Ions whose abundance relative to total hydrogen (H I + H II) are less than this value after the preliminary ion abundance calculation (See Chapter 10) are not included in full multilevel calculation. The default value is . This parameter should be changed with caution owing to the fact that it determines the size of the matrix that xstar tries to solve in calculating the level populations.
In versions of xstar 2.1lxx and earlier this matrix had a maximum size of 2400, and an attempt to solve for more than this number of levels simultaneously would result in xstar stopping with a message `ipmat too large'.
In version 2.2 this limitation has been removed, and arbitrarily small values of critf can be accomodated. Also, a faster algorithm has been adopted for the multi-level calculation, so the speed advantages of large critf have been reduced. Also, the input parameter critf now refers to the fractional ion abundance (i.e. relative to the parent element) rather than the absolute (i.e. relative to H) ion abundance.