  The geometrical and electronic properties of the monolayer (ML) of tetracene
(Tc) molecules on Ag(111) are systematically investigated by means of DFT
calculations with the use of localized basis set. The bridge and hollow
adsorption positions of the molecule in the commensurate $\gamma$-Tc/Ag(111)
are revealed to be the most stable and equally favorable irrespective to the
approximation chosen for the exchange-correlation functional. The binding
energy is entirely determined by the long-range dispersive interaction. The
former lowest unoccupied orbital remains being unoccupied in the case of
$\gamma$-Tc/Ag(111) as well as in the $\alpha$-phase with increased coverage.
The unit cell of the $\alpha$-phase with point-on-line registry was adapted for
calculations based on the available experimental data and the computed
structures of the $\gamma$-phase. The calculated position of the Tc/Ag(111)
interface state is found to be noticeably dependent on the lattice constant of
the substrate, however its energy shift with respect to the Shockley surface
state of the unperturbed clean side of the slab is sensitive only to the
adsorption distance and in good agreement with the experimentally measured
energy shift.
