<p>&nbsp;&nbsp;&nbsp;&nbsp;The attractive
1/r<sup>6</sup> contribution to the Lennard-Jones potential is due to
the induced dipole-dipole interaction of two atoms.
Although each atom is electrically neutral, the instantaneous
fluctuations in the charge distribution can have nonspherical
symmetry. The resulting dipole in one atom can induce a dipole
moment in the other atom. The
resultant attractive potential is called the van der
Waals potential.</p>

<p>&nbsp;&nbsp;&nbsp;&nbsp;The repulsive interaction for small r is a consequence of the
Pauli exclusion principle. The electron wave functions of the
two molecules must distort to avoid overlap, causing some of the
electrons to be in different quantum states. The net effect is an
increase in kinetic energy and an effective repulsive interaction
between the electrons. The
1/r<sup>12</sup> form of the repulsive potential is chosen only for
convenience and is not derived from first principles.</p>