### Alfredo Correa Tedesco (14-ERD-103)

#### Abstract

Our goal is to advance the field of quantum molecular simulations by developing a framework for nonadiabatic electron–nuclear dynamics (processes with energy transfer), which is a fundamental mechanism of state and phase changes in various dynamical processes of physics. Specifically, we will develop and apply a first-principles, parameter-free computational technique that is appropriate for describing materials under strongly driven conditions, such as those induced by electromagnetic radiation in intense laser fields; radiation of alpha and beta particles, protons, and swift ions; and nonlocal thermodynamic-equilibrium effects of different electron and ion temperatures. The Born–Oppenheimer approximation (separation of atomic nuclei and electron motion) and the assumption of thermal equilibrium between ionic and electronic subsystems are ubiquitous in the field of quantum molecular dynamics. However, there are many instances, particularly for matter under extreme conditions, where these basic approximations are not appropriate. We plan to develop a new predictive simulation framework for nonadiabatic molecular dynamics by building upon our current implementation of coupled ion and electron dynamics (Ehrenfest nonadiabatic interactions between electrons and ions), where the time-dependent dynamics of electrons is taken into account explicitly.

With this research project we expect to (1) develop a framework for systematic calculation of electronic and ionic excitations caused by nonadiabatic effects (e.g., electronic stopping power and collisions and ion ranges in solids of arbitrary complex materials); (2) develop fully ab initio simulations of a collision cascade including friction in the electronic environment; (3) implement electromagnetic coupling during simulated laser interaction (and after effects) for metal surfaces or damage in optical materials; (4) perform direct computation of ion–electron coupling for modeling of two-temperature systems in the electronvolt regime; (5) develop a general-purpose computational code to perform Ehrenfest dynamics; and (6) improve the scalability of existing Born–Oppenheimer dynamics via Ehrenfest dynamics.

#### Mission Relevance

Our research into electromagnetic coupling, swift ions, and two-temperature systems has direct relevance to both NNSA and LLNL missions. The outcome of this project will result in the ability to carry out quantitative predictions of the effects of radiation in nuclear materials (as in new reactor designs and fuel containment) and strong laser excitations (as in Inertial Confinement Fusion and optics), in support of Livermore core missions in national and energy security. The core competency in advanced materials and manufacturing will also benefit from our modeling of materials under strongly driven conditions (high-energy-density science). In addition, our code can make efficient use of existing high-performance computing platforms at LLNL, and can be used by other researchers for various programmatic applications.

#### FY15 Accomplishments and Results

In FY15 we (1) established the trade-off between convergence and accuracy necessary to calculate ab initio electronic stopping, (2) quantified the role of core electron excitations in the stopping process, and (3) implemented, with the help of a newly hired postdoctoral researcher, two new features necessary to complete the project. Specifically, we calculated the electronic current and implemented new "exponential" integrators. These integrators incorporate better energy and norm conservation of the effective potential for electronic-structure calculations that are fundamental for long time simulations bridging the gap with atomic molecular-dynamics timescales.

#### Publications and Presentations

- Caro, A., et al., “Adequacy of damped dynamics to represent the electron-phonon interaction in solids.”
*Phys. Rev. B*.**92**, 144309 (2015). http://dx.doi.org/10.1103/PhysRevB.92.144309 - Schleife, A., Y. Kanai, and A. C. Correa, “Accurate atomistic first-principles calculations of electronic stopping.”
*Phys. Rev. B.***91,**014306 (2015). http://dx.doi.org/10.1103/PhysRevB.91.014306 - Schleife, A., et al., "Quantum dynamics simulation of electrons in materials on high-performance computers."
*Comput. Sci. Eng*.**16**(5), 54 (2014).