Ions whose abundance relative to total hydrogen (H I + H II) are less than this
value after the preliminary ion abundance calculation
(See Chapter 9) 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. Currently this matrix has
a maximum size of 2400, and an attempt to solve for more than this number of levels
simultaneously will result in xstar stopping with a message `ipmat too large'.
This is most likely to occur in situations where more than 4 or 5 of the lower ion stages of
iron meet the critf criterion; for problems with zero iron abundance critf can
probably be specified as small as 1.e-15; when iron is non-zero the limit may be reached
if critf is as large as 1.e-8.