Ab inito quantum chemical calculations are performed to quantify the stabilizing role of long C—H···O=C contacts in nucleic acid base pairing, which was suggested by Leonard, McAuley-Hecht, Brown & Hunter [(1995). Acta Cryst. D51, 136–139]. For the Watson-Crick adenine-uracil pair, a contribution of about 6% to the total bond energy is obtained. This weakly bonding effect is primarily a result of electrostatic attraction between the total positive charge of adenine C(2)—H and the negative end of the dipole uracil O(2)=C.