In ion irradiation processes, electronic stopping power describes the energy transfer rate from the irradiating ion to the target material's electrons. Due to the scarcity and significant uncertainties in experimental electronic stopping power data for materials beyond simple solids, there has been growing interest in the use of first-principles theory for calculating electronic stopping power. In recent years, advances in high-performance computing have opened the door to fully first-principles non-equilibrium simulations based on real-time time-dependent density functional theory (RT-TDDFT). While it has been demonstrated that the RT-TDDFT approach is capable of predicting electronic stopping power for a wide range of condensed matter systems, there has yet to be an exhaustive examination of the physical and numerical approximations involved and their effects on the calculated stopping power. We discuss the results of such a study for crystalline silicon with protons as irradiating ions. We examine the influences of key approximations in RT-TDDFT non-equilibrium simulations on the calculated electronic stopping power, including approximations related to basis sets, finite size effects, exchange-correlation approximation, pseudopotentials, and more. Finally, we propose a simple and efficient correction scheme to account for the contribution from core electron excitations to the stopping power, as it was found to be significant for large proton velocities.

Understanding the role of core electron excitation in liquid water under proton irradiation has become important due to the growing use of proton beams in radiation oncology. Using a firstprinciples, non-equilibrium simulation approach based on real-time time-dependent density functional theory, we determine the electronic stopping power, the velocity-dependent energy transfer rate from irradiating ions to electrons. The electronic stopping power curve agrees quantitatively with experimental data over the velocity range available. At the same time, significant differences are observed between our first-principles result and commonly-used perturbation theoretic models. Excitations of the water molecules' oxygen core electrons are a crucial factor in determining the electronic stopping power curve beyond its maximum. The core electron contribution is responsible for as much as one-third of the stopping power at the high proton velocity of 8.0 a.u. (1.6 MeV). K-shell core electron excitations not only provide an additional channel for the energy transfer but they also significantly influence the valence electron excitations. In the excitation process, generated holes remain highly localized within a few angstroms around the irradiating proton path whereas electrons are excited away from the path. In spite of its great contribution to the stopping power, K-shell electrons play a rather minor role in terms of the excitation density; only 1% of the hole population comprises K-shell holes even at the high proton velocity of 8.0 a.u.. The excitation behavior revealed here is distinctly different from that of photonbased ionizing radiation such as X/γ-rays.When a highly energetic ion travels through and interacts with matter, its kinetic energy is transferred into the target material's electronic and nuclear subsystems. This energy loss of the projectile ion can arise from both elastic collisions with nuclei (nuclear stopping) and inelastic scattering events (electronic stopping). When the particle's kinetic energy is sufficiently large (on the order of ~10 keV per nucleon), the major contribution to the energy transfer comprises electronic stopping wherein the projectile ion induces massive electronic excitations in the target matter 1-2 . This electronic stopping phenomenon is at the heart of emerging ion bean cancer therapies. The use of proton beam radiation over more conventional radiation based on X/γ-ray photons is often considered more effective because of the ion's distinct spatial energy deposition profile with a very sharp peak. [3][4] By calibrating the initial kinetic energy of the protons, this energy deposition peak can be tuned to coincide with the location of the tumour. This energy deposition profile is largely determined by electronic stopping power, which measures the rate of energy transfer from the charged particle to electrons in matter per unit distance of the energetic particle's movement. 1,[5][6][7] The stopping power is a continuous function of the particle velocity, and the velocities near the...

Electronic excitations are produced when matter is exposed to ion irradiation comprising highly energetic ions. These electronic stopping excitations are responsible for ion beam-induced DNA damage by energetic protons and α-particles, the chemistry and physics of which are central to burgeoning radiation cancer therapies. By simulating the non-perturbative electronic response of DNA to irradiating protons and α-particles, our first-principles dynamics simulations enable us to test the validity of the commonly used linear response theory description, and they also reveal unprecedented details of the quantum dynamics of electronic excitations. In this work, we discuss the extent to which the linear response theory is valid by comparing to the first-principles determination of electronic stopping power, the energy-transfer rate from ions to electronic excitation. The simulations show that electronic excitations induced by proton and α-particle irradiation cause ionization of DNA, resulting in the generation of holes. By studying the excited hole generation in terms of both the energetic and spatial details in DNA, our work reveals remarkable differences with the excitation behavior of DNA under more commonly used ionizing irradiation sources such as X/γ-ray photons. Furthermore, we find that the generation of excited holes does not directly correlate with the energy-transfer rate as a function of the irradiating ion velocity, in contrast to what is often assumed in the chemistry and physics of radiation oncology.

We present the first-principles determination of electronic stopping power for protons and α-particles in a semiconductor material of great technological interest: silicon carbide. The calculations are based on non-equilibrium simulations of the electronic response to swift ions using real-time time-dependent density functional theory (RT-TDDFT). We compare the results from this first-principles approach to those of the widely used linear response formalism and determine the ion velocity regime within which linear response treatments are appropriate. We also use the non-equilibrium electron densities in our simulations to quantitatively address the long-standing question of the velocity-dependent effective charge state of projectile ions in a material, due to its importance in linear response theory. We further examine the validity of the recently proposed centroid path approximation recently proposed for reducing the computational cost of acquiring stopping power curves from RT-TDDFT simulations.

Real-time, time-dependent density functional theory (RT-TDDFT) has gained popularity as a first-principles approach to study a variety of excited-state phenomena such as optical excitations and electronic stopping. Within RT-TDDFT simulations, the gauge freedom of the time-dependent electronic orbitals can be exploited for numerical and scientific convenience while the unitary transformation does not alter physical properties calculated from the quantum dynamics of electrons. Exploiting this gauge freedom, we demonstrate propagation of maximally localized Wannier functions within RT-TDDFT. We illustrate its great utility through a number of examples including its application to optical excitation in extended systems using the so-called length gauge, interpreting electronic stopping excitation, and simulating electric field-driven quantized charge transport. We implemented the approach within our plane-wave pseudopotential RT-TDDFT module of the QB@LL code, and performance of the implementation is also discussed. arXiv:1903.05081v2 [cond-mat.mtrl-sci]