Electron binding energies of dipole-bound anions at the coupled cluster level with single, double, and triple excitations: $HCN−$ and $HNC−$

The electron binding energies for the weak dipole-bound anions $HCN−$ and $HNC−$ were found to be 13.2 and 35.7 cm⁻¹, respectively, at the coupled cluster level of theory with single, double, and triple excitations [CCSDT]. A more approximate approach, in which the triples contribution is treated perturbatively [CCSD(T)], provides an electron binding energy which is underestimated for $HCN−$ by 25% and overestimated for $HNC−$ by 19%. The new results provide benchmarks for model potentials aiming to reproduce dynamical correlation effects in electron–molecule interactions.