I'm trying to figure out what the Hamiltonian for a simple molecule is using the Born-Oppenheimer approximation. 1) My textbook gives the Hamiltonian for a simple system like H2 when you hold the internuclear distance constant. The only terms that drop out are the ones where you take the Laplacian for the atoms. Since the B-O approximation separates the wavefunction into nuclear and electronic components, I'm guessing this must be the electronic component. 2) But what does the nuclear Hamiltonian look like?