First-principles Free-energy Calculations Research Articles

Deep Learning-Based Prediction of Contact Maps and Crystal Structures of Inorganic Materials.

Crystal structure prediction is one of the major unsolved problems in materials science. Traditionally, this problem is formulated as a global optimization problem for which global search algorithms are combined with first-principles free energy calculations to predict the ground-state crystal structure of a given material composition. These ab initio algorithms are currently too slow for predicting complex material structures. Inspired by the AlphaFold algorithm for protein structure prediction, herein, we propose AlphaCrystal, a crystal structure prediction algorithm that combines a deep residual neural network model for predicting the atomic contact map of a target material followed by three-dimensional (3D) structure reconstruction using genetic algorithms. Extensive experiments on 20 benchmark structures showed that our AlphaCrystal algorithm can predict structures close to the ground truth structures, which can significantly speed up the crystal structure prediction and handle relatively large systems.

Copernicium: A Relativistic Noble Liquid.

The chemical nature and aggregate state of superheavy copernicium (Cn) have been subject of speculation for many years. While strong relativistic effects render Cn chemically inert, which led Pitzer to suggest a noble‐gas‐like behavior in 1975, Eichler and co‐workers in 2008 reported substantial interactions with a gold surface in atom‐at‐a‐time experiments, suggesting a metallic character and a solid aggregate state. Herein, we explore the physicochemical properties of Cn by means of first‐principles free‐energy calculations, which confirm Pitzer's original hypothesis: With predicted melting and boiling points of 283±11 K and 340±10 K, Cn is indeed a volatile liquid and exhibits a density very similar to that of mercury. However, in stark contrast to mercury and the lighter Group 12 metals, we find bulk Cn to be bound by dispersion and to exhibit a large band gap of 6.4 eV, which is consistent with a noble‐gas‐like character. This non‐group‐conforming behavior is eventually traced back to strong scalar‐relativistic effects, and in the non‐relativistic limit, Cn appears as a common Group 12 metal.

Defect segregation to grain boundaries in BaZrO3 from first-principles free energy calculations

Grain boundaries determine the performance of proton-conducting BaZrO3. This contribution investigates the effects of a free energy treatment of defect and dopant segregation.

Spin crossover in liquid (Mg,Fe)O at extreme conditions

We use first-principles free-energy calculations to predict a pressure-induced spin crossover in the liquid planetary material (Mg,Fe)O, whereby the magnetic moments of Fe ions vanish gradually over a range of hundreds of GPa. Because electronic entropy strongly favors the nonmagnetic low-spin state of Fe, the crossover has a negative effective Clapeyron slope, in stark contrast to the crystalline counterpart of this transition-metal oxide. Diffusivity of liquid (Mg,Fe)O is similar to that of MgO, displaying a weak dependence on element and spin state. Fe-O and Mg-O coordination increases from approximately 4 to 7 as pressure goes from 0 to 200 GPa. We find partitioning of Fe to induce a density inversion between the crystal and melt, implying separation of a basal magma ocean from a surficial one in the early Earth. The spin crossover induces an anomaly into the density contrast, and the oppositely signed Clapeyron slopes for the crossover in the liquid and crystalline phases imply that the solid-liquid transition induces a spin transition in (Mg,Fe)O.

First-principles free energy calculations of the structural phase transition in LiBH4with I, Cl, Na, and K substitution

LiBH4 is a fast ionic conductor in its high-temperature phase, which is stabilized at room temperature by various chemical substitutions, making it a potential solid electrolyte material for Li-ion batteries. Using firstprinciples variable-cell-shape molecular dynamics simulations, we reproduce the experimentally observed lowand high-temperature structures. Using the height of a structure-factor-like peak as a collective coordinate, we calculate the free energy differences between the two structures as a function of temperature and substitutional ion concentration. We get good agreement with experiment for I, Cl, and Na, and predict that K is even more effectiveforloweringthecriticaltemperature.Decompositionofthefreeenergyintoenthalpyandentropyreveals that the mechanism driving this lowering varies among substitutional elements. Calculating the full free energy, rather than simply the enthalpy, is therefore crucial to understanding how chemical substitution stabilizes the high conductivity phase.

Constraints on the phase diagram of molybdenum from first-principles free-energy calculations

We use first-principles techniques to reexamine the suggestion that transitions seen in high-$P$ experiments on Mo are solid-solid transitions from the bcc structure to either the fcc or hcp structures. We confirm that in the quasiharmonic approximation the free energies of fcc and hcp structures become lower than that of bcc at $P>325$ GPa and $T$ below the melting curve, as reported recently. However, we show that if anharmonic effects are fully included this is no longer true. We calculate fully anharmonic free energies of high-$T$ crystal phases by integration of the thermal average stress with respect to strain as structures are deformed into each other, and also by thermodynamic integration from harmonic reference systems to the fully anharmonic system. Our finding that fcc is thermodynamically less stable than bcc in the relevant high-$P$/high-$T$ region is supported by comparing the melting curves of the two structures calculated using the first-principles reference-coexistence technique. We present first-principles simulations based on the recently proposed Z method, which also support the stability of bcc over fcc.

First-order phase transitions by first-principles free-energy calculations: The melting of Al

The melting properties of aluminum are calculated from first-principles molecular-dynamics simulations using density-functional theory in the local-density approximation. We calculate a melting temperature of 890 K at zero pressure, to be compared to the experimental value of 933 K. An elaborate discussion of the techniques employed is presented. The solid- and liquid-state free energies are obtained via coupling constant integration. The respective reference systems are the quasiharmonic crystal and the Lennard-Jones fluid. Good quality of the Brillouin zone sampling is shown to be crucial. The strategy followed is expected to be applicable to a wide range of liquid metals.

First-principles free-energy calculations on condensed-matter systems: Lattice vacancy in silicon.

We illustrate a method for performing first-principles free-energy calculations within the Kohn-Sham scheme. We show that the method can be used in cases in which electrons have to be decoupled from the system and illustrate it with results for the formation free energy of the Si vacancy. The results agree well with the available data from experiments and ab initio calculations. \textcopyright{} 1996 The American Physical Society.