A 25088-atom MgO cluster, supported on a BaO(100) substrate, has been simulated using an amorphization and recrystallization technique. The structure of the MgO cluster comprises a central MgO "plateau" region (vicinal MgO(100)/BaO(100)) intersected at each of the four corners by misaligned MgO crystallites, which can be characterized loosely as exhibiting triangular pyramidal morphologies. One particular crystallite is characterized as conforming to vicinal (ca. 10degrees) MgO(111)/BaO(100). indicating a dipolar MgO surface, which comprises a complex combination of mono-, di-, and tri-atomic steps and ledges at one particular face: the other two surfaces remaining perfect MgO {100} faces. The central MgO region is rotated by ca. 6degrees about an angle normal to the interfacial plane with respect to the underlying BaO and comprises various mixed screw-edge dislocations, the atomistic core structures of which are presented graphically. The interfacial region has a reduced ionic density, owing to the many voids and includes significant intermixing of Mg and Ba ions. In addition, significant migration of Ba ions around the sides of the MgO cluster is observed.