Retardation long-range potentials between two helium atoms

The retardation long-range potentials between two 1${\mathrm{}}^1$S (or 2${\mathrm{}}^3$S) helium atoms are calculated. The complex dynamic multipole polarizabilities of 1${\mathrm{}}^1$S and 2${\mathrm{}}^3$S 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${\mathrm{}}^1$S–2${\mathrm{}}^3$S ${\mathrm{He}}_2$ state is estimated. © 1996 The American Physical Society.