Knowledge Bank

The theoretical description of excited electronic states by ab initio SCF and CI techniques has been shown to be quite satisfactory for $\mathrm{<mrow\; data-mjx-texclass="ORD">1,3</mrow>}(n\to \Pi \ast )$, $^{3}(\Pi \rightarrow \Pi^{*}) $ and Rydberg states in a variety of molecules. Also, although this approach has been less satisfactory in treating the $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}(\Pi \to \Pi \ast )$ state, recent developments have shown that satisfactory results may be obtained if allowance is made for the ionic nature of this state. Previous calculations have been concerned with the development of the calculation method and have, of necessity, dealt with molecules whose transitions have been well characterized spectroscopically. We have now applied these calculations to diimide, a molecule which has not been so well characterized. We have employed a basis set flexible enough to describe all the low-lying states of diimide and used CI techniques to calculate the Franck-Condon transition energy to each of these states. The results of these calculations are discussed.