In this study in the Theoretical and Computational Chemistry Program in the Division of Chemistry ab initio electronic structure methods are being used to study the ground and excited states of molecular systems. The focus is on the comparison of various multi-reference correlation methods and on the testing of combined perturbative-variational approaches. Applications include studies of excited states of diatomic molecules near avoided crossings, low-lying excited states of polyenes and other conjugated species, and the ground-state properties of both symmetrical and asymmetrical alkali halide dimers. %%% Modern computer-based computational methods are being used in this study to characterize the energies and shapes of a number of different types of molecules both in their ground and excited states. These molecular types include diatomics such as oxygen, carbon monoxide, and aluminum hydride, various hydrocarbons, and such high-temperature species as that formed by two sodium chloride molecules bound together.