H2 self-shielding models

By default (v2.0) the H2 self-shielding model is Bertoldi & Draine 1996 (https://ui.adsabs.harvard.edu/abs/1996ApJ...468..269D/abstract). One can also use the model by Wolcott-Green and Haiman MNRAS 412, 2603–2616 (2011)

.true. ! Wolcott_Green_shielding

or the older model by Federman, S. R., Glassgold, A. E., & Kwan, J. 1979, ApJ, 227, 466

.true. ! Federman_H2_shielding