Surface structure of liquid Li and Na: an ab initio molecular dynamics study

Orbital-Free Density Functional Theory: An Attractive Electronic Structure Method for Large-Scale First-Principles Simulations

Kohn-Sham Density Functional Theory (KSDFT) is the most widely used electronic structure method in chemistry, physics, and materials science, with thousands of calculations cited annually. This ubiquity is rooted in the favorable accuracy vs cost balance of KSDFT. Nonetheless, the ambitions and expectations of researchers for use of KSDFT in predictive simulations of large, complicated molecular systems are confronted with an intrinsic computational cost-scaling challenge. Particularly evident in the context of first-principles molecular dynamics, the challenge is the high cost-scaling associated with the computation of the Kohn-Sham orbitals. Orbital-free DFT (OFDFT), as the name suggests, circumvents entirely the explicit use of those orbitals. Without them, the structural and algorithmic complexity of KSDFT simplifies dramatically and near-linear scaling with system size irrespective of system state is achievable. Thus, much larger system sizes and longer simulation time scales (compared to conventional KSDFT) become accessible; hence, new chemical phenomena and new materials can be explored. In this review, we introduce the historical contexts of OFDFT, its theoretical basis, and the challenge of realizing its promise via approximate kinetic energy density functionals (KEDFs). We review recent progress on that challenge for an array of KEDFs, such as one-point, two-point, and machine-learnt, as well as some less explored forms. We emphasize use of exact constraints and the inevitability of design choices. Then, we survey the associated numerical techniques and implemented algorithms specific to OFDFT. We conclude with an illustrative sample of applications to showcase the power of OFDFT in materials science, chemistry, and physics.

Interface-governed nanometric machining behaviour of Cu/Ag bilayers using molecular dynamics simulation

The nanometric machining of Cu/Ag bilayers and pure Cu film is performed using molecular dynamics (MD) simulations. The mechanical and tribological properties of Cu/Ag bilayers are investigated by comparing with those of pure Cu film. The effects of machining parameters (indenter radius, tool speed and machining depth) on the subsurface damage and material removal are studied by analyzing the dislocation movement, chipping volume, machining force and average temperature of the workpiece. The results show that the hardness of Cu/Ag bilayers is smaller than that of pure Cu film, due to the dislocation nucleation and emission from the Cu/Ag interface. Meanwhile, the friction coefficient of Cu/Ag bilayers is larger than that of pure Cu film. Furthermore, the metal bonding energy at the Cu/Ag interface is weaker than that in pure Cu film, which causes the low hardness in the Cu/Ag bilayers. The Young's moduli in the Cu/Ag bilayers and pure Cu film are calculated by the Hertz contact mechanism and are close to the experimental result. During nanometric machining of Cu/Ag bilayers, the larger indenter radius or higher tool speed would cause a larger indentation force. The chipping volume, machining force and average temperature would increase with the increment of indenter radius, tool speed and machining depth. The subsurface damage can be reduced by selecting the smaller indenter radius, lower tool speed, and smaller machining depth, where fewer lattice defects are produced. In addition, the selection of lower tool speed also plays a crucial role in improving the smoothness of the ground surface.

The nanometric machining of Cu/Ag bilayers and pure Cu film is performed using molecular dynamics (MD) simulations.

Angular-momentum-dependent orbital-free density functional theory

Orbital-free (OF) density functional theory (DFT) directly solves for the electron density rather than the wave function of many electron systems, greatly simplifying and enabling large scale first principles simulations. However, the required approximate noninteracting kinetic energy density functionals and local electron-ion pseudopotentials severely restrict the general applicability of conventional OFDFT. Here, we present a new generation of OFDFT called angular-momentum-dependent (AMD)-OFDFT to harness the accuracy of Kohn-Sham DFT and the simplicity of OFDFT. The angular momenta of electrons are explicitly introduced within atom-centered spheres so that the important ionic core region can be accurately described. In addition to conventional OF total energy functionals, we introduce a crucial nonlocal energy term with a set of AMD energies to correct errors due to the kinetic energy density functional and the local pseudopotential. We find that our AMD-OFDFT formalism offers substantial improvements over conventional OFDFT, as we show for various properties of the transition metal titanium.

An ab initio study of the structure and dynamics of bulk liquid Cd and its liquid-vapor interface

Several static and dynamic properties of bulk liquid Cd at a thermodynamic state near its triple point have been calculated by means of ab initio molecular dynamics simulations. The calculated static structure shows a very good agreement with the available experimental data. The dynamical structure reveals collective density excitations with an associated dispersion relation which points to a small positive dispersion. Results are also reported for several transport coefficients. Additional simulations have also been performed at a slightly higher temperature in order to study the structure of the free liquid surface. The ionic density profile shows an oscillatory behavior with two different wavelengths, as the spacing between the outer and first inner layer is different from that between the other inner layers. The calculated reflectivity shows a marked maximum whose origin is related to the surface layering, along with a shoulder located at a much smaller wavevector transfer.

