  Field strength correlators are semi-classically evaluated in the dilute gas
model of non-Abelian sources (instantons) and compared with lattice data for
QCD at zero temperature. We show that one of the Euclidean invariant, tensorial
structures vanishes for configurations being purely selfdual or anti-selfdual.
We compute the invariant functions contributing to the correlators within the
two lowest orders in an instanton density expansion. Fitting instanton size and
density for quenched and full QCD, we obtain a reasonable description.
