Radiative Transfer in 3d

The high resolution and sensitivity provided by ALMA will reveal new small scale structures in many sources, e.g. star forming regions. Such inhomogeneities may not have been considered in the analysis of past observations but they will be essential to the understanding of future data. Radiative transfer methods are needed to interpret the observations and the presence of complicated source structures and small scale inhomogeneities requires 3D modelling. We will describe studies of molecular line emission we have made using models of inhomogeneous molecular clouds. These are based on MHD models and include e.g. thermal balance calculations for molecular clouds. Our radiative transfer code is based on Monte Carlo simulation. With current computing resources it has already become possible to handle even large 3-D models. We shall show that Monte Carlo method can be applied also in cases of hight optical depths.