On the breakdown of the Dirac kinetic energy operator for estimating normal mass shiftsJ.G. Li, C. Nazé, M. Godefroid, G. Gaigalas and P. JönssonEur. Phys. J. D, 66 11 (2012) 290DOI: https://doi.org/10.1140/epjd/e2012-30328-5