Convergence behavior of multigrid, showing the maximum relative error and the maximum relative error estimated by Eq. (18) for each V-cycle, as well as the maximum relative change in population per V-cycle. Upper panels: three-level Ca ii atom; lower panels: H i, both for atmosphere Model 1. For three-level Ca ii we used ν₁ = 2, ν₂ = 2, ν₃ = 32 and for H i, we used ν₀ = 15, ν₁ = 2, ν₂ = 25, ν₃ = 32.