A comparative study of theoretical methods for calculating forbidden transitions

In this study simple calculational methods for forbidden transitions are tested. Using different methods we calculated vibrationaly induced electronic transition moments. These D(Q) functions are, to a very good approximation, linear functions of the normal coordinates. A separate calculation of ground and excited state with a 4-31G basis set is the best alternative way to reproduce large basis set+CI results on formaldehyde. Contrary to older results, we show that in formaldehyde mode 5 (and in a less degree mode 6) contributes largely to the total np* oscillator strength. It is argued that the experimental determination of f-values should be reconsidered.