The FWHM of the angular distribution from multiple scattering is calculated via two different models formulated in

Marion and Zimmerman, NIM 51 (1967) 93-101

Sigmund and Winterbon, NIM 119 (1974) 541-557.

Marion and Zimmerman use a quantum approach relying on the Born-Oppenheimer approximation, and so it is theoretically valid when the Born parameter \( \alpha_B \sim (Z_1Z_2)/(137\beta) \) (see the information page for more details) is much less than 1. Sigmund and Winterbon's classical approach is theoretically valid when \( \alpha_B \) is much greater than 1. To aid with estimating a calculation's accuracy, the reported values are accompanied by other parameters that indicate the validity of the model.
To help address situations where the conditions fail to meet the assumptions of the given models, a value based on an experimental fit from Anne et al., NIM B34 (1988) 295-308 is also reported as an extension to the Sigmund formulation.

Each method and calculation is described in detail on the information page here.