The Nucleon-Mass Difference in Chiral Perturbation Theory and Nuclear   Forces
  A new method is developed for treating the effect of the neutron-proton mass difference in isospin-violating nuclear forces. Previous treatments utilized an awkward subtraction scheme to generate these forces. A field redefinition is used to remove that mass difference from the Lagrangian (and hence from asymptotic nucleon states) and replace its effect by effective interactions. Previous calculations of static Class II charge-independence-breaking and Class III charge-symmetry-breaking potentials are verified using the new scheme, which is also used to calculate Class IV nuclear forces. Two-body forces of the latter type are found to be identical to previously obtained results. A novel three-body force is also found. Problems involving Galilean invariance with Class IV one-pion-exchange forces are identified and resolved. 
J. L. Friar, U. van Kolck, M. C. M. Rentmeester, R. G. E. Timmermans