Dear avdw:
I’m a new learner, I would like to know that what kind of system does Cluster Expansion theory/ATAT difficult to deal with and hard to converge? And what kond of materials is suitable to use CE.
Thank you very much in advance!
Kesson
Ideally, your system should be crystalline, have a small number of different lattices, these lattices should have a high symmetry and have a small primitive unit cell. If there are point defects, they should be substitutional (vacancies are ok too). If there are interstitials, you need to add a lattice site to hold those atoms as well.
If there are large atom size mismatches, the CE may need a lot of terms to converge.
Ideally, the atoms should not relax too far away from the "ideal" positions they start in when you relax them with an ab initio method.