Long-range interactions between two excited helium atoms

Using variational wave functions in correlated Hylleraas coordinates, we have precisely calculated the dispersion coefficients $C_n$ (with n up to 10) for the long-range interaction between two excited helium atoms including the effect of finite nuclear mass. In this work, the long-range interaction coefficients reported are $C_3$, $C_6$, $C_8$, $C_9$, and $C_{10}$ for He$(2\,^1\!S)$--He$(2\,^1\!P)$; $C_6$, $C_8$, and $C_{10}$ for He$(2\,^3\!S)$--He$(2\,^1\!P)$; and $C_5$, $C_6$, $C_8$, and $C_ {10}$ for He$(2\,^1\!P)$--He$(2\,^1\!P)$ and He$(2\,^1\!P)$--He$(2\,^3\!P) $ for the $\Delta$, $\Pi$, and $\Sigma$ molecular states.