[Pw_forum] Bandstructure of a metal slab

[Marcos Verissimo Alves]

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...

Thanks,

Marcos