R-matrix calculations on electron-uracil collisions are presented within the static exchange, static exchange plus polarization, and close-coupling approximations. Particularly as input for the close-coupling calculations, a series of target calculations is performed which considers low-lying singlet and triplet excited states of the uracil target. The scattering calculations find three low-lying shape resonances of (2)A(") symmetry and three higher-energy Feshbach resonances of (2)A(') symmetry. In both symmetries the precise resonance parameters are found to be sensitive to the treatment of polarization effects employed. Cross sections are presented for both elastic scattering and electronic excitation. Comparisons are made with energy-dependent, differential cross section measurements at 90 degrees angle and good agreement is found for scattering energies above 0.5 eV.

R-matrix calculations on electron collisions with the purine bases found in DNA and RNA (i.e., adenine and guanine) are presented. Resonant anion states of these systems are identified by employing different approximation levels of ab initio theoretical methods, such as the static exchange, the static exchange plus polarization, and the close-coupling methods. The results are compared with other available calculations and experiments. All of these ab initio approximations, which we refer to as a scattering "model," give four shape resonances of 2 A (π ) symmetry within the energy range of 10 eV for both molecules. For adenine, the most sophisticated method, the close-coupling model, gives two very narrow 2 A (σ ) symmetry Feshbach-type resonances at energies above 5 eV. Quantitative results for the total elastic and electronic excitation cross sections are also presented.

Ab initio scattering calculations for low-energy electron collisions with gas-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, and close-coupling approximations. For both the bases, all of our scattering models find three shape resonances of 2A″ (π) symmetry lying below 10 eV, in accordance with other calculations and experimental observations. In addition to these, a fourth 2A″ resonance is found in all models for thymine and in our best model, i.e. the uncontracted close-coupling approximation, for cytosine. This model places the fourth 2A″ resonance in both systems in the 6–7 eV range. No evidence is found for the presence of low-lying Feshbach resonances in either system. Integral cross sections for electronically elastic and inelastic collisions are also presented.