The spin gap of CaV4O9 revisited
  The large-plaquette scenario of the spin gap in CaV4O9 is investigated on the basis of extensive exact diagonalizations. We confirm the existence of a large-plaquette phase in a wide range of parameters, and we show that the most recent neutron scattering data actually require an intra-plaquette second neighbor exchange integral much larger than the inter-plaquette one, thus justifying the perturbative calculation used in the interpretation of the neutron scattering experiments. 
M. Mambrini and F. Mila