Ionization potentials, electron affinities, and term splittings have been computed for the lowest electronic configurations of atoms with Z from 13 to 18. Following approximate Hartree-Fock calculations, correlation energies are computed by variational solution of one- and two-electron Bethe-Goldstone equations, defined in terms of orbital excitations. All computed energy differences are in good agreement with experiment. The energy differences arise primarily from M-shell correlations, with very little contribution from L-M interactions, and no substantial L-shell effect. Estimates are made of the effects of extending the orbital basis set beyond f orbitals and of including three-particle correlation. © 1975 The American Physical Society.