[Pw_forum] Total energy of two isomeric compounds

[Giuseppe Mattioli]

Dear Romeda
Total energies in a plane wave/pseudopotential framework are meaningless. Differences between total energies are meaningful instead. You could 
calculate, for example, the formation heats from standards (metal unit cells for Ti, Cr and Zn) of your compounds in order to compare the formation 
energies of the two alloys.
HTH
Giuseppe

On Monday 28 October 2013 15:48:23 Romeda Azeen wrote:
> Dear Q.E Users
> Greeting,
> I'm also a new user of Q.E. I am studying the structural
> properties of two isomeric compounds (TiCr, ZrCr). Accordingly, I make a
> geometrical optimization for the these compounds i.e. determine the total
> energy as a function of the lattice constant and then the data fitted to
> the universal Murnaghan equation of state.
> I have a question about the obtained total
> energy of these compounds? When, I have calculated the total energy of both
> compounds to plot the total energy vs. volume/lattice constant, the
> obtained total energy for TiCr is more negative compared to ZrCr?
> Why the computed total energies are more
> negative for TiCr? 
> The obtained equilibrium lattice constants are
> 6.891 and 7.198 Å respectively for TiCr and ZrCr. The more negative energy
> can be related to lattice constant?
> Can you help me?
>  
> Ti-Cr:
> a(a.u.)            E(Ry)
> 12.3965    -929.59332904
> 12.491     -929.61596146
> 12.5855    -929.63374735
> 12.68      -929.64697023
> 12.7745    -929.65590118
> 12.869     -929.66085069
> 12.9635    -929.66198078
> 13.0580    -929.65969906
> 13.1525    -929.65417327
> 13.247     -929.64537400
> 13.3415    -929.63407783
> 13.436     -929.62016514
> 13.5305    -929.60373836
> 13.625     -929.58513132
> 13.719     -929.56461987
> --------------------------------------
> Zr-Cr
> 12.6800     -894.4877     
> 12.7745     -894.52096    
> 12.869      -894.54919    
> 12.9635     -894.57259    
> 13.058      -894.59135    
> 13.1525     -894.60606    
> 13.247      -894.61653    
> 13.3415     -894.62354    
> 13.436      -894.62689    
> 13.5305     -894.62684    
> 13.625      -894.62369    
> 13.719      -894.61753    
> 13.814      -894.60833    
> 13.9085     -894.59663    
> 14.003      -894.58263    
> 14.0975     -894.56609    
> 14.192      -894.54736    
> 14.2865     -894.52646 
>  
> Sincerely,
> Romeda Azeen
> Bhavnagar University Bhavnagar
> 364002 Gujarat, India


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   Giuseppe Mattioli                            
   CNR - ISTITUTO DI STRUTTURA DELLA MATERIA   
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