Abstract: This paper employs the harmonic oscillator model to investigate the origins of inter-molecule potential. We find the interaction between non-polar molecules comes from the quantum effect, while the interaction between polar molecule includes both classical and quantum parts. The classical interaction is not the usual dipolar interaction: on the one hand, it decreases with the distance as 1R6; on the other hand, the interaction energy is proportional to the square of electric moment. Furthermore, the interaction exists even when one of the molecule has zero dipole moment. The appearance of this kind of potential is essentially due to the electric quadrupole moment.

Key words: harmonic oscillator model, electric dipole moment, electric quadrupole moment, long-distance attraction