[Pw_forum] IFCs and Difference between unit cell and supercell

[Jingjie Zhang]

Dear all,

       The material is silicon. The structure is FCC with two atoms. When I
am extracting IFCs, I don't  know which R I should use. I mean R = (k-1)*a1
+ (h-1)*a2+(l-1)*a3, are the a1, a2, a3 are conventional cell vectors or
the primitive cell vectors? If they are lattice vectors, they should be
conventional vectors. Then I will miss the force constant between the atom
at the origin and the atom at the face center.

      When I use supercell: simple cubic with 8 atoms to build silicon
structure, the omega is different at gamma point with the omega from the
structure with FCC. And the thing most confuse me is that the point group
is different in this structure: 43m. Can anyone explain why this happen
please?

----------------------------------------------------------
Jingjie Zhang
Graduate student at University of Virginia
email: jz9wp at virginia.edu
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