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ANNNI

ANNNI: Axial Next-Nearest-Neighbor Ising model
Definition:Method used by ab initio approaches to express the stacking fault energy (→SFE) by a series of density functional theory (→DFT) energies obtained for fully periodic stacking sequences
Explanation:Within the ANNNI model, the internal SFE is expanded in terms of energies of defect-free bulk structures. In face centred cubic (→fcc) materials, the internal SFE is, in the first order approximation, proportional to the energy difference between hcp and fcc structures. The advantage of the ANNNI model is its computational efficiency, which avoids expensive calcuations of supercells containing stacking faults (→SFs) explicitly. However, local phenomena induced by the intrinsic stacking fault (see ISF) formation, such as spatial atomic relaxations or changes in magnetic order in the vicinity of the fault cannot be taken into account.
References:P. J. H. Denteneer and W. van Haeringen, Journal of Physics C: Solid State Physics 20 (1987)