[Wien] Problem in the no. of U(Gx,Gy,Gz) & G-vectors

[Peter Blaha]

By default, lapw0 uses a very dense FFT-mesh (because of the 
xc-potential in GGA). This is not necessary for your purpose.

Use the ifflim routines to find the necessary FFT-dimensions for your 
case (see DO 40 loop in lapw0.F and subsequent ifflim calls). They 
should be (2*Gmax)+1 AND a multiple of 2,3,5 (because of FFT).

Another point which seems to confuse you: Whith higher symmetry, one 
often uses this symmetry and defines "Stars" of G; i.e. a set of 
G-vectors, which can be transformed into each other by symmetry 
operations (see Singh). The corresponding coefficients c(G) are 
identical or differ at most by a phase factor (non-symmorphic groups). 
WIEN2k stores usually these STAR-coefficients, but for an FFT one needs 
to expand that into a full 3D-array.
Expansion and contraction is done by the routines setfft and getfft, and 
in these routines you also see the corrrespondence between a G-vector 
and its position in the FFT-array.

> I have extracted c(G) coefficients from case.output1, so the only remained variable is theta(G).
> M_mn(k1, k2)=<u_m, k1|u_n, k2>= sigma_ij{ (c*(Gi)_m,k1) * theta(Gi-Gj) * (c(Gj)_n,k2) }
> 
> I put these following do loops in the last line of rean0.F in order to extract U(Gx,Gy,Gz) or in other words theta(G).
> do I1=1,IFFT1
>  do I2=1,IFFT2
>   do I3=1,IFFT3
>     write(2229,*) U(I1,I2,I3)
>   end do
>  end do
> end do
> Runnig lapw0 for a sample, the fort.2229 is produced. (including 729000 lines!  90^3=729000)
> 
> On the one hand the number of result of FFT is equal to the mesh which is used for example 90*90*90, On the other hand, "i,j" in the mentioned formula change from 1 to the number of G-vectors. So the number of theta(G) is expected to be of the order of G-vectors no.
> So in this step 729000 theta are not reasonable for me.
> I'm sure I made a mistake in somewhere or maybe I didn't understand something correctly. Would you please guide me in this issue?
> 
> Your help is appreciated like your previous favor.
> 
> With my best regards,
> Fatemeh

-- 
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Peter Blaha
Inst. Materials Chemistry, TU Vienna
Getreidemarkt 9, A-1060 Vienna, Austria
Tel: +43-1-5880115671
Fax: +43-1-5880115698
email: pblaha at theochem.tuwien.ac.at
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