Atoms Configuration interaction method Ground state Wave functions Coalescence Electric fields Helium Mean values Screened Coulomb potentials Three-body systems Two-electron atoms Atomic collision Atomic physics Atomic system Basis sets Computation theory Configuration interactions Convergency rate Correlated electrons Correlation methods Coulomb singularities Cusp conditions Degrees of accuracy Electrons Excited states First excited state Global properties Ground state wavefunctions Ground-state energies Harmonic potential