Dear wien users, I'd like to calculate a binding energy of a defect in crystal lattice, for example vacancy (hole). The system is hcp be, and I do not count for any effects of lattice distortions, and vibrations. The first way is through a supercell approach, the supercell should be quite big to neglect for interaction of defects through supercells. The energy of supercell with N-1 atoms and 1 vacancy is E[N-1]. The energy of Be atom in crystal is E[1]. Thus the binding energy Ev_super is Ev_super = E[N-1] - (N-1)*E[1].
The second way is through energy of isolated Be atom, Ei. Then binding energy Ev_atom is Ev_atom = Ei - E[1]. What I have got is Ev_super= 0.939 eV Ev_atom= 4.021 eV This is quite close to what I have from a pseudopotential code Ev_super= 1.056 eV Ev_atom= 3.786 eV Some details, the lattice is hcp be (2 atoms per unit cell), GGA-13, supercell is 4x4x2 (64 atoms). Energy of isolated was obtained through large box. I'm not sure about reached asymptotic (the energy was obtained from volume expanded by factor 108), but what I have seen, the energy of isolated atom is only increasing vs volume expansion, thus making the difference between Ev_super and Ev_atom only larger. A pseudopotential test on a 96 atoms supercell showed that a supercell provides energy accuracy within 0.06 eV, Ev_super[96-1]= 0.999 eV Any ideas for this conundrum? -- Bogdan Yanchitsky Institute of Magnetism Vernadsky Blvd., 36-b 03142 Kiev UKRAINE Tel. (+380-44) 444 34 20 Fax. (+380-44) 444 10 20
