  In one dimension density-density interactions of particles reduce their
mobility and hence the Drude weight, which controls the divergence of the
optical conductivity at zero frequency, decreases. We study effects of pair
hopping events on this result in a 1D system of spinless fermions. The
considered model consists of the usual single-particle hopping and pair hopping
terms. In the absence of the density-density interactions, we first show that a
variation of the pair hopping amplitude results in a monotonic change of the
Drude weight. We next demonstrate that weak nearest-neighbor density-density
interactions increase the Drude weight, whereas in the regime of strong
interactions the Drude weight decreases as expected. Our numerical findings are
supported by bosonization results.
