[Pw_forum] (no subject)

Giuseppe Mattioli giuseppe.mattioli at ism.cnr.it
Tue Jun 19 12:27:20 CEST 2012


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

-- 
********************************************************
- Article premier - Les hommes naissent et demeurent
libres et ègaux en droits. Les distinctions sociales
ne peuvent être fondèes que sur l'utilitè commune
- Article 2 - Le but de toute association politique
est la conservation des droits naturels et 
imprescriptibles de l'homme. Ces droits sont la libertè,
la propriètè, la sùretè et la rèsistance à l'oppression.
********************************************************

   Giuseppe Mattioli                            
   CNR - ISTITUTO DI STRUTTURA DELLA MATERIA   
   v. Salaria Km 29,300 - C.P. 10                
   I 00015 - Monterotondo Stazione (RM)          
   Tel + 39 06 90672836 - Fax +39 06 90672316    
   E-mail: <giuseppe.mattioli at ism.cnr.it>



More information about the users mailing list