Nuclear-Structure Effects in Pion Double Charge Exchange

We show that nuclear structure can have a strong effect on the calculated double-charge-exchange cross section. The measured ratio of cross sections at forward angles on an ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$ target relative to an ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ target is $R=2.3\mathrm{}\pm 1$. Our calculated value is $R=16\mathrm{}$ with the most simple nuclear model, but a well known, more sophisticated model leads to $R=3\mathrm{}$.