[Pw_forum] *.fc file in PW_SCF help.

[xirainbow]

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 anditself
.
What the meaning of "force constant between one atom and its self"?


> >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.

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