[Pw_forum] QHA applicatiom - F_QHA.x

[Eleftheria Gkogkosi]

 Dear all

 Reference is made to the output of the executable F_QHA.x;

 AA.
 The output of F_QHA.x starts with a header like the following
 QUOTE
 # Zero vibration energy: 0.0036240239 (Ry/cell)
 # Phonon DOS norm : 6.036119 ! 3N for check purpose, N number of atoms 
 in the unit cell
 # 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
 UNQUOTE
 For test purposes, I run F_QHA.x for the cases of alpha- and beta- Tin.
 I can see that "F_vibration vs T" graphs cross above 300 K (in 
 cosnsistency with the phase transition).
 This does not hold for the sum E_internal+F_vibration (which I assumed 
 to be the Heelmholtz free energy in the first place).
 So, is it safe to conclude that F_vibration is actually the Helmholtz 
 free energy? And if so, what does E_internal stand for?

 BB.
 As a prerequisite to QHA, the PHDOS.out is obtained.
 What does it mean when one gets negative frequencies in tjis file?
 Is it a matter of poor k-point sampling or an indication of unstable 
 structue at a given Temperature?
 Is it fair to errase the negative parts and proceed withy QHA for the 
 thermodynamical properties?

 Thank you very much,
-- 
 Eleftheria Gkogkosi

 Applied Mathematics & Physics Dept.
 National Technical University of Athens
 http://users.ntua.gr/elefthe