  We compute the deterministic approximation of products of Sobolev functions
of large Wigner matrices $W$ and provide an optimal error bound on their
fluctuation with very high probability. This generalizes Voiculescu's seminal
theorem [Voiculescu 1991] from polynomials to general Sobolev functions, as
well as from tracial quantities to individual matrix elements. Applying the
result to $\exp(\mathrm{i} tW)$ for large $t$, we obtain a precise decay rate
for the overlaps of several deterministic matrices with temporally well
separated Heisenberg time evolutions; thus we demonstrate the thermalisation
effect of the unitary group generated by Wigner matrices.
