AN R-MATRIX APPROACH FOR ELECTRON-SCATTERING OFF POLYATOMIC-MOLECULES

A variational R-matrix approach from first principles-the selected sta tes R-matrix method (SSRM)-is proposed for the study of electron scatt ering off polyatomic, non-linear molecules. Cartesian Gaussian-type or bitals are used exclusively to represent both valence and continuum or bitals. The two-electron integrals along with the nuclear-attraction i ntegrals are evaluated over the entire space. In a subsequent step the se integrals are effectively confined to the R-matrix sphere by subtra cting their contributions from outside the sphere using a multipole ex pansion which approximates the projectile-target interaction in the ou ter region. This approach allows the use of efficient integral codes a vailable for the calculation of molecular bound states. Correlation an d polarization effects are only taken into account for selected, energ etically low-lying (n + 1)-particle CI roots, corresponding to the R-m atrix 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 (2PI(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 refer ence data from the literature.