where the first two sums are over all N at in the unit cell and the third summation is over all translations L of the unit cell, i ̸ = j for L = 0. C 6i j is the dispersion coefficient for atoms i and j. The values for C 6ii are parameterized for each element and are invariant to the chemical environment, meaning C 6ii for carbon in ethene is the same as carbon in ethane . A modification of this method is called Grimme D3 :