ABSTRACT
The hydrogen atom is a system of two particles subjected to electromagnetic interaction of which the electrostatic part is the most important contribution and depends only on the electron–nucleus distance. The ionization energy is the minimal energy needed to remove an electron from the isolated neutral atom in its ground state, leaving the cation in its ground state as well. The "specular" quantity is the electron affinity, which is the energy given off by a neutral atom in its ground state when an electron is inserted, thereby forming an anion in its ground state as well. According to Koopmans' theorem, the atomic orbitals of the ion are the same as already computed for the neutral atom, so that the Hartree–Fock single-electron energies are approximations to the ionization energy and electron affinity. The reliability of the Hartree–Fock calculation depends on how judicious is the choice of the input single-electron wavefunctions.
