[Pw_forum] About F_QHA.x

[Eyvaz Isaev]

Hi, 

Let me finish this discussion about  the heat capacity conversion. 
Well, I have calculated C_v for graphite with 4 atoms in the unit cell (hexagonal). Calculated C_v(295K)= 4.0758 (R).  Now converting it to J/mol/K we get 4.0758*8.3145/4=8.472 J/mol/K which is very good agreement with   8.517 J/mol/K.

Please pay attention that in this case C_v is per atom, not for the unit cell. Applying the same procedure for Si we get: 4.6966*8.314/2 = 39.048/2 = 19.524 J/mpl/K that matches very well 19.789 you updated (see below). 
 
> I would like to correct a mistake in last post, for Si,  C (298 K) = 
> 19.789 J.mol^{-1}.K^{-1} (and not 9.789 J.mol^{-1}.K^{-1} ) (Sorry for > it.)

> Also, from PRB, 50, 14587, C_v {~300} ~ 20 J.mol^{-1}.K^{-1}

> For Si, C_p - C_v < 0.004 (Ref.:Flubacher et.al, Phil Mag, 4: 39, 273 > — 294)

Yes, sure there will be not too much difference in between C_p and C_v at room temperature.

> Can anybody could explain the reason behind this huge error in the 
> calculation of specific heat ?
 
Hopefully, now it is explained. Just be sure in comparison with an experimental data whether C_v(Exp) is for per atom or per cell or formula unit, etc.

Bests,
Eyvaz

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Prof. Eyvaz Isaev, 
Department of Physics, Chemistry, and Biology (IFM), Linkoping University, Sweden 
Theoretical Physics Department, Moscow State Institute of Steel & Alloys, Russia, 
isaev at ifm.liu.se, eyvaz_isaev at yahoo.com