= Bond potential-well depth
{title2=$D_e$}
For a <diatomic molecule>, let $V(r)$ be the electronic <potential energy> at <bond length> $r$, with the <atomic nuclei> treated as fixed while finding the electronic <energy>. Choose zero at infinitely separated <chemical atoms> in the specified electronic <quantum states>. If the minimum occurs at $r_e$, the <bond potential-well depth> is
$$
D_e=V(\infty)-V(r_e)=-V(r_e).
$$
The bottom of this curve describes an equilibrium <bond length>, not an attainable motionless vibrational <quantum state>. Changing the zero of <potential energy> shifts both endpoints equally and leaves $D_e$ unchanged. A <quantum harmonic oscillator> approximates the curve near its minimum but rises without limit, so that local approximation alone cannot supply a dissociation asymptote. A <Morse potential> supplies both a minimum and a finite separated-fragment limit.<IUPAC Compendium of Chemical Terminology (Gold Book) by International Union of Pure and Applied Chemistry/dissociation energy>[definition: subscripts e and 0 distinguish the potential-energy minimum and ground state]{ref}
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