[Pw_forum] heat of formation for Mg3N2

[Stefano Baroni]

Dear Xiangmei:

On Aug 23, 2006, at 6:19 AM, Xiangmei Duan wrote:

>
> Dear all,
>
> I've opotimized \alpha-Mg3N2 with cubic anti-bixbyite structure (J.  
> Mater. Chem., 12, 2475, 2002)using espresso-2.1.5,
> and got the reasonable lattice constant 9.896 Ang. (LDA) (exp:  
> 9.953 Ang).
> When I calculated the heat of formation (in terms of the bulk Mg  
> and N2),
> I was expected to get an absolute value larger than the  
> experimental one,
> for the well known overbinding of LDA.

this I do not quite understand. it seems to me that you are comparing  
*TWO* binding energies: that of \alpha-Mg3N2 with the appriopriate  
average of those of Mg and N2. When the two terms of a difference are  
(supposedly) both too large, why do you expect the difference to be  
too large too? For N2, you mean N2 in the gase phase, I guess (and  
not "bulk")?

> In fact, I got -2.91 eV (exp. -4.80 eV).
> (we tested with LDA calculation using other code like Dmol^3, and  
> got -5.38 eV with the lattice parameter 9.942 A)

This seems to me too large a difference: same structure, same  
functional ???

> For N2, the calculated bond length is 1.103 AA, and the binding  
> energy is 11.11 eV, and the exp. values 1.098 A and  9.91 eV

binding of what with respect to what? molecular binding wrt isolated  
atoms?

> For Mg bulk, the lattice constants are 3.1317 A (a) and 5.21 (c).  
> (comparing with 3.21 and 5.21)

how about the binding energy?

Cheers - Stefano

---
Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center -  
Trieste
[+39] 040 3787 406 (tel) -528 (fax) / stefanobaroni (skype)

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