Validation of diffusion. In all panels the analytical solution is compared with the mean for 10 (A-C) or 30 (D) iterations of the STEPS simulation at three different times after start of the simulation. A. 3D diffusion in an infinite volume from a point source. B. 1D diffusion in a finite tube: all the molecules are positioned at the border (distance = 0) initially. C. 1D diffusion in a semi-infinite tube with the concentration at the border (distance = 0) clamped. D. 1D diffusion in a finite tube with a constant and equal influx of the same species of molecule at both ends (displacement = −5 and +5).