  Since metallic surface states on (111) noble metals are free-electron like,
their propagators can be evaluated analytically. Since they are well-screened,
one can use simple tight-binding formalism to study their effects. The needed
phase shifts can be extracted from experiment. Hence, one can now make
quantitative predictions of these slowly-decaying, oscillatory indirect
interactions. For the (isotropic!) pair interactions (which decay as the
inverse square of adatom-adatom separation), remarkable agreement has been
obtained with experiments by two groups. We have extended the formalism to
consider the full indirect ("triple") interaction of 3 adsorbates, which is the
sum of the 3 constituent pair interactions plus the non-pairwise "trio"
contribution, which tends to decay with the 5/2 power of perimeter. Here, we
concentrate on interactions due to ordered overlayers and to linear defects,
relating the latter to the interactions of (nx1) ordered overlayers and to the
constituent pair interactions. We compare with experimental studies of
interactions of adatoms with adchains and of consequent 1D motion of adatoms
trapped between two such parallel chains. We discuss implications for step-step
interactions (on vicinal surfaces), with attention to the modification of the
surface state itself for small terrace widths.
