Born-Oppenheimer approximation & distance

[QuarkDecay]

According to the Born-Oppenheimer approximation, what does the internuclear distance Req depend on?

• Atomic number Z?
• Rotational Energy of the nuclei?
• Electrons' kinetic energy?
• Coulomb interaction between the two nuclei?
• Coulomb interaction between the electrons?
• Vibrational energy of the molecule?

There could be more than one right answer. I'm confused because my books just show the calulations, but don't correlate the rest of the different types of interactions on them

 

[DrDu]

You can find arguments for basically all of them.