We present an application of pseudopotentials to ab initio calculations of elastic and electronically inelastic electron-molecule-collison cross sections. We use the Schwinger multichannel method (SMC) implemented with the local-density norm-conserving pseudopotentials of Bachelet, Hammann, and Schlüter [Phys. Rev. B 26, 4199 (1982)]. In our procedure, the core electrons and protons are replaced by the nonlocal but single-particle pseudopotential and the valence electrons are treated in a many-body framework, as in the SMC method. Our calculated integral and differential cross sections are in very good agreement with previous all-electron calculations.

The Schwinger multichannel method [K. Takatsuka and V. McKoy, Phys. Rev. A 30, 1734 (1984)], which is based on the Schwinger variational principle for the scattering amplitude [J. Schwinger, Phys. Rev. 72, 742 (1947)], was designed to account for exchange, polarization and electronically multichannel coupling effects in the low-energy region of electron scattering from molecules with arbitrary geometry. The applications of the method became more ambitious with the availability of computer power combined with parallel processing, use of norm-conserving pseudopotentials and improvement of the description of target excited states (minimal orbital basis for single configuration interaction). The most recent applications involving 33 and 45 electronically open channels for phenol and ethylene molecules, represent good examples of the present status of the method. In this colloquium, we review the strategy and point out new directions to apply the method in its full extension.

Measured and calculated differential cross sections for elastic (rotationally unresolved) electron scattering from two primary alcohols, methanol $\left(\mathrm{C}{\mathrm{H}}_3\mathrm{O}\mathrm{H}\right)$ and ethanol $\left({\mathrm{C}}_2{\mathrm{H}}_5\mathrm{O}\mathrm{H}\right)$, are reported. The measurements are obtained using the relative flow method with helium as the standard gas and a thin aperture as the collimating target gas source. The relative flow method is applied without the restriction imposed by the relative flow pressure conditions on helium and the unknown gas. The experimental data were taken at incident electron energies of 1, 2, 5, 10, 15, 20, 30, 50, and $100\mathrm{eV}$ and for scattering angles of 5°–130°. There are no previous reports of experimental electron scattering differential cross sections for $\mathrm{C}{\mathrm{H}}_3\mathrm{O}\mathrm{H}$ and ${\mathrm{C}}_2{\mathrm{H}}_5\mathrm{O}\mathrm{H}$ in the literature. The calculated differential cross sections are obtained using two different implementations of the Schwinger multichannel method, one that takes all electrons into account and is adapted for parallel computers, and another that uses pseudopotentials and considers only the valence electrons. Comparison between theory and experiment shows that theory is able to describe low-energy electron scattering from these polyatomic targets quite well.

We report elastic differential, integral, and momentum transfer cross-sections for H2X molecules (X: O, S, Se, and Te) obtained at the static exchange level of approximation. The energy range considered was from 2 up to 30 eV for H2O and from 5 up to 30 eV for the other molecules. Our calculations were performed with the Schwinger multichannel method with pseudopotentials [M. H. F. Bettega, L. G. Ferreira, and M. A. P. Lima, Phys. Rev. A 47, 1111 (1993)], combined with a Born closure procedure in order to account for the long-range potential due to the permanent dipole moment of the targets. Our calculated cross-sections for H2O and H2S are in good agreement with other theoretical results. Agreement with available experimental data is also encouraging. It was found that molecular size plays a crucial role in the scattering process. The influence of heavy and H atoms in the collisions is also discussed. For the integral cross-sections of the heavier molecules we also investigated incident energies below 5 eV, looking for possible shape resonances. Through the symmetry decomposition of the integral cross-sections and the eigenphase sum analysis, we found shape resonances for H2S, H2Se, and H2Te at the B2 symmetry. For H2Te, we have also found a shape resonance at the A2 symmetry. For all molecules a very broad structure was found at the A1 symmetry. This is the first work to report such resonances for H2Se and H2Te.

We report elastic integral and differential cross sections for electron scattering from the aza-derivatives of pyrrole, furan, and thiophene, namely, pyrazole, imidazole, isoxazole, oxazole, isothiazole, and thiazole. The calculations were performed within the Schwinger multichannel method with pseudopotentials, with inclusion of static, exchange, and polarization interactions, for energies up to 10 eV. We found two π* shape resonances and a high-lying σ* shape resonance in each system. A sharp low-energy σ* resonance was also identified in isothiazole and thiazole. Pyrazole and imidazole presented yet a broad low-lying σ* resonance. The positions of the resonances agree very well with existing experimental results. We discuss the similarities and differences among the resonances of these compounds.

We report on the shape resonance spectra of uracil, 5-fluorouracil, and 5-chlorouracil, as obtained from fixed-nuclei elastic scattering calculations performed with the Schwinger multichannel method with pseudopotentials. Our results are in good agreement with the available electron transmission spectroscopy data, and support the existence of three π* resonances in uracil and 5-fluorouracil. As expected, the anion states are more stable in the substituted molecules than in uracil. Since the stabilization is stronger in 5-chlorouracil, the lowest π* resonance in this system becomes a bound anion state. The present results also support the existence of a low-lying \documentclass[12pt]{minimal}\begin{document}$\sigma _{\rm CCl}^*$\end{document}σ CCl * shape resonance in 5-chlorouracil. Exploratory calculations performed at selected C–Cl bond lengths suggest that the \documentclass[12pt]{minimal}\begin{document}$\sigma _{\rm CCl}^*$\end{document}σ CCl * resonance could couple to the two lowest π* states, giving rise to a very rich dissociation dynamics. These facts would be compatible with the complex branching of the dissociative electron attachment cross sections, even though we cannot discuss any details of the vibration dynamics based only on the present fixed-nuclei results.