Very recently, we obtained well-resolved scanning tunneling microscopy (STM) images of what appear to be Ga tetramers on $\mathrm{Ag}\mathrm{}\left(001\right).$ To clarify this finding, we apply two related semiempirical theoretical techniques [atom superposition electron delocalization (ASED) and electronic scattering quantum chemistry (ESQC)], previously applied with success to other adsorbate systems, to single Ga adatoms and Ga tetramers adsorbed on $\mathrm{Ag}\mathrm{}\left(001\right).$ The dependence of STM images on the bonding site and on various tip configurations is computed by the ESQC method. These simulations agree with the observed STM images confirming the tetramer model, but show no clearcut bonding-site dependence, and hence do not allow a determination of the Ga adsorption site by comparison with experimental STM images alone. However, ASED calculations for a single Ga adatom yield an adsorption energy for the fourfold hollow site which is about 1 eV higher than for the bridge and top sites, indicating that the hollow site is also the preferred bonding site for Ga adatoms forming a tetramer.

• Received 15 October 1997

DOI:https://doi.org/10.1103/PhysRevB.57.10035