Electron propagator test of atomic natural orbital basis sets

Electron propagator calculations of Sc and Ti ionization energies employing contractions based on atomic natural orbitals reveal the soundness of this approach for choosing basis sets. Errors in total energies greatly exceed those for ionization energies, where contraction errors are reduced to less than 1 mhartree. Systematic improvements involve retention of natural orbitals with lower occupation numbers and augmentation with diffuse and polarization functions.