To quantify the agreement of models and observations we compare the spectra over the full radius of the models. Here, an example of B 68 is shown. The solid lines represent observations, the dotted lines the synthetic spectra of the best models. On the left panels, we show the spectra of the ¹³CO (J = 2−1) (gray) and C¹⁸O (J = 2−1) (black) transitions. On the right panels, we show the spectra of the N₂H⁺ (J = 1−0) transition. The top line shows the spectra at the position of the core center, the bottom line the spectra at a radial distance of 7000 AU.