[Pw_forum] Bandstructure of a metal slab

Marcos Verissimo Alves verissim at df.ufscar.br
Fri Jan 16 21:15:36 CET 2004

Hi all,

This e-mail has nothing to do with pwscf specifically, but it is
something I am having trouble with. Right now I want to calculate the
phonon spectrum of PbO at the Gamma point, and study its behavior with
pressure. I don't want the frequencies, only; I want the eigenmodes as
well, to see which of them (obtained by DFT) are Raman active (through
group theory analysis), and which are not.

I performed calculations with a large supercell (180 atoms) using finite
displacements of the atoms in the primitive cell to generate the force
constants matrix and then diagonalized it, and got a set of phonon
frequencies and a set of phonon eigenmodes. Then, I performed a phonon
calculation with a single unit cell instead of a supercell. The
eigenfrequencies thus obtained are practically identical to the ones
from the supercell calculation, but the eigenmodes are not.

My questions are: why do the eigenmodes differ? My guess is that the
forces on the other atoms have not decayed, in the calculation using a
single unit cell. However, why are the frequencies at Gamma identical
for the two calculations?

Second, can I trust the eigenmodes obtained from a single-cell
calculation? Is there any way to relate them to the ones obtained from
the supercell calculation?

Third, is there some method I can use to use a single unit cell for a
phonon calculation and still obtain good eigenmodes? Or is it inevitable
to use a supercell, when using finite displacements to calculate the
force constant matrix?

I would greatly appreciate any help you can give me, including basic
literature on the subject, as well as papers you might know that you
find relevant to clear this subject...



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