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The local structure and the g factors for the tetragonal Er3+ center in ThO2 are theoretically studied from the perturbation formulas of the g factors for a 4f(11) ion in tetragonal symmetry. This defect center can be attributed to the substitutional Er3+ associated with a positive compensator in the center of the adjacent cube along the [100] direction. The four oxygen ions within the layer between the impurity Er3+ and the compensator are found to displace by about 0.18 angstrom towards the compensator due to their electrostatic attraction. The calculated g factors show reasonable agreement with the observed values. The validity of the assignment of the positive compensator is also discussed. (c) 2007 Elsevier B.V. All rights reserved.