Microscopic calculation of the van der Waals interaction between small metal clusters

The nonretarded van der Waals coefficients ${\mathit{C}}_6$ and ${\mathit{C}}_8$ are determined for all pairs of neutral sodium microclusters with 1, 2, 8, 20, and 40 atoms. These small systems are described self-consistently within the jellium model and their multipole dynamical polarizabilities calculated making use of the time-dependent local density approximation. It is found that quantum size effects, in particular Landau fragmentation, play a crucial role in the determination of these coefficients. Furthermore, it is found that the van der Waals coefficients are very insensitive to the internal temperature of the clusters, in the range of temperatures presently reachable experimentally. A brief discussion on the accuracy of the predicted values is carried out.