Long-range interactions between a He(2 ³S) atom and a He(2 ³P) atom for like isotopes

For the interactions between a He($2 ^3S$) atom and a He($2 ^3P$) atom for like isotopes, we report perturbation theoretic calculations using accurate variational wave functions in Hylleraas coordinates of the coefficients determining the potential energies at large internuclear separations. We evaluate the coefficient $C_{3}$ of the first order resonant dipole-dipole energy and the van der Waals coefficients $C_{6}$, $C_{8}$, and $C_{10}$ for the second order energies arising from the mutual perturbations of instantaneous electric dipole, quadrupole, and octupole interactions. We also evaluate the coefficient $C_{9}$ of the leading contribution to the third order energy. We establish definitive values including treatment of the finite nuclear mass for the ${}^3$He($2 ^3S$)--${}^3$He($2 ^3P$) and ${}^4$He($2 ^3S$)--${}^4$He($2 ^3P$) interactions.