We consider the action of
semigroups e−tH, with H = −Δ + V on L2(Rν), on the scale of Sobolev spaces ℋα.
We show that while e−tH maps L2= ℋ0 to ℋ2 under great generality, there exist
bounded V so that, for all β > 0, e−tH[ℋβ] is not contained in any ℋα with
α > 2.