The ABINIT Users Mailing List ( CLOSED )

[Matteo Giantomassi <gmatteo@pcpm.ucl.ac.be>]

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