[Pw_forum] *.fc file in PW_SCF help.

[Stefano de Gironcoli]

xirainbow wrote:
> Dear Stefano de Gironcoli:
>  
> First, thanks for you answer:)
> I draw a lot of inspiration from your reply:P
>  
> But I am still confused about one thing.
> "   4   4   4
>    1   1   1   1
>    1   1   1   2.24396850738E-01"
> I think F_{1,1}^{x,x}(R(1,1,1)) is force constant between the 1th atom 
> and itself.
> What the meaning of "force constant between one atom and its self"?

think of one spring connecting 2 balls.. E=1/2 K (x1-x2)^2
what is the force between one atom and itself ?
 
>
>     >Notice also that the force constant grid is written as these were
>     >periodic ...
>     >so in a 4x4x4 grid R(1,1,1) = 0,  R(2,1,1,) = a1,  and  R(4,1,1)=-a1
>
>  
> You said that "force constants are periodic".
>  I think you are right.But I do not know why it is periodic.

because they are obtained by a FFT on a discrete regular grid of q vectors.
look (google or numerical recipes) for spectral analysis by Fourier 
transform.
An understanding of aliasing errors in FFT may also be useful.
There is a relationship between the density of q-point grid and the 
extent to which interatomic force constants are accurately described in 
real space... or rather  the opposite ...  between the extent of IFC in 
real space and the density of q-point grid needed to accurately describe 
them.

best,
stefano de Gironcoli  - SISSA and DEMOCRITOS



> ____________________________________
> Hui Wang
> School of physics, Nankai University, Tianjin, China
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