Comment on ``BCS to Bose-Einstein crossover phase diagram at zero   temperature for a d_{x^2-y^2} order parameter superconductor: Dependence on   the tight-binding structure''
  The work by Soares et al. [Phys. Rev. B 65, 174506 (2002)] investigates the BCS-BE crossover for d-wave pairing in the 2-dimensional attractive Hubbard model. Contrary to their claims, we found that a non-pairing region does not exist in the density vs coupling phase diagram. The gap parameter at T=0, as obtained by solving analytically as well as numerically the BCS equations, is in fact finite for any non-zero density and coupling, even in the weak-coupling regime. 
A. Perali, P. Pieri, G. C. Strinati