[Pw_forum] (no subject)

[Giuseppe Mattioli]

Dear Mourad
No special keywords, only a bit of physical chemistry...
Let us suppose we are calculating The formation heat (aka formation enthalpy, DeltaH_f) of wurtzite 
ZnO. First of all, what is it the formation enthalpy? Suppose that we are not very rigorous, we may 
say "the energy gained to form ZnO from Zn and O in their standard state". We may consider therefore 
this chemical process:

Zn(metal)+O2(gas)-->ZnO(wurtzite)

How can we calculate such an energy? Well, all chemical "Delta" quantities are defined as a difference 
between some final state quantity (free energy, enthalpy, ...) and the related initial state 
quantity. If we consider negligible the PV contribution to the above chemical process (H = U+PV), we 
can estimate formation heats as differences of total energies calculated in supercells:

DeltaH_f (ZnO) = E(ZnO) - E(Zn) - E(O2)

You must only be sure that the number of atoms in all the supercells is fully consistent with your 
calculation:

ZnO(wurtzite): 4 atoms (2 ZnO units)
Zn(metal, hcp): 1 atom
O2 molecule: 2 atoms

Your final equation is therefore

DeltaH_f (ZnO) = E(ZnO) - 2*E(Zn) - E(O2)

Modify it according to your purpose...

HTH

Giuseppe


On Tuesday 19 June 2012 11:21:54 debbichi mourad wrote:
> Dear pwscf users.
> How to calculate the formation enthalpy, which command  I can added to the
> input? best regards
>  
> DEBBICHI Mourad
> Unité de Recherche Physique des Solides,99/UR/13-19,
> Département de Physique, Faculté des Science de Monastir,
> Avenue de l'Environnement 5019, Monastir Tunisie.
> tél:+21697487042
> mourad_fsm at yahoo.fr

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   Giuseppe Mattioli                            
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