Another problem which emerges for long timesteps concerns boundaries: for reactions, the algorithm assumes free diffusion in the space around the entities. This assumption is mostly correct when the timestep is small but at large timestep, the chance of encountering a boundary during that timestep become significant. At that point the free diffusion assumption assumed in the previous equations breaks down leading the reaction happening too fast in the simulation. Simply put, entities close to boundaries have less volume in which to diffuse and, therefore, a higher chance of encounter than entities far from any boundary. We can expect this effect to become important when the scale of the system becomes comparable to the typical distance travelled during a timestep. For biological systems on the μm scale, and chemical entities diffusing with diffusion constants ~μms-1, this sets an absolute upper limit on the timesteps at ~0.1 s. Properly taking into account these boundary effects is beyond the scope of the present work. However, boundary effects are not expected to play an important role as the timescale for these effects is ~0.1 s, which is a much longer timescale than the limit previously set by single particle interaction at ~0.002 s.