The dispersion corrections investigated here most substantially impact the HCN + O( 3 P) ® HCNO reaction energy [ΔEe(rxn) = -De(HCN-O)]. Over the entire data set, the mean shift in the reaction energy is -0.37 kcal mol -1 and with an attendant standard deviation of 1.09 kcal mol -1 , but these statistics are misleading. As expected, the dispersion corrections decrease ΔEe(rxn) and hence increase De(HCN-O) with the exception of D3(BJ), which yields shifts of the opposite sign in all cases. The (BJ) damping schemes more strongly influence the dissociation energy than their (0) counterparts, and the (M) variations influence results even greater still. Six dispersion schemes yield a maximum change in ΔEe(rxn) greater than 3.9 kcal mol -1 ! For example, the reaction energy is decreased by a conspicuous -6.2 kcal mol -1 in the case of HF-D3M(BJ)ATM and increased by a dramatic 7.8 kcal mol -1 with DSD-PBEB95-D3(BJ). Our recent benchmark research with definitive wavefunction methods gives a ΔEe(rxn) = -51.08 kcal mol -1 . Because ΔEe(rxn) = (14.4, -109.4) kcal mol -1 for (HF, DSD-PBEB95), the [D3M(BJ)ATM, D3(BJ)] corrections do move the reaction energy in a favorable direction, but the magnitude of the change is too large to be a consequence of intramolecular dispersion.