1 INTRODUCTION  
2 METHOD  
2.1 Hydrodynamical Simulation  
2.2 Chemical Modelling  
2.3 Radiative Transfer  
3 RESULTS  
4 DISCUSSION  
4.1 The (In)accuracy of Isolated Core Models  
4.2 Sulfur Depletion in Molecular Gas  
4.3 Linear Intensity Scaling in Optically-Thick Lines  
5 CONCLUSIONS