In the first part of this work, a distribution function P(012) for the interelectronic angle in atomic systems is suggested and examined for the ground states of 2- and 4-electronions. For He, several excited states have also been examined and comparisons made between P(012) functions resulting from different correlated wavefunctions. Various expectation values and energies have been evaluated in order to assist our study of the behaviour and relative importance of angular correlation. Some ideas of mathematical statistics have been applied in this examination. The 'positive' correlation effects predicted by Kutzelnigg et al, for the 1p state of He, are shown to be artifacts of insufficiently-flexible wavefunctions. Information concerning the relative shielding of electrons between states, and hence the diffuseness of 'charge clouds' is obtained from the corresponding P(012) functions. In part II, the method of configuration interaction (CI) and its application to the calculation of diatomic molecular wavefunctions is described. The ground states and many excited states of the molecules F2, F2-, F2+, F2++, NF and NF- have been examined using the complete CI and complete valence CI approximations with minimal basis sets of Slater orbitals. Several experimentally-unknown states are included in this study. Potential energy curves have been determined and, where applicable, used to obtain theoretical values of the spectroscopic constants by means of Dunham Analysis. In some instances other energy-related properties are presented. The F2++ system is found to be unbound. Where possible our results are compared with previous calculations and also with experiment. The suitability of molecules for treatment by CI calculations within minimal basis sets is discussed.