The dynamics of highly excited electronic systems: Applications of the electron force field

Abstract

Highly excited heterogeneous complex materials are essential elements of important processes, ranging from inertial confinement fusion to semiconductor device fabrication. Understanding the dynamics of these systems has been challenging because of the difficulty in extracting mechanistic information from either experiment or theory. We describe here the electron force field ͑eFF͒ approximation to quantum mechanics which provides a practical approach to simulating the dynamics of such systems. eFF includes all the normal electrostatic interactions between electrons and nuclei and the normal quantum mechanical description of kinetic energy for the electrons, but contains two severe approximations: first, the individual electrons are represented as floating Gaussian wave packets whose position and size respond instantaneously to various forces during the dynamics; and second, these wave packets are combined into a many-body wave function as a Hartree product without explicit antisymmetrization. The Pauli principle is accounted for by adding an extra spin-dependent term to the Hamiltonian. These approximations are a logical extension of existing approaches to simulate the dynamics of fermions, which we review. In this paper, we discuss the details of the equations of motion and potentials that form eFF, and evaluate the ability of eFF to describe ground-state systems containing covalent, ionic, multicenter, and/or metallic bonds. We also summarize two eFF calculations previously reported on electronically excited systems: ͑1͒ the thermodynamics of hydrogen compressed up to ten times liquid density and heated up to 200 000 K; and ͑2͒ the dynamics of Auger fragmentation in a diamond nanoparticle, where hundreds of electron volts of excitation energy are dissipated over tens of femtoseconds. These cases represent the first steps toward using eFF to model highly excited electronic processes in complex materials.

High-temperature high-pressure phases of lithium from electron force field (eFF) quantum electron dynamics simulations

“…We performed the eFF dynamics for total simulation times of 1-2 ps with 1-2 ps of preequilibration and a time integration step of dt ¼ 5 attoseconds. With eFF the time-dependent Schrödinger equation uses an empirical exchange term with three universal parameters that are exactly the same as in previous publications (6)(7)(8). Individual trajectories were integrated in the microcanonical ensemble.…”

“…The eFF makes it practical to describe the nonadiabatic quantum dynamics of extended systems containing highly excited electrons at a computational cost comparable to that of classical molecular dynamics (6). This allows the dynamics of electrons and nuclei to be treated on an equal footing, without the adiabatic assumption of first solving for stationary states of the electrons.…”

The ReaxFF reactive force-field: development, applications and future directions

“…The electron and hole particles interact with atomic centers through a single Gaussian function. 139 We implemented charge-valence coupling, which allows the electron or hole particle to modify the number of valence electrons in a host atom, thus ensuring the appropriate change in valence when calculating the degree of over or under coordination in the host atom. To demonstrate the capability of the electron-explicit version of ReaxFF (eReaxFF), we trained our forcefield to capture the electron affinity (EA) of various hydrocarbon species.…”

Molecular dynamics simulation of strong shock waves propagating in dense deuterium, taking into consideration effects of excited electrons

“…In this work, the excitation of electrons is described by the electron force field (eFF) method 25,[36][37][38] , which is a further development of the WPMD method. In addition to the Gaussian wave packet approximation to electronic wave functions, the eFF method provides a simplified parameterization with improved accuracy to the Pauli's exclusion force between electrons of the same spin, which is the necessary part in the description of electronic structures, e.g., the shell structure and chemical bonds.…”