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["Matteo Giantomassi" <Matteo.Giantomassi@uclouvain.be>]

> we can obviously have  that ecuteps  controls the dimension of
> $G\times G$ for  $\chi_0$ in reciprocal space.
> My question is what does ecutwfn  account for $\chi_0$ ?  From the
> equation
> for $\chi_0$ i donot find any extra parameters  to reach final result.
> This
> setting really makes me confusion, any hints and comments are appreciated.

 ecutwfn defines the number of plane waves used to represent the wave
functions
 entering the definition of the oscillator matrix elements

> \begin{equation}
> M_{kij}(q+G)=<k-qi|e^{-i(q+G)r}|kj>
> \end{equation}

 The oscillators are calculated using a two-step procedure.

 1) First one performs a FFT, from G to r space, of the two kets
    |k-q,i>  and |kj>  where the number of employed G-vectors
   is equal to npwwfn.

 2) The product u^*_{k-q,i}(r) u_{k,j}(r)
   is constructed on the real space FFT mesh,
   and Fourier transformed to reciprocal space to obtain M_{kij}(q,G)

 Regards,
 Matteo Giantomassi