[QE-users] vacancy formation energy

[mehrdad zamzamian]

Thanks for considering to my case
I calculated the isolated Fe in a BCC crystal: E= -55.74720381 Ry
again i have
total energy of 64 atoms= -2860.35431865 Ry
total energy of 63 atoms (removing a Fe atom)= -2804.44525448 Ry
so, E(vacancy)=-2860.35431865+2804.44525448+55.74720381= -0.16186Ry=2.2 eV!
This time it was so closer to 1.6 eV, but it still has almost 37% error!!
I checked various configurations for isolated Fe (changing magnetization,
E_cut, cell size and...) but the best result was this 2.2 eV.
I appreciate any suggestions.
Thanks

Seyed Mehrdad Zamzamian
Sharif University of Technology, Tehran, Iran
Energy engineering department
E-mail: mehrdad.zamzamian at gmail.com


On Thu, May 9, 2019 at 3:25 PM Giuseppe Mattioli <
giuseppe.mattioli at ism.cnr.it> wrote:

>
> Dear Mehrdad Zamzamian
> Please sign always your posts to the forum with name and scientific
> affiliation, we appreciate it.
> I suppose that the error is in the calculation of the single Fe atom.
> You should calculate the formation energy with respect to the
> reference energy of a Fe atom in its standard state, that is the bcc
> crystal.
> HTH
> Giuseppe
>
> Quoting mehrdad zamzamian <mehrdad.zamzamian at gmail.com>:
>
> > with regard
> > I want to calculate vacancy formation energy in Fe3C. I calculated the
> > following results by using these two pp:
> > Fe     55.84500  Fe.pbe-nd-rrkjus.UPF
> > C      12.01070  C.pbe-rrkjus.UPF
> > total energy of 64 atoms= -2860.35431865 Ry
> > total energy of 63 atoms (removing a Fe atom)= -2804.44525448 Ry
> > total energy of a single Fe atom= -55.45181620 Ry
> > using two ways i can calculate:
> > the first one) E(vacancy)= -2804.44525448 +(63/64)*2860.35431865
> > =11.21602794 Ry =152.60188965 eV!!!!!!
> > the second way) E(vacancy)= +2860.35431865-2804.44525448-55.45181620=
> > 0.45724797 Ry =6.2211778211 eV!!!!!!
> >
> > but in references, it must be ~1.61 eV ( and also i reached to this value
> > using MD simulation). what is wrong? Which calculation way is correct?
> >
> > Thanks
>
>
>
> GIUSEPPE MATTIOLI
> CNR - ISTITUTO DI STRUTTURA DELLA MATERIA
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