  We study post-Lie algebra structures on pairs of Lie algebras (g,n), and
prove existence results for the case that one of the Lie algebras is
semisimple. For semisimple g and solvable n we show that there exist no
post-Lie algebra structures on (g,n). For semisimple n and certain solvable g
we construct canonical post-Lie algebra structures. On the other hand we prove
that there are no post-Lie algebra structures for semisimple n and solvable,
unimodular g. We also determine the generalized $(\al,\be,\ga)$-derivations of
n in the semisimple case. As an application we classify post-Lie algebra
structures induced by generalized derivations.
