Grid of HCO⁺(1−0) spectra obtained from RATRAN modelling of a collapsing cloud (see text). All input parameters are fixed with the exception of the infall velocity (V_inf) and velocity dispersion (σ_turb). Each modelled spectrum (in red) has been obtained for the corresponding V_inf − σ_turb displayed in the top and right-hand sides of the figure. The numbers in the top right-hand corner of each spectrum give a relative idea of the fit quality. The HCO⁺(1−0) spectrum observed at the centre of SDC335 is plotted in black. Since we aimed to keep the observed spectra displayed in scale, we applied a 0.5 factor to the modelled spectra to take into account the Mopra main-beam efficiency.