Polarizable potentials for metals: The density readjusting embedded atom method (DR-EAM)

Abstract

In simulations of metallic interfaces, a critical aspect of metallic behavior is missing from the some of the most widely-used classical molecular dynamics force fields. We present a modification of the Embedded Atom Method (EAM) which allows for electronic polarization of the metal by treating the valence density around each atom as a fluctuating dynamical quantity. The densities are represented by a set of additional fluctuating variables (and their conjugate momenta) which are propagated along with the nuclear coordinates. This "Density Readjusting EAM" (DR-EAM) preserves nearly all of the useful qualities of traditional EAM, including bulk elastic properties and surface energies. However, it also allows valence electron density to migrate through the metal in response to external perturbations. We show that DREAM can successfully model polarization in response to external charges, capturing the image charge effect in atomistic simulations. DREAM also captures some of the behavior of metals in the presence of uniform electric fields, predicting surface charging and shielding internal to the metal. We further show that it predicts charge transfer between the constituent atoms in alloys, leading to novel predictions about unit cell geometries in layered L1 0 structure.

Thermal Transport in Citrate-Capped Gold Interfaces Using a Polarizable Force Field

“…The metallic layer below this largely reacts with a net positive charge density, conserving total charge in the metal and effectively neutralizing local fields at sites internal to the metal structure. In the original work parametrizing DR-EAM, this behavior was also observed in layered metallic structures responding to a uniform electric field …”

“…For both planar and nanoparticle interfaces, polarizing the gold surface leads to an increase in G. Our explanation for this is the same as that of Bhattarai et al The image charge and image dipole interactions present in the DR-EAM potential enhance the binding strength of all polar species, leading to enhanced coupling between the motion of solid and liquid phases, thereby enhancing thermal transport. 23 For the planar interfaces, polarization of the gold surface does not appear to impact either solution-phase conductivity or interfacial thermal conductance. However, polarization of the spherical nanoparticles causes a significant increase in G (see Table 4).…”

“…This partial charge is then propagated along with the nuclear degrees of freedom, allowing the charge distribution in the metal to respond to the local environment. This density readjusting embedded atom model (DR-EAM) is a superset of EAM-style potentials with the addition of Coulomb and self-potential terms . In a study of the Au(111)–water interface, polarization of the gold surface using DR-EAM increased thermal conductance at the interface relative to EAM.…”

“…This density readjusting embedded atom model (DR-EAM) is a superset of EAM-style potentials with the addition of Coulomb and self-potential terms. 23 In a study of the Au(111)−water interface, polarization of the gold surface using DR-EAM increased thermal conductance at the interface relative to EAM. This increase was likely due to an alteration in water ordering at the gold surface as well as the image charge and image dipole interactions present in the DR-EAM potential.…”

Embedding Quantum Statistical Excitations in a Classical Force Field

“…The embedded-atom method (EAM) for metals, , the modified EAM (MEAM) for covalent materials, , and their many extensions, are among the most widely used interaction potentials for simulating materials structure, defects, and phase diagrams. , Examples of empirical models proposed for improving descriptions of local structure, chemistry, and fluctuating charges in EAM include ES+, − SEAM, the charge-transfer EAM of Zhou et al, MEAM-BO, , DR-EAM, and A-EAM . An early formulation of the present ensemble DFT CT-EAM, expressed in terms of empirically modeled charge-transfer densities, was successfully applied by Muralidharan et al to describe the structure of water clusters.…”