipol)) + 1
>            if (m(ipol) .lt. 1) m(ipol) = m(ipol) + nr(ipol)
>         end do
>         if (.not.lq) call errore('init','q not allowed',1)
> %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
> I know the parameters at(icar, ipol) means the components of the
> crystal vectors, q(icar, np) is the coordinates of the qpoints and
> nr(ipol) are the dimensions
> of the FFT grid formed by the q-point grid. What I don't understand is
> why the three parameters multiply and sum for x, y and z
> components:that is,
>           do icar=1,3
>               xq = xq + at(icar,ipol) * q(icar,nq) * nr(ipol)
>            end do
> and then there is another line: lq = lq .and. (abs(nint(xq) - xq) .lt. eps
> what makes (abs(nint(xq) - xq) should be less than eps.
> Is there anyone familiar with this part? Any small suggestions may be
> a great help for me.
> Thanks for your help!
> Best wishes,
> Guoying
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