Electron nuclear dynamics of H+ + uracil at ELab = 1.47 and 4.00 keV: a prototype of ion cancer therapy reactions.

We present a complete simplest-level electron nuclear dynamics (SLEND) investigation of H+ + C4H4N2O2 (uracil) at ELab = 1.47 and 4.00 keV. These reactive systems are computationally tractable prototypes to simulate ion-induced DNA damage during proton cancer therapy (PCT). SLEND is a time-dependent, variational, on-the-fly, and non-adiabatic method that describes nuclei with classical mechanics and represents electrons with a single-determinantal wavefunction in the Thouless representation. Current simulations with the 6-31G** basis set involve 3136 simulated trajectories per collision energy from 32 symmetry-inequivalent projectile-target orientations. Unlike some alternative theoretical methods, SLEND/6-31G** does not impose constraints on the nuclei motions and can therefore describe the H+ projectile scattering and the collision-induced chemical reactions occurring in these systems. Specifically, SLEND/6-31G** predicts six exit channels: one simple projectile scattering channel and five reactive ones corresponding to (1) poly-cyclization, (2) dehydrogenation, (3) ring opening, (4) complete fragmentation, and (5) multi-process reactions of the C4H4N2O2 target upon collision. Each of these reactive channels encompasses an astonishing number of reaction subtypes that render reactive chemical species that can subsequently damage DNA. We reveal and analyze all these processes through various computer animations ("movies") and plotting schemes. All this valuable chemical information is inaccessible by experiments and alternative theoretical methods. We quantify the reactive processes in terms of relative abundances of the produced chemical species and of integral cross sections (ICSs) of the fragmentation and target-to-projectile electron-transfer reactions. These ICSs can be used as input data in subsequent Monte Carlo simulations of PCT phenomena of clinical relevance. Scattering angle functions in the low impact parameter range reveal cases of considerable forward and backward scattering; in that range, those functions display fluctuating patterns of maxima and minima that reflect the projectiles' head-on collisions with the target nuclei. Scattering angle functions in the high impact parameter range reveal slight forward scattering, and display glory and rainbow scattering signatures. We analyze our results in comparison with available theoretical and experimental data and discuss future investigations of PCT systems.