  We have calculated long-range molecular potentials of the $0_g^{+}$,
$0_u^{-}$ and $1_u$ symmetries between highly-excited rubidium atoms. Strong
$np+np$ potentials characterized by these symmetries are important in
describing interaction-induced phenomena in the excitation spectra of high $np$
Rydberg states. Long-range molecular resonances are such phenomena and they
were first reported in S.M. Farooqi {\it et al.}, Phys. Rev. Lett. {\bf 91}
183002. One class of these resonances occurs at energies corresponding to
excited atom pairs $(n-1)d+ns$. Such resonances are attributed to $\ell$-mixing
due to Rydberg-Rydberg interactions so that otherwise forbidden molecular
transitions become allowed. We calculate molecular potentials in Hund's case
(c), use them to find the resonance lineshape and compare to experimental
results.
