Hydrogen neutral fraction x_H as a function of radius r for the Strömgren volume tests. The simulation results were binned into 100 radial bins in log space. The full lines show the average in each bin, while the shaded regions underneath the curves correspond to the standard deviation in each bin. For reference, the semi-analytic reference solution derived from Eq. (2) is also shown, as well as the Strömgren radius for both versions of the test.