We propose a simple method for calculation of low-lying shape electronic resonances of polyatomic molecules. The method introduces a perturbation potential and requires only routine bound-state type calculations in the real domain of energies. Such a calculation is accessible by most of the free or commercial quantum chemistry software. The presented method is based on the analytical continuation in a coupling constant model, but unlike its previous variants, we experience a very stable and robust behavior for higher-order extrapolation functions. Moreover, the present approach is independent of the correlation treatment used in quantum many-electron computations and therefore we are able to apply Coupled Clusters (CCSD-T) level of the correlation model. We demonstrate these properties on determination of the resonance position and width of the (2)Πu temporary negative ion state of diacetylene using CCSD-T level of theory.

This paper discusses the use of quantum scattering methods for the evaluation of the elastic (rotationally summed) integral and differential cross sections produced in collisions at low energies (E coll 20 eV) of electrons and positrons with polyatomic gaseous targets. The calculations use close-coupled equations and a parameter-free model of electron and positron interactions with the molecules in a single-collision situation. Comparison with existing experiments on the collision processes with the two projectiles helps us to know more detail about the features of the forces in play during the scattering events. Computed results are also found to be in good accord with experiments.

We introduce a computational method developed for study of long-range molecular Rydberg states of such systems that can be approximated by two electrons in a model potential of the atomic cores. Only diatomic molecules are considered. The method is based on a two-electron Rmatrix approach inside a sphere centered on one of the atoms. The wave function is then connected to a Coulomb region outside the sphere via multichannel version of the Coulomb Green's function.This approach is put into a test by its application to a study of Rydberg states of the hydrogen molecule for internuclear distances R from 20 to 400 bohrs and energies corresponding to n from 3 to 22. The results are compared with previous quantum chemical calculations (lower quantum numbers n) and computations based on contact potential models (higher quantum numbers n).

Absolute elastic cross sections, integral and differential, are computed for electron-cyclopropane collisions at energies from 1.0 up to 15.0 eV, spanning the region where possible shape resonance structures were detected by earlier experiments and calculations. The present results confirm the above findings and locate a resonance of A2' symmetry around 6.0 eV and a broader, weaker resonance of A2'' symmetry around 8 eV. The calculations are further shown to be in good agreement with the existing measurements of the angular distributions of the scattered electrons at energies from 2.0 up to 10.0 eV. The presently computed cross sections also agree with earlier R-matrix calculations (Beyer T, Nestmann B M, Sarpal B K and Peyerimhoff S D 1997 J. Phys. B: At. Mol. Opt. Phys. 30 3431) and with the available integral cross section measurements. A model local exchange potential of very simple usage is shown to provide quite good accord with the exact calculations at the static-exchange level.

Inelastic low-energy (0-1 eV) collisions of electrons with HeH + cations are treated theoretically, with a focus on the rovibrational excitation and dissociative recombination (DR) channels. In an application of ab initio multichannel quantum defect theory (MQDT), the description of both processes is based on the Born-Oppenheimer quantum defects. The quantum defects were determined using the R-matrix approach in two different frames of reference:the center-of-charge and the center-of-mass frames. The results obtained in the two reference systems, after implementing the Fano-Jungen style rovibrational frame-transformation technique, shows differences in the rate of convergence for these two different frames of reference.We find good agreement with the available theoretically predicted rotationally inelastic thermal rate coefficients. Our computed DR rate also agrees well with available experimental results. Moreover, several computational experiments shed light on the role of rotational and vibrational excitations in the indirect DR mechanism that governs the low energy HeH + dissociation process. While the rotational excitation is several orders of magnitude more probable process at the studied collision energies, the closed-channel resonances described by the high-n, rotationally excited neutral molecules of HeH contribute very little to the dissociation probability. But the situation is very different for resonances defined by the high-n, vibrationally excited HeH molecules, which are found to dissociate with approximately 90%

The kinetic energy distribution in the reactive mode in transition states correlates the asynchronicity of CPET with relative kinetic isotope effects.

We present a new computational implementation of a discrete-basis representation for the bound-continuum exchange interaction in electron scattering from polyatomic targets of arbitrary geometry. Both bound and continuum electrons are described within a single-centre expansion framework, the ensuing static interaction is obtained exactly and correlation-polarization effects are included via a parameter-free, density-functional-based model potential. Coupled scattering equations are solved efficiently using a Volterra integral form and convergence tests on the size of the additional basis needed to represent exchange forces are carried out extensively for H2O and O3 as benchmark systems.

Experimental data are presented for the scattering of electrons by H2O between 17 and 250 meV impact energy. These results are used in conjunction with a generally applicable method, based on a quantum defect theory approach to electron-polar molecule collisions, to derive the first set of data for state-to-state rotationally inelastic scattering cross sections based on experimental values.