On the breakdown of the Dirac kinetic energy operator for estimating normal mass shifts J.G. Li, C. Nazé, M. Godefroid, G. Gaigalas and P. Jönsson Eur. Phys. J. D, 66 11 (2012) 290 Published online: 27 November 2012 DOI: 10.1140/epjd/e2012-30328-5