where s r,8 and α n in D3(0) have fixed values (s r,8 = 1, α 6 = 14, α 8 = 16) and s r,6 is optimized for each functional. The parameters α 1 and α 2 are adjustable nonlinear parameters in the D3(BJ) scheme that also depends on the DFT functional. The D3(op) adds one extra nonlinear fitting parameter β n with the following constraint: