Ab initio method to calculate C-C bond dissociation energy? I'm trying to review how to calculate bond dissociation energy for C-C in ethane, a very simple calculation (or should be).

I get the following Hartree energies for SCF 6-31G(p) for methane, ethane, and H2:

ch4_spe            -40.194639920746,
ethane             -79.228125042573,
h2                 -1.126456057886,


I would think that to get the C-C bond dissociation energy, I would simply do (Perl code)

$energies{ethane} - $energies{ch4_spe} * 2 - $energies{h2}


correct?

The reason that I'm asking is that this give an answer of 2.2876 Hartree using NWChem, which converts to 6006 kJ/mol, which is off by about a factor of 17 from the correct value of about 346 kJ/mol.  I know that SCF isn't that accurate, but it shouldn't be this far off. I optimized the bond lengths using Avogadro.

This is the correct method to calculate the C-C bond energy?
This "experiment" is about determining bond dissociation energy.

The general way to solve this isn't like calculating enthalpies of reaction, as I initially thought, but rather through calculating the energy of two CH3 radicals, and comparing it with ethane.  As another user pointed out, this should be done with unrestricted Hartree Fock and with diffuse basis sets.

Thus, energy(ethane) - 2*energy(methyl radical) is the solution.