This Java applet demonstrates the effects of two atoms approaching each other. It is an interactive MO scheme for H₂ and He₂ and shows the orbital contours, energies and bonding energies as a function of the distance between the two selected atoms. The scrollbar allows you to change the atom-atom distance.
The orbitals shown below are the bonding combination of the two 1s-AOs of H and He, which differ slightly in size. For this qualitative demonstration, however, we can use the same orbitals for the two atoms. Also, the energy profiles and levels are only qualitative, although the bonding description given is quite realistic.

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H₂ and He₂ -The Simplest Examples