This input parameter allows the user to control the size of the matrix solved when calculating level populations.

In versions 2.1lxx and before 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'. 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 less than 1.e-8.

With version 2.2 this is no longer true and small values of this parameter are recommended.

Tim Kallman 2018-12-14