  In their recent paper, Kholmetskii, Missevitch, and Yarman "reanalyze the
usual classical derivation of spin-orbit coupling in hydrogenlike atoms" and
find a result "in qualitative agreement with the solution of the Dirac-Coulomb
equation for hydrogenlike atoms." However, the authors' result is based on an
equation of translational motion of the electron that omits any contribution
due to the existence of "hidden" momentum of the electron intrinsic magnetic
dipole moment in the electric field of the nucleus. Accounting for hidden
momentum is necessary to obtaining conservation of linear momentum in the
interaction of a magnetic dipole with a point charge. If hidden momentum is
omitted from the description, the force on the nucleus due to the electron will
differ from the force on the electron due to the nucleus. Thus, omitting the
hidden momentum contribution, the binding energy including the spin-orbit
coupling cannot be consistently calculated, while including hidden momentum,
the value obtained is in disagreement with experiment.
