Retardation long-range potentials between two helium atoms

The retardation long-range potentials between two 1¹S (or 2³S) helium atoms are calculated. The complex dynamic multipole polarizabilities of 1¹S and 2³S states of helium are obtained through a variation-perturbation approach with B-spline and Slater-type basis functions in a configuration-interaction scheme. They are used to calculate the retardation long-range potentials. The retardation coefficients are examined. Their relative magnitudes are different for different systems. The possibility of an excited state of helium dimer from the 1¹S--2³S He₂ state is estimated. {} 1996 The American Physical Society.