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

[Miguel Martínez Canales]

Dear Paolo,

Thank you very much for your answers. I am particularly thankful for the 
reference to Nicola Marzari's paper.


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


PG> hard to say, especially without seeing

OK, as I've seen attachments sometimes fail, I'll later provide the first 
21 files of my phdos. Anyway, I think I've found the issue. It was related 
to the nk's I was using to get the phonon density of states. It seems that 
a regular 6,6,6 or 8,8,8 mesh is not enough, although with that mesh the 
IFC seems to have converged. Using a 20,20,20 or higher mesh would yield a 
more logical behaviour.

$ head -21 na.phdos
  -0.2601E-05  0.0000E+00
   0.1000E+01  0.4175E-05
   0.2000E+01  0.1670E-04
   0.3000E+01  0.3758E-04
   0.4000E+01  0.6680E-04
   0.5000E+01  0.1044E-03
   0.6000E+01  0.1503E-03
   0.7000E+01  0.2046E-03
   0.8000E+01  0.2672E-03
   0.9000E+01  0.3382E-03
   0.1000E+02  0.4175E-03
   0.1100E+02  0.5052E-03
   0.1200E+02  0.6012E-03
   0.1300E+02  0.7056E-03
   0.1400E+02  0.8183E-03
   0.1500E+02  0.2017E-02
   0.1600E+02  0.2252E-02
   0.1700E+02  0.2474E-02
   0.1800E+02  0.2683E-02
   0.1900E+02  0.2880E-02
   0.2000E+02  0.3064E-02

One can easily see the hop and the lack of parabolic behaviour above 14 
cm^-1. On a side note, the difference between a 40 40 40 and a 12 12 12 
phonon dos is like night and day. Is such difference to be expected? I 
suppose it's related to the way the phonon dos is computed within the code.

Finally, it might interest you to know that fitting a parabola to the low w 
regime and using the pseudopotential's equilibrium lattice parametre, the 
Debye temperature is 152 K, very close to the value presented in Table 23.3 
of Ashcroft's and Mermin's book.

Thanks again,

Miguel


-- 
----------------------------------------
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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