Interatomic potentials that can model ligand field effects were used to investigate the properties of vacancies in orthorhombic LaMnO3. The minimum energy structures of LaMnO3-delta (where delta=1/192) were calculated for an oxygen vacancy on either the O1 or O2 site, respectively. It is predicted that the "degenerate" activation energy (and pathway) for oxygen diffusion in cubic LaMnO3-delta is lifted after a cubic-orthorhombic phase transition. Within the orthorhombic phase, one of the once triply degenerate activation energies is lowered, indicating that there is a preferred migration pathway, while one is increased, indicating an increased activation energy for unrestricted oxygen migration (and a much closer agreement to that observed in strontium doped systems). The lowest energy pathways within the orthorhombic, as opposed to the cubic perovskite structure, are no longer symmetric. The activation energies of migration indicate a preferential vacancy migration between the O2 and O1(s) sites, where the migrating oxygen ion would simultaneously arc around the central manganese ion with a bond length, Mn-O, which varied between 1.72 and 1.77 A. This type of pathway suggests that vacancies migrate along O2(m)-O1(s)-O2(m)-O1(s) chains. (C) 2003 American Institute of Physics.