The Cartesian Hessian matrix ( K , 3n x 3n) of the second energy derivatives may be viewed as (n x n) block matrix where each AB k block (3 x 3) is a pair interaction matrix as exposed in the work by Seminario. The full set of matrix elements between atoms A and B and can be written symbolically as the dyad of and BA F  vectors (see Appendix, p. 9.4):