An R-matrix approach for electron scattering off polyatomic molecules

A variational R-matrix approach from first principles-the selected states R-matrix (SSRM)-is proposed for the study of electron scattering off polyatomic, non-linear molecules. Cartesian Gaussian-type orbitals are used exclusively to represent both valence and continuum orbitals. The two-electron integrals along with the nuclear-attraction integrals are evaluated over the entire space. In a subsequent step these integrals are effectively confined to the R-matrix sphere by subtracting their contributions from outside the sphere using a multipole expansion which approximates the projectile-target interaction in the outer region. This approach allows the use of efficient integral codes available for the calculation of molecular bound states. Correlation and polarization effects are only taken into account for selected, energetically low-lying (n+1)-particle CI roots, corresponding to the R-matrix poles. Higher-lying roots are approximated by a static exchange calculation. In order to test this method, electron scattering off N2 is considered in detail for both resonant (2 Pi g) and non-resonant (2 Sigma g+) scattering symmetries. Good accordance is found between the eigenphase sums from the SSRM calculations and results obtained with a standard R-matrix package for linear molecules as well as with reference data from the literature.