Formula for success

Researchers in Belgium and the US have managed to make classical electrostatics work down to the atomic level by formulating a model in which each atom is represented by both a net electric charge and a dipole. These atomic charges, which in the model are free to move from one atomic site to another through bonds between atoms, are not accounted for by macroscopic models merely based on polarizable elements of material. The consideration of these charges makes the calculation of electric fields reliable down to the atomic level, which is the scale required to study the interactions with small molecules or atomic clusters.

To examine the idea in more detail, the group has considered pairs of tetrahedral Ag20, Ag84 and Ag120 clusters, which are placed 'tip-to-tip' along the z axis. Silver clusters have attracted widespread interest because of their ability to magnify, locally, the fields that are applied to them and because of the enhancement that they induce in the Raman scattering of small molecules placed in their vicinity.

The frequency-dependent polarizabilities obtained using the model turned out to be in excellent agreement with reference data obtained using a high-level quantum method – time-dependent density-functional theory. These results will be extended to the simulation of Raman scattering. They suggest a wider application for this type of model.

The group published its work in Nanotechnology.

•  The DCDA software used for the simulations presented in this article is available at