[Wien] stability issue

[Xavier Rocquefelte]

Dear Chin

Your question is a little aside from the WIENLIST. Indeed, it is 
thermodynamics.

In equation 1, you evaluate the enthalpie of formation of the compound 
ABO3 from the reactants AO and BO2 using internal energies. Indeed, you 
neglect the volumetric effect (P deltaV). From your DFT calculation, if 
correct, you can simply conclude that the following reaction is 
endothermic.

Equation 2 gives you a different reaction and in your case is 
exothermic. We cannot answer in more details based on your message which 
do not provide any details. I can simply say that depending on the 
experimental conditions, the atmosphere can be oxygen rich / oxygen poor 
... you can start from pure solids A and B ... or other compounds ... In 
all case you will estimate a total energy difference which will give you 
an idea of the endo/exo-thermic nature of the reaction. You will not 
estimate the Gibbs energy (because you also neglect the entropic part T 
deltaS). However, in many articles we consider that estimation of 
internal energies are sufficient for solids because both deltaV and 
deltaS are negligeable (but such an approximation can be really bad).

Hope it will help you.

Cheers

Xavier




Le 22/12/2017 à 10:32, chin Sabsu a écrit :
> Dear Sir,
>
> I am working on the system ABO3 .
>
> I have calculated formation energy in two ways:
>
> Delta_E= Et[ABO3]  - Et[AO]  - Et[BO2]  ------ (1)
>
>  Delta_E= Et[ABO3] - Et[A] - Et[B] - 3/2Et[O2]    ----- (2)
>
> The Delta_E is positive 0.04 Ry from Eq (1) while is negative from Eq 2.
>
> Why is the energy difference calculated from Eq (1) positive?
>
>
> Thanks in advance!
>
>
> Regards
>
> Chin
>
>
>
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