The second derivative (K constant) in atomic resolution will be extracted from the second term the expansion in eq. 25. Thanks to the applied vector formulation, it has been proved to be of an electronic nature only, which makes it superior for the chemical analysis over the conventional reaction force constant. K can be related to the DF Connectivity Matrix elements CAB rather that to the Cartesian Hessian elements by using the result for the second term proved with eq. 26 (the transformation of the product of vectors /