Figure 1: Scattering length and effective range for the 𝑠 -wave scattering of fermionic 4 0 K atoms around the Feshbach resonance at 𝐵 0 = 2 0 2 . 1 G demonstrating the divergence in a coupled-channel calculation (symbols) [33] and in a zero-range model (full lines). The difference in the zero-range and coupled-channel models is caused by the presence of a bound state close to threshold in the open channel.