The energy shift of a nucleon in a static periodic magnetic field is
evaluated at second order in the external field strength in perturbation
theory. It is shown that the measurement of this energy shift on the lattice
allows one to determine the unknown subtraction function in the forward doubly
virtual Compton scattering amplitude. The limits of applicability of the
obtained formula for the energy shift are discussed.Comment: The explicit factor e is restored in the equations. The conclusions
are unchange
