A study of electron correlation in two-electron atoms.

In Part I of this work the properties of H-, He, Li+ and Be++ calculated using a large series of wavefunctions are examined. Values Comparisons of the results obtained from explicitly correlated (EC) and configuration interaction (CI) wavefunctions with those obtained from independent particle functions enable us to discuss the correlation effects arising from specific terms within an EC wavefunction and from changes in the orbital basis set of a CI function. The relative performance of EC and CI wavefunctions with respect to one- and two-electron properties is also examined. In Part II electron correlation effects in the scattering of electrons, protons and hydrogen-like atoms from He and Li+ are examined. Both elastic scattering and the l'S -> n'P (n = 2,3) excitation reactions axe considered. The first Born approximation is used throughout. Generalized oscillator strengths, total cross sections and atomic form factors are presented; all calculations were performed using both the "length" and "velocity" formulae for the scattering amplitude. The natural expansion of a 35-configuration CI wavefunction, truncated to include only the first X terms, was used for the ground state wavefunction of the target atom. For X = 1 this wavefunction closely resembles the Hartree Fock function. The influence of a systematic introduction of correlation is examined by taking X = 1, 2, 3 .. etc. In addition, results calculated from the complete CI function are compared with others obtained from an EC function of comparable energy. In Part III there is a change of emphasis. Here we investigate the usefulness of certain novel 'cusp orbitals' in one-centre calculations on molecules. Several calculations applied to the H2+ molecule as a test system are described.