R-matrix study of elastic and inelastic electron collisions with cytosine and thymine

Scattering calculations for low-energy electron collisions withgas-phase DNA pyrimidine bases (cytosine and thymine) are performed using the R-matrix method. The low-lying resonant states of the anions, formed due to electron capture by the bases, are identified using different levels of theory,including static exchange, static exchange plus polarization, andclose-coupling approximations. For both the bases, all of our scattering modelsfind three shape resonances of $^2A''$ ($\pi$) symmetry below 10 eV, inaccordance with other calculations and experimental observations. For bothbases good agreement is found between the observed position of the lowestresonance and the value obtained with our best model, i.e., the uncontractedclose-coupling approximation. In addition, this model finds a fourth $^2A''$resonance in both systems, lying in the $6-7$ eV range. No evidence is found for the presence of low-lying Feshbach resonances in either system. Quantitative integral cross sections for electronically elastic and inelastic collisions are also presented.