In Eq. (2.4), 𝑘 𝑛 is the elastic constant for normal contact, 𝛾 𝑛 is the viscoelastic damping constant for normal contact, 𝑣 𝑟 𝑛 is the normal relative velocity of particles i and j. In Eq. (2.5), 𝜌 𝑖 is the density of particle i and 𝑔 is the gravitational acceleration.