  We investigate the effect of assisted hopping in the Anderson impurity model.
We use the flow equation method, which, by means of unitary transformations,
generates a sequence of Hamiltonians in order to eliminate the assisted hopping
terms. This approach yields a renormalized on-site energy $\epsilon_d^*$, a
renormalized correlation energy $U^*$ and other terms, which include pair
hopping. For some parameter values, the initial Hamiltonian flows towards an
attractive Anderson model. We argue that this result implies a tendency towards
local pairing fluctuations.
