 ANNNI: Axial NextNearestNeighbor 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 defectfree 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) 

