879 (2011). C. T. Nelson, B. Winchester, Y. Zhang, S. J. Kim, A. Melville, C. Adamo, C. M. Folkman, S. H. Baek, C. B. Eom, D. G. Schlom, L. Q. Chen, X. Pan, [ Nano Lett.]{} [**11**]{}, 828 (2011). L. D. Landau, E. M. Lifshitz, and L. P. Pitaevskii, [Electrodynamics of Continuous Media]{}, Second Edition: Volume 8 (Course of Theoretical Physics), (Elsevier, New-York 1984). J. R. Tyldesley, [ An introduction to Tensor Analysis: For Engineers and Applied Scientists.]{} (Longman, Niw-York 1973). A. Malashevich, and I. Souza, [ Phys. Rev. B]{} **82**, 245118 (2010). R. E. Raab, and O. L. De Lange, [*Multipole Theory in Elecromagnetism.*]{} (Clarendon Press - Oxford 2005). Note that ${\bf {\cal P}(r)}$ and ${\bf {\cal M}_0(r)}$ are technically ill-defined in the sense that they depend on the choice of a gauge [@Hirst]. However, all the different gauges result in the same current density, ${\bf {\cal J}(r)}$, [@Hirst] which is the physical quantity that appears in Eqs. (12) and (13). As a result, the choice of the gauge does not modify our results, in general, and Eq. (21), in particular. Such conclusion can also be reached by realizing that the quantity appearing in the left hand side of 