The first sum runs over all unique pairs of atoms A and B. The second sum runs over the n th -order dispersion term, which is calculated with Cn AB , isotropic n th order coefficient for each atomic pair AB, and rAB, which is the distance between atoms A and B. The coefficients sn are density-functional dependent scaling parameters. Since a given density functional already includes local electron correlations, the dispersion correction should be damped in a functional-specific way at small distances. For this reason, a damping function, 𝑓 𝑑𝑎𝑚𝑝,𝑛 (r AB ), is introduced. Various options for damping functions have been proposed, with the most popular and widely used being zero-damping (D3(0)) and Becke-Johnson damping (D3(BJ)), where the latter consistently outperforms the former. While not so widely used, the optimal power, D3(op), forms can exhibit even better performance . The forms of the damping functions are as follows: