The performance of the standard hierarchy of ab initio models—that is, Hartree–Fock theory,
second-order Møller–Plesset theory, coupled-cluster singles-and-doubles theory, and coupled-cluster
singles-doubles-approximate-triples theory—in combination with correlation-consistent basis sets is
investigated for equilibrium geometries of molecules containing second-row elements. From an
analysis on a collection of 31 molecules yielding statistical samples of 41 bond distances and 13
bond angles, the statistical errors mean deviation, mean absolute deviation, standard deviation, and
maximum absolute deviation are established at each level of theory. The importance of core
correlation is examined by comparing calculations in the frozen-core approximation with
calculations where all electrons are correlated
