To move each particle, two random numbers are drawn: one is used to update the x-axis position, the other to update the y-axis position. The random numbers themselves are produced from an algorithm that calculates its output according to a bell-shaped (Gaussian) curve. The peak probability of this Gaussian distribution is set at zero, with decreasing probability of obtaining either a positive or negative value further and further from zero. For the simulations used here, the Gaussian distribution for single particles, lipids and water molecules has a standard deviation (SD) of 5 units, while the Gaussian distribution for the movement of larger ion/water assemblies and proteins is much more tightly clustered around the mean, having an SD of 2 units. As with all Gaussian distributions, +/- 1 SD accounts for about two-thirds of all the values; +/- 3 SD includes 99% of all possible values.