The papers collected in this volume were presented at an international symposium on Computational Methods in Chemistry. This symposium was sponsored by IBM Germany and was held September 17-19, 1979, in Bad Neuenahr, West Germany. According to Graham Richards [ Nature 278, 507 (1979)] the "Third Age of Quantum Chemistry" has started-;-where the results of quantum chemical calculations have become so accurate and reliable that they can guide the experimentalists in their search for the unknown. The particular example highlighted by Richards was the suc cessful prediction and subsequent identification of the relative energies, transition probabilities and geometries of the lowest triplet states of acetylene. The theoretical predictions were based chiefly upon the work of three groups: Kammer [ Chern. Phys. Lett. ~, 529 (1970)] had made qualitatively correct predictions; Demoulin [ Chern. Phys. 11, 329 (1975)] had calculated the potential energy curves for the two lowest triplet states (3 and 3 ) of B A acetylene; and Wetmore and Schaefer III [ J. Chern. Phys. ~~ 1648 (1978)] had determined the geometries of the cis (3B and ~A ) and the trans (3B and 3A ) isomers of these two sta~es. Inua 2 2 guided search, Wendt, Hunziker and Hippler [ J. Chern. PHys. 70, 4044 (1979)] succeeded in finding the predicted near infrared absorption of the cis triplet acetylene (no corresponding absorp tion for the trans form was found, which is in agreement with theory), and the resolved structure of the spectrum confirmed the predicted geometries conclusively.
