= Ground-state dissociation energy
{title2=$D_0$}
The <ground-state dissociation energy> starts at the lowest vibrational <quantum state>, with the lowest allowed rotational <quantum state> and the fragment states explicitly specified. For a <diatomic molecule> whose fragments are ground-state <chemical atoms>, omitting any additional rotational correction,
$$
D_0=D_e-E_{\rm ZP},
$$
where $E_{\rm ZP}$ is the vibrational <zero-point energy> measured upward from the well minimum. The initial <molecule> already has this <energy>, so less additional <energy> is needed to reach the same final limit.<IUPAC Compendium of Chemical Terminology (Gold Book) by International Union of Pure and Applied Chemistry/bond-dissociation energy in theoretical chemistry>[definition of bond-dissociation energy in theoretical chemistry: D0 = De minus zero-point vibrational energy]{ref}
For fragments that are themselves <molecules>, include their <zero-point energies> too. Define $D_e$ as the electronic <energy> difference between separately relaxed products and reactant; then, with consistent rotational conventions,
$$
D_0=D_e+\sum_{\rm fragments}E_{\rm ZP}-E_{\rm ZP,parent}.
$$
Thus “subtract the parent's <zero-point energy>” is sufficient for atomic products but generally incomplete for molecular products. An excited final <quantum state> raises its channel threshold by its excitation <energy>; that threshold does not become the ground-product $D_0$ just because a detector selects it.
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