The third derivative of energy over reaction progress (an anharmonicity parameter), has not yet been analyzed as such. Our novel approach allows to the more natural description of the anharmonic effects with the use of the Reaction Fragility concept that has served as a leverage for the idea a of the Reaction Fragility Spectra. Eq. 25 and eq. 26 allow for identifying all three energy derivatives in the energy expansion with ξ and to discover their informative potential for a chemical reaction. The Taylor expansion for the energy function E() reads: