A many-body expansion of the correlation energy has been formulated in terms of localized molecular orbitals, and the CEEIS (correlation energy extrapolation by intrinsic scaling) method has been used for the evaluation of local full configuration interaction energies in this context. No truncations were applied in the correlating virtual space. With this combination of methods, the full valence Cl energy of the molecule NCCN has been calculated within 0.1 mhartree for cc-pVDZ basis sets (configuration space dimension = 10(19)) With appropriate choice of localized orbitals, the many-body expansion was converged to this accuracy with the four-body terms The individual many-body terms exhibited a rapid decrease with the distance between the localized orbitals. For the orbitals chosen, the conjugated triple bonds presented no problems. The analysis of the various contributions furnishes rigorous quantitative ab initio benchmarks for the range of intramolecular electron correlations