[Pw_forum] Debye temp, phonon DOS and sum rules.

[Miguel Martínez Canales]

Dear People,

To save you from reading a boring tale, I'll put my questions first and the
long boring tale later. My questions are:

1) Is trying to calculate the Debye temperature from the phonon DOS too
naïve?

2) What are the practical differences between simple and crystal asr? Could
be translated to: What does "optimized corrections of the IFC" in
pwtools/qwr.f90 mean?

3) Is the hoop I'm seeing at around 17cm-1 a van Hove singularity I haven't
covered in my dispersion curves or something more profound?

Thanks a lot for your patiente and attention.

I am currently interested in calculating some Debye temperatures for some
materials. How would I calculate it? The way that strikes more obvious to
me is calculating the phonon density of states. Remembering that as w->0
g(w) should be proportional to w2, one could fit a parabola to the curve
and then associate the parameters to the Debye density of states. Once you
know c, T_d comes inmediately.

Here I find the first issue: at q->0, w doesn't ->0 (unless you have an
infinite k-point grid). I thought that, when calculating the interatomic
force constants, one can impose sum rules so that when q->0 the forces will
vanish and w->0.

After checking convergence (Na.pz-n-vbc.UPF PP, up to 45 Ry and meshes of
10, 12, 14, 16 and 20 20 20 k point grids), I've calculated the phonon dos
with asr=no, simple and crystal (both in matdyn and q2r). There was no
difference (at first sight, less than 1% above 17cm-1), except in the w->0
regime. This was expected. It seems to me that at large omegas, asr=crystal
is closer to asr=no than simple asr.

Anyway, in all three cases, there appeared a weird jump in g(w) at around
15 cm-1. It doesn't seem to be due to a van Hove singularity, though. I've
mapped the phonon dispersion curve (as seen in Phys. Rev. 128 1112 (1962))
and the first \nabla w(k)=0 appears at roughly 29 cm-1.

Thanks again for your infinite patiente and attention,

Miguel

-- 
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Miguel Martínez Canales
    Dto. Física de la Materia Condensada
    UPV/EHU
    Facultad de Ciencia y Tecnología
    Apdo. 644
    48080 Bilbao (Spain)
Fax:  +34 94 601 3500
Tlf:  +34 94 601 5437
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