Validity of the BCS model Hamiltonian in the limit of small sizes

Finite-size effects when the electronic level spacings become comparable to the bulk superconducting gap can suppress pairing correlations in small me tallic particles. We examine an alternative mechanism for finite-size suppr ession of superconductivity: the role of the nonpair portion of the interac tion, which could become important in small systems. We show that the cross over size at which the nonpair contribution becomes significant is typicall y already within the regime wherein finite lever spacing suppresses the pai ring correlations; however, the nonpair terms could become relevant in cert ain weak-coupled superconductors.