**B.2. Radial grid**

Essentially two types of radial grids have been used : one kind for the uniform
(r^{0}) density distribution and another for the r^{-1}
case. This was necessary because, the code CSDUST3 has convergence
problems, if the distance between successive
radial grid points is such that the optical depth
1 at any
of the frequencies of the grid. This problem
is most severe at the two boundaries of the cloud, where some UV radiation
(stellar or ISRF) is incident, so that a finer grid is required there.
Hence for the uniform density distribution, the entire radial range has
been divided into two parts, with 71 points in a logarithmically
increasing grid spacing in the first part ( i.e., from the inner
boundary to mid point of the cloud), and an exact mirror image on the
second part. This gives a total of 141 radial grid points with a finer
grid at both the boundaries. It may be noted, that the necessity for
having a fine grid towards the outer boundary of the cloud, exists
only for uniform density distribution, since in this case alone there
remains enough dust grains to produce high optical depth at the outer
boundary. For the case of
r^{-1} density distribution, the dust density falls off so
rapidly, that towards the outer edge, the optical depth between successive
radial grid points is never large enough. As a result, a smooth
convergence is achieved even with a logarithmically increasing
grid spacing right from the inner to the outer boundary of the cloud.