Exact Treatment of the Long-Range Dipole–Dipole Interaction between Two Helium Atoms

We present a new expression for the long-range dipole–dipole interaction coefficient in terms of a polarizabilitylike response function. Using certain results of Schwartz, we are able to evaluate this coefficient in the case of the interaction between two helium atoms and, by careful error analysis, to set accurate error bounds to this quantity. We obtain C6 = 1.4605 ± 0.0025.