I assume you're looking for the bound state energy of two spin-1/2 fermions in the singlet state, near the zero-energy resonance.

Take the lowest order EFT, 

= g

Above, = (2pi)^3 delta(k-k') , and k,k' are three-dim. wave vectors.

Write down the Lippmann-Schwinger for the T-operator T(z)

T(z)=V+VG_0(z)T(z)

where G_0(z) is the non-interacting Green's function (non-relativistic) after separation of centre of mass and relative coordinates. 

Inserting the identity 1 = (2pi)^(-3) \int d^3q |q>

thank you so much.!