Calculation of the dispersion interaction between two atoms

A general procedure for systematically evaluating the long range interaction between two hetero-nuclear atoms in arbitrary states is outlined. The $C_6$, $C_8$ and $C_{10}$ dispersion coefficients for the excited states of a number of alkali and alkaline atoms interacting with H and He are evaluated. One useful result concerns the lowest order $C_6$ coefficient for a pair of hetero-nuclear atoms. This can always be written in terms of sum rules only involving the oscillator strength. In addition, the coefficients for the long-range interaction between two homo-nuclear lithium atoms in a variety of excited states are presented.