R -matrix calculation of low-energy electron collisions with uracil

Abstract

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.

Elastic and inelastic low-energy electron collisions with pyrazine

“…From now on we will refer to the basis set 6-311+G** as "diffuse" and the cc-pVDZ basis set as "compact." Our scattering calculations do not exhibit convergence of the positions of resonant structures with respect to the number of virtual orbitals included; we observe a behaviour for the positions of the three 2 A u , 2 B 2g and 2 B 3u π * resonances quantitatively very similar to the one observed by Dora 21 on uracil. Therefore, the optimal number of virtual orbitals used was chosen to give, within a specific model (SEP or closecoupling), positions of the three π * resonances as close to the experimental ones as possible without overcorrelating any of them.…”

“…In order to obtain optimal target orbitals for the description of the scattering process, several basis sets were tested. We have found our scattering results particularly sensitive to the quality of the target orbital description, and we therefore extended the analysis of basis set choice carried out by Dora et al 21 Specifically Dora et al used the basis set ccpVDZ for their calculations on uracil, while for their calculations on pyrazine Winstead and McKoy 26 used a larger 6-311++G** basis set, containing diffuse functions on all atoms; we tested these and another three similar basis sets. We used the Hartree-Fock SCF and the state-averaged CASSCF (SA-CASSCF) methods to generate the target orbitals; the calculations were performed using MOLPRO 2009.1.…”

“…43 The use of the modified virtual orbitals (MVO) (Ref. 42) in SE scattering models of our type had already been tested 21 and no significant differences found in the scattering results when compared with calculations using the HF orbitals.…”

“…We investigate the whole energy region below the ionization threshold of the molecule, with particular care devoted to the higher energy range, in order to look for the presence of core-excited shape or Feshbach resonances not reported for this molecule before. As a starting point for the construction of our scattering models, we choose those devised by Dora et al in their study 21 of electron collisions with uracil.…”

“…To the best of our knowledge the only inelastic and fully ab initio calculations of electron collisions with pyrimidinelike molecules are those of Winstead and McKoy 20 using the Schwinger multichannel method (SMC) and Dora et al 21 using the R-matrix method, on electron collisions with uracil.…”