1D simulations of gas freely diffusing into vacuum. For simplicity, all quantities are presented in dimensionless computational units, where both the initial density and pressure are normalised to unity. In this simulation, the left side of x = 0 is initially a constant non-vacuum state, while the right side is set to a vacuum state. Three solutions are presented: the exact solution, the numerical solution involving real vacuum, and the numerical solution involving mimic vacuum. All curves correspond to the same simulation time.