Strong-correlation effects in Born effective charges

Large values of Born effective charges are generally considered to be reliable indicators of the genuine tendency of an insulator towards ferroelectric instability. However, Born effective charges can also be strongly influenced by proximity to metallicity and by strong electron correlation, properties which tend not to occur in ferroelectric materials. In this paper we compare the Born effective charges of some prototypical ferroelectrics with those of magnetic, nonferroelectric compounds using a novel, self-interaction free methodology that improves on the local-density approximation description of the electronic properties. We show that the inclusion of strong-correlation effects systematically reduces the magnitudes of the Born effective charges and the electron localization lengths. Furthermore, we give an interpretation of the Born effective charges in terms of band energy structure and orbital occupations which can be used as a guideline to rationalize their values in the general case.