[Thermo_pw-forum] result of debye temperature, entropy, free energy and heat capacity returned NaN

[Andrea Dal Corso]

It seems that your system is unstable. One of the elastic constants C_44
turns out to be negative. In this case the speed of sound gives NaN
because the code takes the square root of a negative number.

There are three possibilities: 
1) your system is actually stable but you are simulating it with too few
k points, too low cut-off etc. So increasing the computational accuracy
may transform it into a stable system.

2) your system is really unstable and the code cannot solve this
situation. 

3) there is still a problem in the calculation of elastic constants and
some error occurs in the calculation of C_44. I will investigate this
possibility.

In any case, thank you for reporting this. I will remove the printing of
NaN.

HTH,

Andrea



On Tue, 2016-08-09 at 08:40 +0200, agbaoye at physics.unaab.edu.ng wrote:
> Good day to everyone
> 
> I did some scf_eleastic_constant and mur_lc_elastic_constants calculations
> on thermo_pw but the result of entropy, free energy heat capacity and
> energy result data returned NaN also making the debye temperature return a
> NaN value. I have tried to figure it out but i am not sure of the cause of
> this anomaly and the possible solutions.
> I will be glad if this anomaly can be resolved
> 
> Thank you
> 
> Agbaoye Ridwan Olamide
> Federal University of Agriculture, Abeokuta
> 
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Andrea Dal Corso                    Tel. 0039-040-3787428
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