to be taken into account is the one of the simple Breit–Wigner propagators. We emphasize that our approach goes significantly beyond a simple “effective coupling”-type approximation where all external momenta at the loop level are neglected. While the latter approximation would disregard the important absorptive parts of the loop functions, those contributions are taken account in our approach through the evaluation of the Z-factors at the complex poles. We will demonstrate in particular that expanding the full propagators around all of their complex poles indeed results in the sum of Breit–Wigner propagators in combination with on-shell Z-factors. Approximating the full propagator matrix in this simple and convenient way has several conceptual advantages. In particular, the formulation in terms of Breit–Wigner propagators and Z-factors facilitates the implementation of a more precise total width than the width obtained from the imaginary part of the complex pole with self-energies evaluated at the same order. Furthermore, for states with masses that are nearly degenerate (i.e. the difference between the masses of the two states is of the same order as the sum of the total widths of the two states) the mixing becomes resonantly enhanced and the resonances overlap, see e.g. Refs. [@Pilaftsis:1997dr; @Frank:2006yh; 