What you calculated is electronic energy, rather than energy of complete dissociation reaction of the molecule.

The concept in quantum chemistry field closest to the so-called "energy of complete dissociation reaction of a molecule" is atomization energy. For example, for H2O molecule you should calculate 2*E(H) + E(O) - E(H2O).

Best,

Tian

I have a question about the energy value obtained from the Gaussian output file.

I've done opt + freq calculation on dichloromethane at B3LYP 6-31+G* as an example and I get an E(B3LYP)=-959.700528 HARTREE/PARTICLE, I know that this energy is related to complete dissociation reaction of the molecule. 

If i use a 1 Hartree = 627.5 kcal/mol i get a huge number to be an energy value, is this correct?

Or is there another way to obtain an energy value in kcal/mol more reliable by common sense?

Thanks a lot !

Alessio Macorano