  The success of density functional theory for the description of the
adsorption of atoms on surfaces is well established, and based on recent
calculations using gradient corrections, it has been shown that it also
describes well the dissociative adsorption of molecules at surfaces -
admittedly however, the data base for reactions at surfaces is still somewhat
small. In the present paper the power of density functional theory calculations
is demonstrated by investigations for two different adsorption systems, namely,
one with a strongly electropositive adsorbate [Na on Al(111)] and one with a
strongly electronegative adsorbate [O on Ru(0001)]. In each case, new hitherto
not expected adsorbate phases have been predicted by the theory: For Na on
Al(111) the stability of a "four-layer" surface alloy was identified while for
O on Ru(0001) it was predicted that the formation of a (1 x 1)-O adlayer should
be possible which implies that the apparent saturation coverage of 1/2 is due
to kinetic hindering.
