How Does Dipole-Dipole Interaction Affect H₂ Molecule's Ground State Energy?

[avkr]

Homework Statement

Consider an $H_2$ molecule where the protons are separated by a wide
distance R and both are located on the z-axis. Ignoring the spin degrees
of freedom and treating the dipole-dipole interaction as a perturbation,
use perturbation theory to estimate an upper limit for the ground state
energy of this molecule.

Homework Equations

The Attempt at a Solution

What I think is that since hydrogen molecule has 0 dipole moment there should be no perturbation to calculate.

 

[vela]

The problem is asking you about the interaction between the two atoms that make up the molecule: how does one atom affect the other through a dipole-dipole interaction?