An isodesmic reaction is one in which the total number of each type of chemical bond is the same in both reactants and products. Because of this equality in bond type, results of calculations using such a reaction should benefit from cancellation of errors that are present in the theory.

Build each of the molecules shown above, and perform a geometry optimization using: (1) Gaussian/AM1, (2) Gaussian/PM3, and (3) Gaussian/DFT B3LYP/631G(d). For AM1 and PM3 methods, you will see the Heat of Formation (dfH) value listed in the Calculated Quantities window. To find the drH value, take the sum of the product dfH values, and subtract the sum of the reactant dfH values:
For the B3LYP/631G(d) values, you will see the RB3LYP energy listed in the Calculated Quantities window. To find drH and convert the energy units from hartrees to kcal mol-1
, use:

Place your results in the table below and compare with the experimental value.