where h is the height of the atom i and calculated by the projection length of the atom coordinate x i on the c vector. h max and h min represent the maximum and minimum heights of surface atoms, respectively. Specifically, δ i = 1 indicates that the atom i is located at the topmost layer, while δ i = 0 means that the atom i is located at the bottommost layer. Then, δ i is expanded via a set of exponential normal radial basis functions e RBF to compute the positional embedding ζ i of surface atom i: