Thermal Properties of Impurity-Doped Clusters: Orbital-Free Molecular Dynamics Simulations of the Meltinglike Transition in Li1Na54 and Cs1Na54

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.

Impurity effects on solid-solid transitions in atomic clusters

We use the harmonic superposition approach to examine how a single atom substitution affects low-temperature anomalies in the vibrational heat capacity (C_V) of model nanoclusters. Each anomaly is linked to competing solidlike "phases", where crossover of the corresponding free energies defines a solid-solid transition temperature (T_s). For selected Lennard-Jones clusters we show that T_s and the corresponding C_V peak can be tuned over a wide range by varying the relative atomic size and binding strength of the impurity, but excessive atom-size mismatch can destroy a transition and may produce another. In some tunable cases we find up to two additional C_V peaks emerging below T_s, signalling one- or two-step delocalisation of the impurity within the ground-state geometry. Results for Ni₇₄X and Au₅₄X clusters (X = Au, Ag, Al, Cu, Ni, Pd, Pt, Pb), modelled by the many-body Gupta potential, further corroborate the possibility of tuning, engineering, and suppressing finite-system analogues of a solid-solid transition in nanoalloys.

Thermodynamic properties of Ga27Si3 cluster using density functional molecular dynamics

Density functional molecular dynamical calculations have been carried out to explore the effect of silicon impurities on thermodynamic properties of Ga(30). We have obtained 500 distinct low energy equilibrium geometries of Ga(27)Si(3) in order to obtain reliable ground state geometry. The specific heat has been calculated using multiple histogram techniques and compared with that of Ga(30). We demonstrate that silicon impurities have a dramatic effect on the thermodynamic properties of the host cluster. In contrast to Ga(30), the specific heat of Ga(27)Si(3) shows a clear melting peak at ≈500 K, changing the character of Ga(30) from a nonmelter to a melter.

One-pot synthesis of AuPt alloyed nanoparticles by intense x-ray irradiation

We synthesized AuPt alloyed nanoparticles in colloidal solution by a one-pot procedure based on synchrotron x-ray irradiation in the presence of PEG (polyethylene glycol). The exclusive presence of alloyed nanoparticles with fcc structure was confirmed by several different experiments including UV-vis spectroscopy, x-ray diffraction (XRD) and transmission electron microscopy (TEM). The composition of the AuPt alloyed nanoparticles can be varied in a continuous fashion by simply varying the feed ratios of Au and Pt precursors. The nanoparticles exhibited colloidal stability and biocompatibility, important for potential applications.

Melting-point depression by insoluble impurities: a finite size effect

Melting-point depression by soluble impurities is an entropy-driven phenomenon. Studying partially oxidized free sodium nanoparticles, we found an additional mechanism, which is caused by insoluble impurities. Oxidization of sodium clusters with 135-192 atoms by a single oxygen molecule causes a melting-point depression of 17+/-6 K; additional oxygen amplifies the effect. This is in contrast to the behavior of bulk sodium, where the melting point does not change upon partial oxidization. First-principles density functional theory calculations reveal the interaction between the pure and the oxidized part of the cluster to be responsible for the effect. There is more structural freedom in a liquid cluster to optimize the interface between the two parts. This stabilizes the liquid phase and causes the observed, cluster-specific melting-point depression.

Single impurity effect on the melting of nanoclusters

We show by molecular dynamics simulations that the melting temperature of clusters can be tuned by selective doping. In fact, a single Ni or Cu impurity in Ag icosahedral clusters considerably increases the melting temperature even for sizes of more than a hundred atoms. The upward shift is correlated to the strain relaxation induced by a small central impurity in icosahedral clusters.