Fig. 2 and 3 show, respectively, the calculated infrared and Raman spectra of methanimine for the analytical and numerical approaches. In the calculated infrared spectrum, using a step length of δx = 10–3 Å in the numerical differentiation, not only are the anharmonic corrections to the frequencies in poor agreement with the analytic ones, but so are also the corrections to the intensities, most strikingly so for the low-frequency peaks. In this case, for the IR spectrum, the δx = 10–3 Å numerical differentiation reproduces the analytic anharmonic spectral intensities almost perfectly for the peaks of frequency above 2900 cm–1 but overestimates (in absolute value) drastically the intensity for the other peaks, the lower the frequency the larger the overestimation.