[Pw_forum] heat of formation for Mg3N2

[Stefano Baroni]

On Aug 23, 2006, at 10:07 AM, Xiangmei Duan wrote:

>
> Dear stefano,
>
> Many thanks for your reply.
>
> On Wed, 23 Aug 2006, Stefano Baroni wrote:
>
>>> 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")?
>
> For the formation energy (H), I used the fomula
> H = E_tot(Mg3N2) - 3E_tot(Mg:bulk) - E_tot(N2) (N2 from the pas phase)

Mg is hexagonal, two atoms/cell, so shouldn't you divide E_tot 
(Mg:bulk) by 2 ??? Below, you do compare the energy of bulk Mg with  
that of 2 isolated atoms ...

> The lattice constant is smaller than the experimental value, which  
> indicates the overbinding. Am I right ?

I think so. My point was that, strictly speaking, the fact that you  
obtain too large a formation energy as defined above cannot be  
ascribed to any overbinding effects, because overbinding affects all  
the terms of the formula which have different signs ... do different  
PP's give the same results? how about the kin ene cutoff? (60 Ry  
should not be too bad).


>>> 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 ???
>
> me too.
> SAME structure, both LDA, DMol^3 use all-electrons for the basis set.

that cannot justify a difference of 2.5 eV between PP and AE  
calculations ...

If nothing else works, you may want to consider non-linear charge  
corrections on Mg?

Cheers - Stefano

>
>>> 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?
> E_b = -[E_tot (N2)- 2E_tot (N-atom)]  both in gas phase
>
>>> 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?
> E_b = -[E_tot(Mg_bulk)- 2 E_tot(Mg-at)(gas)] = 4.95 eV
> The total energies for free atoms (N and Mg) include the spin- 
> polarization energies.
>
> Best wishes,
> Xiangmei
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---
Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center -  
Trieste
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