MOCASSIN 2.02 manual
This keyword specifies the path of the nebular composition data file. If the default 'solar' composition (defined in the file solar.dat) is to be used, this keyword can be omitted. However the solar.dat file includes all elements with Z<30: this will result in a more memory expensive simulation. It is therefore advised to set to zero those elements which are not needed in the simulation. Otherwise the nebular composition can be specified in the user-defined
string file to be found in the current directory. All composition input files must be in a format consisting of one column containing the abundances by number and relative to hydrogen for the first thirty elements in order of ascending atomic number. The abundances of elements which are not to be included in the simulations must be set to zero (this will automatically exclude them by flagging them out throughout the program). If the multiChemistry keyword is specified the nebComposition keyword must be omitted.
* if gas is present and no multiChemistry