Multiconfiguration Hartree-Fock theory and density-functional theory both show that the negative ions ${\mathrm{Ca}}^{\mathrm{-}}$ and ${\mathrm{Sc}}^{\mathrm{-}}$ have stable ground states, the extra electron being a 4p electron. The multiconfiguration Hartree-Fock method, with a relativistic shift correction, predicts an electron affinity of 0.045 eV for Ca, a result in excellent agreement with recent experiment. A less accurate calculation for Sc yields an electron affinity of 0.152 eV. Density-functional theory gives a simple explanation of why the extra electron goes into the 4p orbital rather than a 3d orbital.

• Received 24 February 1987

DOI:https://doi.org/10.1103/PhysRevLett.59.2263