January 8, 1997. Accuracy of very low rates (less than 10**(-20))
is improved by using double precision in exp.
January 6, 1997. Minor modification which affects very low rates
(less than 10**(-30)) to avoid negative Na+ rates at logT < 4.2
November 12, 1996. An improved presentation
for the first exponential integral.
The previous version used the approximation suggested by
Arnaud & Rothenflug (1985) which led to a jump in the
rates at kT=(E_ionization)/10.0 for low-ionized species.
Arnaud, M., & Raymond, J. 1992, ApJ, 398, 394
Arnaud, M., & Rothenflug, R. 1985, A&AS, 60, 425
Kingdon, J. B., & Ferland, G. J. 1996, ApJS, 106, 205
Last updated: 1996 November 12