  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.
