  The van der Waals coefficients, from C_11 through to C_16 resulting from 2nd,
3rd and 4th order perturbation theory are estimated for the alkali (Li, Na, K
and Rb) atoms. The dispersion coefficients are also computed for all possible
combinations of the alkali atoms and hydrogen. The parameters are determined
from sum-rules after diagonalizing the 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 percent whenever the
inter-nuclear spacing is larger than 20 a_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.

Atomic Physics;Chemical Physics