Dipole, quadrupole, octupole, and dipole–dipole–quandrupole polarizabilities and second hyperpolarizabilities at real and imaginary frequencies for helium in the 2 3 S state: Dispersion‐energy coefficients for interactions between He(2 3 S ) and H(1 2 S ), He(1 1 S ), He(2 3 S ), or H 2 (X 1 ∑ g + )

We have determined the dynamic dipole (α 1 ), quadrupole (α 2 ), octupole (α 3 ), and dipole–dipole–quadrupole (B) polarizabilities and the second hyperpolarizability tensor (γ) for the helium atom in its lowest triplet state (2 3 S ). We have done so for both real and imaginary frequencies: in the former case, for a range of frequencies (ω) between zero and the first electronic‐transition frequency, and in the latter case for a 32‐point Gauss–Legendre grid running from zero to ħω = 20 E h . We have also determined the dispersion‐energy coefficients C 6 , C 8 , and C 10 for the systems H(1 2 S )He(2 3 S ), He(1 1 S )He(2 3 S ), and He(2 3 S )He(2 3 S ) and the C , C , C , C , and C coefficients for the interaction He(2 3 S )H 2 ( X 1 ∑ ). Our values of the higher‐order multipolar polarizabilities and of γ for the 2 3 S state of helium are, we believe, the first to be published.