Hi
I obtain the gass phase reaction energy of -234 kcal/mol for the following simple reaction using Dmole3, while the experimental ggas phase gibbs free energy of this reaction is -221 kcal/mol. I used the frequency calculations to correct the gibbs free energies at ambient temperature but it does not change considerably the calculated energy. I tried different functionals, Quality, core treatments and basis sets and it remains the same. Does anyone have an idea about the source of the error.

 Reaction : H2O+H2O==H3O(+) + (OH)-

 Thank you