To get closer to the source of the problem, power spectra for isolated fragments in water were calculated from the velocity autocorrelation function. By doing so, emerging frequencies within one fragment become visible. As examples, power spectra of the fragments HS3 and HS30 are shown in Figure . The frequency range from 0 to 125 cm -1 is also shown zoomed-in. For the HS30 fragment, a high peak at around 400 cm -1 is visible. The dashed vertical line corresponds to a frequency with a period being 5 times 20 fs long, which is further explained below. Frequencies in the zoomed-in region can be identified as noise, which looks very similar to other fragments. Due to the coarse-graining, the characteristic vibrational lines of the molecules are averaged out, leaving a noisy signal behind, which is determined by the parameters of the bonded interactions.