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[Xiaohong Zheng <zheng@chips.ncsu.edu>]

Hello, Dear Matteo,

Thank you very much for your great help. In my mind, GW calculation will 
use the wave functions and eigenvalues obtained in the SCF step.  But 
according to your explanation, it seems that the wave functions created 
in the SCF step will not be used in screening and sigma calculations. 
The wave functions |k-q,b1> and |k,b2> in M := <k-q,b1| e^{-i(q+T).r} 
|k,b2>  will be obtained by another diagonalization of the the 
Hamiltonian matrix after the scf has been reached? And the size of 
Hamiltonian matrix for getting |k-q,b1> and |k,b2> will be much smaller 
that the one in the original scf calculations? And this Hamiltonian 
matrix is determined by self-consistent hamiltonian and ecutwfn? Is ecut 
only used to get the self-consistent potential and charge density and no 
wave functions of the output of the SCF step will stored or used in GW 
calculations?

In the tutorial, there several exercises, like

*"Convergence on the number of planewaves in the wavefunctions to 
calculate the Self-Energy."
*


     *"Convergence on the number of planewaves to calculate Sigma_x."*

*"Convergence on the number of planewaves in the wavefunctions to 
calculate the screening (epsilon^-1)."

Does it mean that different wave functions will be used **and be 
recalculated **be separate diagonalization of the Hamiltonian matrix 
determined by ecutwfn and ecutsigx when we can calculate these quantities?

*Thank you again!

Xiaohong

Matteo Giantomassi wrote:
> On Sun, 28 Oct 2007, Xiaohong Zheng wrote:
>
> > Hello, every one,
> >
> > what is the difference between "ecut" and "ecutwfn" in GW 
> > calculations? They look like two cutoff energies of wave functions. 
> > What are the difference between them? Why do we need two different 
> > sets of wave functions in the GW calculations?
>
> Dear Xiaohong,
>
> To perform a GW calculation we need to evaluate matrix elements in the 
> form
>
> M := <k-q,b1| e^{-i(q+T).r} |k,b2>
>
> where the number of reciprocal lattice vectors T is defined by ecuteps 
> in case of a screening calculation (optdriver 3) or npwsigx in case of 
> sigma calculations (optdriver 4).
>
> Ecutwfn (nwpwfn or nshwfn) is instead used to define the G basis set 
> employed to describe the two wavefunctions |k-q,b1> and |k,b2>, and is 
> usually smaller than ecut since the matrix elements M
> converge faster that the wavefunctions, see the tutorial.
>
> In you particular case, the variable ecut is only used during the GS 
> or NSCF calculation that precedes the generation of the KSS file. The 
> value of ecut, roughly speaking, defines the number of Fourier 
> coefficients written on the KSS file.
>
> Note that ecut has no meaning during a screening or sigma run, anyway 
> you have to specify a value in the input
> file otherwise the code crashes before entering the GW part.
>
> Hope this helps,
> Best Regards
> Matteo Giantomassi