Contribution of electron correlation to the stability of dipole-bound anionic states

It has long been assumed that electron correlation effects are relatively unimportant for describing dipole-bound anionic states. It is shown here that this assumption is incorrect: high-level electronic structure calculations on the dipole-bound anion states of ${\mathrm{CH}}_3$CN, ${\mathrm{C}}_3$ ${\mathrm{H}}_2$, and ${\mathrm{C}}_5$ ${\mathrm{H}}_2$ reveal that for these species a large fraction of the electron binding energy derives from the dispersion-type interaction between the loosely bound electron and the neutral molecule. The predicted values of the electron affinities of the dipole bound states of ${\mathrm{CH}}_3$CN and ${\mathrm{C}}_3$ ${\mathrm{H}}_2$, 108 and 173 ${\mathrm{cm}}^{\mathrm{-}1}$, respectively, are in excellent agreement with the recent experimental results, 93 ${\mathrm{cm}}^{\mathrm{-}1}$ and 171±50 ${\mathrm{cm}}^{\mathrm{-}1}$, respectively. The predicted value for ${\mathrm{C}}_5$ ${\mathrm{H}}_2$ is 614 ${\mathrm{cm}}^{\mathrm{-}1}$. © 1996 The American Physical Society.