Therefore, if each of the class 𝒙 ! and 𝒙 " contain Nd number of datapoints (here Nd is the same as the number of MD generated trajectories m, see section 2.2.3.), the 𝒙 $ matrix is a 6 × 2𝑁 $ matrix (this "6" comes from the fact that each of these Nd datapoint in class 𝒙 ! or 𝒙 " has a dimension of 6 or the number of features).