For the interactions between a He(23S) atom and a He(23P) 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 C3​ of the first order resonant dipole-dipole
energy and the van der Waals coefficients C6​, C8​, and C10​ for
the second order energies arising from the mutual perturbations of
instantaneous electric dipole, quadrupole, and octupole interactions. We also
evaluate the coefficient C9​ of the leading contribution to the third order
energy. We establish definitive values including treatment of the finite
nuclear mass for the 3He(23S)--3He(23P) and 4He(23S)--4He(23P) interactions.Comment: This article has been accepted by Physical Review