Higher-order Cn dispersion coefficients for the alkali-metal atoms -: art. no. 042701

The van der Waals coefficients, from C-11 through to C-16 resulting from second-, third-, and fourth-order perturbation theory are estimated for the alkali-metal (Li, Na, K, and Rb) atoms. The dispersion coefficients are also computed for all possible combinations of the alkali-metal atoms and hydrogen. The parameters are determined from sum rules after diagonalizing a semiempirical fixed core Hamiltonian in a large basis. Comparisons of the radial dependence of the C-n/r(n) potentials give guidance as to the radial regions in which the various higher-order terms can be neglected. It is seen that including terms up to C-10/r(10) results in a dispersion interaction that is accurate to better than 1% whenever the inter-nuclear spacing is larger than 20a(0). This level of accuracy is mainly achieved due to the fortuitous cancellation between the repulsive (C-11,C-13,C-15) and attractive (C-12,C-14,C-16) dispersion forces.