  We investigate the adsorption of a single tetracyanoethylene (TCNE) molecule
on the silver (001) surface. Adsorption structures, electronic properties, and
scanning tunneling microscopy (STM) images are calculated within
density-functional theory. Adsorption occurs most favorably in on-top
configuration, with the C=C double bond directly above a silver atom and the
four N atoms bound to four neighboring Ag atoms. The lowest unoccupied
molecular orbital of TCNE becomes occupied due to electron transfer from the
substrate. This state dominates the electronic spectrum and the STM image at
moderately negative bias. We discuss and employ a spatial extrapolation
technique for the calculation of STM and scanning tunneling spectroscopy (STS)
images. Our calculated images are in good agreement with experimental data.
