In order to establish the relationship between the electronic structure and the site occupancy within Cu-O chains, total energy calculations were carried out for the sub-stoichiometric compound, Cu3Ba2YO6.5, for various patterns of vacancy ordering. Super-cells which contained up to 4 formula units were considered. The effective interactions of O(I) atoms were deduced from the total energies by means of the cluster expansion technique. The most stable configuration was found to be the experimentally observed ortho-II structure. This was confirmed by Monte-Carlo simulations of a rigid lattice, performed using pair interactions. The effect of vacancy ordering upon the total and partial densities of states was considered.

Ab initio Study of Vacancy Ordering in YBa2Cu3O7. P.A.Korzhavyi, C.Amrosch-Draxl, B.Johansson: Journal of Low Temperature Physics, 1999, 117[3-4], 395-400