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.
Kemper, M. J. H., Lemmens, A. M. C., & Buck, H. M. (1981). A comparative study of theoretical methods for calculating forbidden transitions. Chemical Physics, 57(1-2), 245-252. https://doi.org/10.1016/0301-0104(81)80038-9