Reference-plane position for the atom-surface van der Waals interaction

We present a detailed study of the dynamic image plane which characterizes the response of a metal surface to external fields varying slowly in space. The image plane enters directly in the definition of the reference-plane position of the van der Waals interaction between an atom and a metal surface. Calculations of the reference-lane position for He and ${\mathrm{H}}_2$ on simple (jelliumlike) and noble-metal surfaces have been performed and improve earlier estimates of this important parameter. In the case of He on the noble metals, the quantitative change in the reference-plane position is relatively small and does not affect significantly the form of previously derived interaction potentials.