This research will contribute to the understanding of the electronic structure of molecules of biological significance. This will aid in understanding their interactions and reactions. Excited states will be examined in an attempt to understand the effect of light and other radiation on biomolecules. Perturbations caused by excited states are also involved in interpretation of biophysical measurements on ground states, as for example, in the g-tensor of radicals. The correct interpretation of these perturbations will be explored for some model systems. The method to be used will be ab initio quantum chemical calculations (with relativistic corrections where needed). This will include molecular orbital, configuration interaction, and perturbation theory calculations for ground and excited states using an extensive Gaussian orbital basis set. A specific aim of this grant is the study of the spectra of polyenes with terminal -CH=O or - CH=NH2+ groups which are important in models of vision. Studies will also be made of some bicyclo heteroatom systems such as indole. A program to calculate g-tensors will be developed and applied to some simple metal centered compounds to test the conventional interpretation and the utility of the ab initio approach.