where 𝐺 𝑒𝑓 is the effective shear modulus of particles i and j (see formula below), 𝑅 𝑒𝑓 is the effective radius of particles i and j, 𝑒 is the coefficient of restitution of particles i and j, 𝑚 𝑒𝑓 is the effective mass of particles i and j and 𝐸 𝑒𝑓 is the effective Young modulus of particles i and j. Their respective expressions are