The escape probability method are employed to decouple the radiative transfer calculations from the level populations.
The user can choose the following methods:
method = 1: uniform sphere
method = 2: expanding sphere (LVG)
method = 3: plane parallel slab (shock)
For detail see van der Tak et al. 2007. Noted that the method 1 has been used in the previous version.

Method: Uniform sphere; LVG(expanding sphere); SLAB (plane parallel slab)