[QE-users] vacancy formation energy

[mehrdad zamzamian]

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
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