WEAK OVERLAP AND SPIN RECOUPLING: APPLICATIONS OF THE CAS SCF METHOD

The electronic structure of many molecular systems in their ground and excited states show characteristic features due to electrons occupying weakly overlapping atomic-like orbitals and to complex spin couplings. Such systems can be conveniently represented with CAS multiconfiguration wavefunctions which are able to capture these essential non-dynamical electron correlation effects and to yield semi-quantitative structures, spectra, and other properties for such systems. Multireference perturbation theoretical treatments of the dynamical electron correlation effects can then be applied to obtain quantitative accuracy. In this chapter we discuss selected examples of such molecular systems, including the design of CAS wavefunctions and we analyze the calculated structural and spectroscopic data. We end with a comparison of the present CAS approach with the broken symmetry UHF methodology. The resuls indicate that the UHF approach reproduces the CAS results well.