A method is presented for calculating electron-hole pair excitation due to an incident atom or molecule interacting with a metal surface. Energy loss is described using an textit{ab initio} approach that obtains a position-dependent friction coeffici
ent for an adsorbate moving near a metal surface from a total energy pseudopotential calculation. A semi-classical forced oscillator model is constructed, using the same friction coefficient description of the energy loss, to describe excitation of the electron gas due to the incident molecule. This approach is applied to H and D atoms incident on a Cu(111) surface, and we obtain theoretical estimates of the `chemicurrents measured by Nienhaus et al [Phys. Rev. Lett. textbf{82}, 446 (1999)] for these atoms incident on the surface of a Schottky diode.
A method for the calculation of the damping rate due to electron-hole pair excitation for atomic and molecular motion at metal surfaces is presented. The theoretical basis is provided by Time Dependent Density Functional Theory (TDDFT) in the quasi-s
tatic limit and calculations are performed within a standard plane-wave, pseudopotential framework. The artificial periodicity introduced by using a super-cell geometry is removed to derive results for the motion of an isolated atom or molecule, rather than for the coherent motion of an ordered over-layer. The algorithm is implemented in parallel, distributed across both ${bf k}$ and ${bf g}$ space, and in a form compatible with the CASTEP code. Test results for the damping of the motion of hydrogen atoms above the Cu(111) surface are presented.
