[Pw_forum] About F_QHA.x

[Yamit]

Dear All

I have attempted to calculate the specific heat of Si using "F_QHA.x", and the calculated Silicon phonon dos  as an input.
(The calculated Si phonon frequencies are in excellent agreement with available experimental data. )

As the output of "F_QHA.x", I have obtained the following results, (in QHA.out file)

############################################################################################################

# T in K, F_vib in Ry/cell, C_v in R (the universal gas constant by 3N modes), S in k_B
############################################################################################################
#   T         E_internal        F_vibration          Specific heat (C_v)       Entropy
############################################################################################################
:
:
  295.00      0.0139740852      0.0055747646           4.6621714979          4.4956255367
 300.00      0.0141222695      0.0054311560           4.6966380876          4.5742737931
 305.00      0.0142715257      0.0052850687           4.7298757363          4.6521815403
 310.00      0.0144218157      0.0051365259           4.7619351485          4.7293532915
:
:
:
############################################################################################################

>From the above results, at 300 K:

4.6966380876  * 8.314 = 39.048
(where, Gas constant, R = 8.314 J/(mol K) ) and for Silicon N = 2, so 3N=6 modes

So, Specific heat, C_v= 38.761 / 3N = 6.5080  J/(mol K)

Experimentally the Specific heat, C_v of Si at 25 °C (~298) is found equal to 9.789 J·mol−1·K−1. 

Can anybody could explain the reason behind this huge error in the calculation of specific heat ?

Warm Regards
Amit N. Harode
C R Labs INDIA
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