Cold Bose Gases near Feshbach Resonances
  The lowest order constrained variational method [Phys. Rev. Lett. {\bf 88}, 210403 (2002)] has been generalized for a dilute (in the sense that the range of interatomic potential is small compared with inter-particle spacing) uniform gas of bosons near the Feshbach resonance using the multi-channel zero-range potential model. The method has been applied to Na (F=1, m_F=1) atoms near the 907G Feshbach resonance. It is shown that at high densities there are significant differences between our results for the real part of energy per particle and the one-channel zero-range potential approximation. We point out the possibility of stabilization of the uniform condensate for the case of negative scattering length. 
Yeong.E. Kim and Alexander L. Zubarev