# Ground state energy of H2 vs experimental value

In the A brief overview of VQE | PennyLane Demos, we found out that the ground state e of H_2 is -1.13619 Ha = -30.917 eV. However the source at Floria University claims that the number is 4.52 eV.

I think I must have misunderstood one of the sources. Could you help me clear that up

Hi @mchau,

It seems to me that they’re referring to the bond dissociation energy or binding energy. From our demo on modelling chemical reactions you can see that the calculated bond dissociation energy is 0.198772 Hartrees = 5.4eV. As mentioned in the demo this calculation can be improved (at a higher computational cost).

From what I see here it looks like the experimental value should be 4.75 eV though, so I’m not sure why Florida mentions it’s less. I don’t know what conditions or assumptions are made in each case so you’ll have to investigate further to find which is the correct value.

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I see. About FCI ground energy, assume that it is 4.75 eV then it is still different from A brief overview of VQE | PennyLane Demos, which is −30.917eV right?

Hi @mchau,

It’s a question of terminology. If you look at the first figure in the Potential Energy Surfaces section of the Modelling chemical reactions demo then you see that we have different energies: repulsion, equilibrium, and dissociation. What we call the ground state in the VQE demo is the lowest energy, which is the equilibrium energy. For H2 this is about -1.1357 Hartree = -30.9eV. The bond dissociation energy, as mentioned in the Modelling chemical reactions demo, is around 5ev.

FCI stands for Full Configuration Interaction. It models all possible interactions between all of the electrons in the molecule. It gives you a very precise view of the energy of your molecule at all points of the curve. It doesn’t denote a specific point. So if you’re reading that the FCI ground energy is about 5eV this actually refers to the dissociation energy instead of the equilibrium one (the lowest energy of the curve).

I hope this helps clarify things!

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