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Crystal structure prediction algorithms, including ab initio random structure searching (AIRSS), are intrinsically limited by the huge computational cost of the underlying quantum-mechanical methods. We have recently shown that a novel class of machine learning (ML) based interatomic potentials can provide a way out: by performing a highdimensional fit to the ab initio energy landscape, these potentials reach comparable accuracy but are orders of magnitude faster. In this paper, we develop our approach, dubbed Gaussian approximation potential-based random structure searching (GAP-RSS), towards a more general tool for exploring configuration spaces and predicting structures. We present a GAP-RSS interatomic potential model for elemental phosphorus, which identifies and correctly "learns" the orthorhombic black phosphorus (A17) structure without prior knowledge of any crystalline allotropes. Using the tubular structure of fibrous phosphorus as an example, we then discuss the limits of free searching, and discuss a possible way forward that combines a recently proposed fragment analysis with GAP-RSS. Examples of possible tubular (1D) and extended (3D) hypothetical allotropes of phosphorus as found by GAP-RSS are discussed. We believe that in the future, ML potentials could become versatile and routine computational tools for materials discovery and design.
Crystal structure prediction algorithms, including ab initio random structure searching (AIRSS), are intrinsically limited by the huge computational cost of the underlying quantum-mechanical methods. We have recently shown that a novel class of machine learning (ML) based interatomic potentials can provide a way out: by performing a highdimensional fit to the ab initio energy landscape, these potentials reach comparable accuracy but are orders of magnitude faster. In this paper, we develop our approach, dubbed Gaussian approximation potential-based random structure searching (GAP-RSS), towards a more general tool for exploring configuration spaces and predicting structures. We present a GAP-RSS interatomic potential model for elemental phosphorus, which identifies and correctly "learns" the orthorhombic black phosphorus (A17) structure without prior knowledge of any crystalline allotropes. Using the tubular structure of fibrous phosphorus as an example, we then discuss the limits of free searching, and discuss a possible way forward that combines a recently proposed fragment analysis with GAP-RSS. Examples of possible tubular (1D) and extended (3D) hypothetical allotropes of phosphorus as found by GAP-RSS are discussed. We believe that in the future, ML potentials could become versatile and routine computational tools for materials discovery and design.
In pursuit of this goal, atomic-scale computer simulations have long been a central approach, and two major families of methods are routinely used today. On the one hand, there are quantum-mechanical simulations, in which we solve Schrödinger's equation for the electronic structure of molecular and periodic systems, most widely based on density-functional theory (DFT). [6][7][8] These methods provide (largely) reliable results for structural models of materials that normally contain a few tens or hundreds of atoms. State-of-the-art DFT methods can be applied to many material classes, and they are increasingly used for high-throughput screening and "in silico" (computer-based) design of materials: new compositions and previously unknown structures have been identified in DFT searches and subsequently experimentally realized. [5,[9][10][11] On the other hand, interatomic potential models ("force fields"), parameterizing interactions between atoms with (relatively) simple functional forms, are widely used in materials science to describe matter in molecular dynamics (MD) simulations. These simulations grant access to larger time and length scales, reaching system sizes of up to hundreds of thousands of atoms. [12] In parameterizing these potentials, a certain physical form of the atomic interactions is assumed, often in terms of bond distances, angles, and so on, and physical properties such as equilibrium lattice parameters or elastic constants enter the fitting of the potential. For this reason, such potentials are often called "empirical." They are several orders of magnitude faster than DFT, but necessarily less accurate and less easily transferable.In this Progress Report, we highlight recent developments in "machine-learned" interatomic potentials, which represent a rapidly growing field that promises to do away with the aforementioned trade-offs. Over the last year, there has been a surge of interest in machine learning (ML) methodology: part of it is due to the dramatic growth of ML throughout the scientific disciplines, and part of it is due to tangible success stories of ML-based interatomic potentials that are now beginning to emerge. We will argue that this is an exciting development with very practical implications, currently on the verge of moving from a somewhat specialized new technology to everyday applicability, poised to enhance and complement the communities' existing strengths in computational materials modeling. We will show selected applications of ML potentials to problems in materials science, discuss the current limitations (and possible pitfalls), and outline what we expect to be interesting directions for the development of the field in the coming years.Atomic-scale modeling and understanding of materials have made remarkable progress, but they are still fundamentally limited by the large computational cost of explicit electronic-structure methods such as density-functional theory. This Progress Report shows how machine learning (ML) is currently enabling a new degree of realism in materi...
Most of the large variety of 2D materials are derived from their parent van der Waals (vdWs) crystals, thus their atomic struc tures are identical to the bulk counterparts with some minor lattice constant differ ence due to the reduced dimensionality and vanished interlayer interaction. Simi larly, some 2D monolayers, of which a typical example would be silicene, [10] stem from bulk materials with covalent bonds. Although their bonding may significantly change (silicene: sp 2 ; bulk silicon: sp 3 ) when they go from the bulk material to the 2D monolayer, their atomic structures are analogous. However, a slight atomic relaxation may occur in order to balance the reduced bonding coordination in the 2D forms. Therefore, in these cases, the determination of the 2D atomic structures is quite straightforward.In contrast, most metal oxides feature strong interlayer ionic bonds. The lack of a strong interlayer interaction in their 2D forms usually introduces dangling bonds, leading to strong surface polarization which induces surface instability of 2D metal oxides. Pronounced lattice relaxation, prominent struc tural reconstruction and substrate effects have been identified as the main mechanisms for compensating such strong sur face polarization in 2D metal oxides, as have been observed for a Pd 5 O 4 overlayer on Pd(111), [22] a strained PdO(101) layer on Pd (100), [23] a Ag 1.83 O trilayer on Ag(111), [24] a RhO 2 trilayer on Rh(111), [25] multiple phases of 2D Mn oxides on Pd(100), [26] and TiO 2 on rutile TiO 2 (011). [27] All of these significant changes increase the difficulty of synthesizing 2D metal oxides, as well as pose a challenge to computational structure prediction methods. Notwithstanding, more recently, spectacular progress has been made in prediction, design, preparation, and charac terization of oxide monolayers owing to the advancement of growth technologies and novel synthesis routes, as well as the development of computational and theoretical methods. [28][29][30][31][32][33] The structural reconstructions in combination with the elec tron confinement in 2D and the large surfacetovolume ratio endow 2D transition metal oxides (TMOs) with stunning physical/chemical properties. Moreover, the 2D TMOs showThe discovery of graphene has stimulated dramatic research interest on other 2D materials including transition metal oxide (TMO) monolayers in order to realize novel functionalization and applications. Due to reduced bonding coordination and strong surface polarization, the structures of most TMOs in the monolayer limit are very different from their bulk counterparts, as well as their physical and chemical properties. In this brief review, the authors sum marize recent research progress on atomically thin TMO layers. The focus is on the structural properties of the TMOs and their interaction with the sub strates from the computational point of view. The authors also introduce the potential applications of the TMO 2D materials on supercapacitors, photo catalysts, batteries, and sensors.
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