Test of the iterative numerical method to solve the diffusion equation without chemical sources and sinks. We consider a δ-peak for the initial concentration x_mol = n_i,k/n_k at the start of the iterations in a Gaussian density structure (see text). The upper figure shows the concentrations as a function of the number of iterations carried out. The lower figure shows the relative deviations between iterations (see Eqs. (28) and (29)).