Long-range interactions of metastable helium atoms

The electric dipole, quadrupole, and octupole polarizabilities of a helium atom and the associated dispersion coefficients for the interaction of two helium atoms are calculated for the atomic $n=2$ singlet and triplet states, respectively, He(2¹S) and He(2³S). The calculations utilize highly accurate, variationally determined, wave functions composed of doubled basis sets. The effects of retardation due to the finite speed of light are calculated for the dispersion potentials of two He(2¹S) or two He(2³S) atoms and for the atom-surface interaction potentials of a He(2¹S) or a He(2³S) atom with a perfectly conducting or a dielectric wall.