Structural and dynamical properties of liquid Mg. An orbital-free molecular dynamics study

Several static and dynamic properties of liquid magnesium near melting have been evaluated by the orbital-free ab initio molecular dynamics method. The calculated static structure shows good agreement with recent experimental data, including an asymmetric second peak in the structure factor which has been linked to the existence of an important icosahedral short-range order in the liquid. As for the dynamic structure, we obtain collective density excitations with an associated dispersion relation which closely follows recent experimental results. Accurate estimates have also been obtained for several transport coefficients.

Structure of the liquid-vapor interfaces of Ga, In and the eutectic Ga-In alloy-an ab initio study

We report the results of ab initio molecular dynamics simulations for the liquid-vapor interface of the liquid metals Ga, In and the eutectic binary alloy Ga-In (16.5% In) for which experimental data are available. The study was performed by using samples of 3000 particles in a slab geometry with periodic boundary conditions. In those systems, the total ionic density distributions along the normal to the interface display some layering and in the case of the Ga-In alloy there appears a highly enriched layer of the lower surface tension component located outermost at the interface. The results are compared with the available experimental data.

The intrinsic structure of the water surface

An operational procedure to obtain the intrinsic structure of liquid surfaces is applied here to a molecular dynamics simulation of water, with a model of point charges for the molecular interactions. The method, which had been recently proposed and used for simple fluids, is successfully extended to a molecular liquid with the complex bond structure of water. The elimination of the capillary wave fluctuations, in the intrinsic density and orientation profiles, gives a new overall view of the water surface, at the sharpest molecular level, and without the size-dependent broadening observed in the mean profiles. The molecules belonging to the outer liquid layer are clearly identified, and we find that only these molecules exhibit a clear preferential orientation to lie flat on the surface. Moreover, there is a strong correlation between the dipolar structure and the local curvatures of the intrinsic surface, so that at the extrusions of the intrinsic surface the molecular dipoles point preferentially toward the vapor side of the interface. Finally, we have found an intrinsic density layering structure, although the inner structure is strongly damped beyond the second layer.

Ab initio studies of layering behavior of liquid sodium surfaces and interfaces

We have studied the liquid surface of sodium with extensive ab initio molecular dynamics simulations based on ensemble density-functional theory. We find clear evidence of layering in the direction perpendicular to the surface that persists to temperatures more than 100 K above the melting point. We also observe clear Friedel oscillations in the electronic density response to the presence of a surface, but their direct effect on atomic layering is ruled out. A careful finite-size effect analysis accompanies our results, showing that liquid slabs 20-25 A thick capture the essential details of the surface structure. We conclude that geometrical confinement is the common cause for layer formation, which is similar to what happens at a liquid-solid interface: at a free liquid surface, the rapid decay of the electronic density from the bulk liquid value to zero in the vapor forms a hard wall against which the atoms pack. Finally, we predict x-ray reflectivities from ab initio molecular dynamics data that include some of the large surface-normal wave vector-transfer regions that, for alkali metals, are not accessible to experiments.

Interplay between the ionic and electronic density profiles in liquid metal surfaces

First-principles molecular-dynamics simulations have been performed for the liquid-vapor interfaces of liquid Li, Mg, Al, and Si. We analyze the oscillatory ionic and valence electronic density profiles obtained, their wavelengths, and the mechanisms behind their relative phase shift.

Competing Thermal Activation Mechanisms in the Meltinglike Transition of NaN (N = 135−147) Clusters

The meltinglike transition in unsupported icosahedral Na(N)() clusters, with N = 135-147, has been studied by isokinetic molecular dynamics simulations based on an orbital-free version of density functional theory. A maximum in the melting temperature, T(m), is obtained for Na141, while the latent heat, deltaE, and entropy of melting, deltaS, are maximal for Na147. These observations are in close agreement with calorimetric experiments on N clusters. The size evolution of deltaS is rationalized by the emergence of important premelting effects associated with the diffusive motion of atomic vacancies at the cluster surface. The precise location of the maximum in T(m) is explained in terms of two different thermally activated structural instability mechanisms which trigger the meltinglike transition in the size ranges N = 135-141 and N = 141-147, respectively.

Anomalous size dependence in the melting temperatures of free sodium clusters: an explanation for the calorimetry experiments

The meltinglike transition in unsupported Na(N) clusters (N = 55, 92, 147, 181, 189, 215, 249, 271, 281 and 299) is studied by first-principles isokinetic molecular dynamics simulations. The irregular size dependence of the melting temperatures Tm observed in the calorimetry experiments of Schmidt et al. [Nature (London) 393, 238 (1998)] is quantitatively reproduced. We demonstrate that structural effects alone can explain all broad features of experimental observations. Specifically, maxima in Tm(N) correlate with high surface stability and with structural features such as a high compactness degree.