Ab initio calculations of low-energy electron scattering by HCN molecules

We report results for vibrationally elastic scattering over the energy range 0.0006--11.6 eV. The interaction potential is composed of a near-Hartree-Fock static term plus a parameter-free model of the correlation-polarization potential. The exchange interaction is included exactly through a separable expansion. Results with a model-exchange potential (free-electron-gas plus orthogonalization) are also reported. A resonance appears in {Π} symmetry near 2.7 eV (width 1.9 eV) that may be the same feature observed in several experiments. In the model-exchange calculation the {Π} resonance is shifted toward higher energy (3.8 eV, width 2.4 eV). The {Σ} symmetry was also found to be very sensitive to the treatment of exchange and to the effect of polarization. Differential and rotational excitation cross sections are evaluated in the multipole-extracted adiabatic-nuclei approximation. Results are compared with the available experimental and theoretical data.