Retardation long-range potentials between two helium atoms

The retardation long-range potentials between two 1^1S (or 2^3S) helium atoms are calculated. The complex dynamic multipole polarizabilities of 1^1S and 2^3S 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 1^1S--2^3S He₂ state is estimated. 1996 The American Physical Society.