# [Pw_forum] heat of formation for Mg3N2

Xiangmei Duan duan at physics.usyd.edu.au
Wed Aug 23 10:07:42 CEST 2006

Dear stefano,

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

>> 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.

>> 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