Extraction of single-particle energies from experiment

We examine the prescription for obtaining single-particle splittings at a closed minor shell, i.e., epsilon-epsilon' = E(N + 1, Z) + E(N-1, Z) -2E(N, Z). We find that to understand the value obtained, especially if the minor shell nucleus is deformed, e.g., 12C, we have to introduce many-particle--many-hole correlations. These correlations are less important, although not negligible, in the vicinity of a good closed shell, like 48Ca