van der Waals forces between atoms without permanent poles may be described as resulting from the interactions of multipoles associated with quantum transitions of the atoms. When the atoms are far apart, the dipole interaction is the only appreciable one. But at distances of the order of the kinetic theory radius, higher poles, usually neglected, must be considered. This paper examines and evaluates three terms of the series of interactions: the dipole-dipole, the dipole-quadrupole, and the quadrupole-quadrupole term, instead of the customary first term alone. The last appears to be in general small, but the dipole-quadrupole term requires consideration. Its effect is illustrated by plotting the potential energy curve for two helium atoms (a) neglecting the term, and (b) including it. The method outlined permits an easy estimate of dipole and quadrupole forces for all substances for which the wave function of only the normal state is known.

• Received 29 June 1931

DOI:https://doi.org/10.1103/PhysRev.38.747