The program PRO2 (Werner 1986; Werner et al. 2003; Rauch & Deetjen 2003) calculates plane parallel NLTE model atmospheres in radiative and hydrostatic equilibrium. It is based on the “Accelerated Lambda Iteration (ALI) method (Werner & Husfeld 1985). It consists mainly out of two iteration cycles. The “outer” cycle is called “Scharmer iteration” (this name is taken from Scharmer 1981 who worked on solutions of radiative transfer problems using approximate lambda operators) and iterates the radiation field according to
The source function S
of the actual iteration n is depending on the radiation field J which
is to be calculated. To calculate S, the non-linear statistical equations have to be solved
under consideration of the radiative and hydrostatic equilibrium. This is done in the “inner”
iteration cycle by a [quasi-] Newton-Raphson iteration. In the following this procedure is called
“linearizations”.