Configuration interaction studies of small molecules.

The method of Configuration Interaction (CI) has been used in a number of studies of small atomic and molecular systems. In Part I, CI wave functions for a series of pseudomolecular ions 2HZ2Z-1 are reformulated in terms of natural orbitals. Changes in the electron density as a function of the nuclear charge Z and bond angle ZHZ are investigated by means of an electron population analysis based on the natural orbitals. Contour diagrams of the electron density in the plane of the molecule are obtained. The total energy for each system is analysed in terms of the kinetic energy, nuclear attraction energy, and the electron and nuclear repulsion energies. In Part II, the CI method is examined in detail. Techniques for obtaining CI wave functions of atoms and molecules are discussed in detail. Computer programs based on these techniques are described, and listed in the Appendices. The Valence Configuration Interaction (VCI) method is also examined as a means of reducing the size of the secular equation, and results of VCI calculations are given for various states of atomicand diatomic oxygen and sulphur. Spectroscopic constants for the states of S2 are given. The methods by which these results can be obtained are also discussed. Finally the results of the VCI calculations are compared with some recent SCF calculations on O2 and S2.