# [Pw_forum] Help regarding Cv value using phdos

bramha pandey pandey.bramha at gmail.com
Mon Feb 20 13:51:05 CET 2012

```Dear Developers and Usres of QE.

Here i am trying to compare  the experimental Cv value to calculated (using
phdos by ph.x,q2r.x and matdyn.x) Cv, but i am shocked that i am obtaining
very large value of Cv and the sum(dos)=5.9782=3*N=3*2 in my system(AlN
with 2 atoms per unit cell in fcc structure ) which is very correct.
For calculation of Cv,
Cv=3*R*integration( D(w)dw (hbar^2w^2/K^2T^2)*exp(x)/(1-exp(x))^2) or
3*R*integration of f(w).where x=hbar*w/K*T.

>>experimental Cv approx Cp at 300K=30.239(J/mol.K)
>>therotical Cv at 300K=147.4268(J/mol.K).
Sir if i will take one degree of freedom then Cv at 300K=147.4268/3=49.1422
which is also not near about 30.239(J/mol.K).

I have also plot the graph between Cv/3R vs T/theta which shows good shape
which is in litecture but i got y-axis starting from 0 to 5.9104.
So should i calculate the phonon dos for lesser value of q point or what
should i do for getting correct result.

Any kind of help will be appreciable.

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Thanks and Regards