MOCASSIN 2.02 manual
Number density [cm-3] of dust grains. If
string1='constant' then it must be followed by
real1, containing the value of Ndust to be applied homogeneously to all cells in the grid. If
string1=file then it must be followed by
string2, the name of the file defining Ndust at each location. Note that MdMg and Ndust are mutually exclusive. This file must consists of four columns, with the first three columns containing the x-, y-, and z- coordinates of the grid cell in [cm] and the fourth columns containing the number density of dust in [cm-3] at the particular grid cell. The x, y and z axis do not to be equally spaced; irregular grids are perfectly acceptable by MOCASSIN and also the extent of each axis can vary (as long as this is consistent with the values given in the nx, ny and nz fields).
# nx ny nz
and the keywords can be omitted from