Classical diffusion coefficient D_c of particle 1 versus m for ε = 0.1 (triangles), 2 (squares), and 4 (circles). The dashed line (in red) denotes the classical diffusion coefficient of unperturbed case, i.e., ε = 0. Inset: mean square of classical momentum of particle 1 versus time for m = 0.001. Dashed line (in red) denotes the fitting function of the form 〈p12(t)〉=Dct with D_c = 0.087. The parameters are K₁ = 1.8, K₂ = 0, λ = 10, and L = π.