  We study the properties of ultrasmall metallic grains with sizes in the range
of 20 up to 400 electrons. Using a particle-hole version of the DMRG method we
compute condensation energies, spectroscopic gaps, pairing parameters and
particle-hole probabilities of the ground state wave function. The results
presented in this paper confirm that the bulk superconducting regime (large
grains) and the fluctuation dominated regime (small grains) are qualitative
different, but show that the crossover between them is very smooth with no
signs of critical level spacings separating them. We compare our DMRG results
with the exact ones obtained with the Richardson solution finding complete
agreement. We also propose a simplified version of the DMRG wave function,
called the Particle-Hole BCS ansatz, which agrees qualitatively with the DMRG
solution and illustrates what is lacking in the PBCS wave function in order to
describe correctly the crossover. Finally we present a new recursive method to
compute norms and expectation values with the PBCS wave function.
