  The measurement of alloying core-level binding energy (CLBE) shifts has been
used to give a precise meaning to the fundamental concept of charge transfer.
Here, ab-initio density-functional calculations for the intermetallic compound
MgAu are used to investigate models which try to make a connection between the
core levels shifts and charge transfer. The calculated CLBE shifts agree well
with experiment, and permit an unambiguous separation into initial-state and
screening contributions. Interestingly, the screening contribution is large and
cannot be neglected in any reasonable description. Comparison of the calculated
results with the predictions of simple models show that these models are not
adequate to describe the realistic situation. On the positive side, the
accuracy of the density-functional calculations indicates that the combination
of experiments with such calculations is a powerful tool to investigate unknown
systems.
