Density Matrix Renormalization Group Study of Ultrasmall Superconducting Grains

We apply the density matrix renormalization group (DMRG) method to the BCS pairing Hamiltonian which describes ultrasmall superconducting grains. Our version of the DMRG uses the particle (hole) states around the Fermi level as the system block (environment). We observed a smooth logarithmic-like crossover between the few electron regime and the BCS-bulk regime.