  We establish a limiting absorption principle for some long range
perturbations of the Dirac systems at threshold energies. We cover multi-center
interactions with small coupling constants. The analysis is reduced to study a
family of non-self-adjoint operators. The technique is based on a positive
commutator theory for non self-adjoint operators, which we develop in appendix.
We also discuss some applications to the dispersive Helmholzt model in the
quantum regime.
