Search

This paper presents the results of an extensive structural search of ternary solids containing lutetium, nitrogen and hydrogen. Based on thousands of thermodynamically stable structures, available online, the convex hull of the formation enthalpies is constructed. To obtain the correct energetic ordering, the highly accurate RSCAN DFT functional is used in high quality all-electron calculations. In this way possible pseudopotential errors are eliminated. A novel lutetium hydride structure (HLu$_2$) that is on the convex hull is found in our search. An electron phonon analysis however shows that it is not a candidate structure for near ambient superconductivity. Besides this structure, which appears to have been missed in previous searches, possibly due to different DFT methodologies, our results agree closely with the results of previously published structure search efforts. This shows, that the field of crystal structure prediction has matured to a state where independent methodologies produce consistent and reproducible results, underlining the trustworthiness of modern crystal structure predictions. Hence it is quite unlikely that a structure, that would give rise within standard BCS theory to the superconducting properties, claimed to have been observed by Dasenbrock-Gammon et al. 10.1038/s41586-023-05742-0 , exists. This solidifies the evidence that no structure with conventional superconducting properties exists that could explain the experimental observation made by Dasenbrock-Gammon et al. 10.1038/s41586-023-05742-0

Machine learning potentials have become an important tool for atomistic simulations in many fields, from chemistry via molecular biology to materials science. Most of the established methods, however, rely on local properties and are thus unable to take global changes in the electronic structure into account, which result from long-range charge transfer or different charge states. In this work we overcome this limitation by introducing a fourth-generation high-dimensional neural network potential that combines a charge equilibration scheme employing environment-dependent atomic electronegativities with accurate atomic energies. The method, which is able to correctly describe global charge distributions in arbitrary systems, yields much improved energies and substantially extends the applicability of modern machine learning potentials. This is demonstrated for a series of systems representing typical scenarios in chemistry and materials science that are incorrectly described by current methods, while the fourth-generation neural network potential is in excellent agreement with electronic structure calculations., 13 pages, 11 figures

To elucidate the geometric structure of the putative room temperature superconductor, carbonaceous sulfur hydride, at high pressure, we present the results of an extensive computational structure search of bulk C-S-H at 250 gigapascals. Using the minima hopping structure prediction method coupled to the GPU accelerated Sirius library, more than 17,000 local minima with different stochiometries in large simulation cells were investigated. Only 24 stochiometries are favourable against elemental decomposition, all of them are carbon doped H$_3$S crystals. The absence of van Hove singularities or similar peaks in the electronic density of states of more than 3.000 candidate phases rules out conventional superconductivity in C-S-H at room-temperature.

Efforts toward stabilization of the Si$_{20}$ fullerene through different schemes have failed despite several theoretical predictions. However, recently Tillmann {\it et. al.} succeeded to stabilize the Si$_{20}$ fullerene through exohedral decoration with eight Cl substituents and twelve SiCl$_3$ groups on the surface and enclosing Cl$^-$ ion. A deeper understanding on what factors lead to stabilization will open the path for stabilizing other systems of interest. Here, we employ the minima hopping method within density functional theory to understand the potential energy surface. The study shows that the exo-endo halide decoration of the cage alters the glassy nature of the potential energy surface of pure cage to structure seeker. Further analysis of different properties of the global minima, reveal that the extra electron instead of residing on the central encapsulated atom in the cage, it is distributed on the cage and increases the encapsulation energy; thereby stabilizing the system. We also provide estimates of the stability for different kind of exo-endo halide decorations and their feasible realization in experiments.

The decahedral cage is the theoretically established ground state of the hydrogen saturated Si 20 H 20 fullerene. However it has never been observed experimentally. Based on an extensive exploration of the potential energy surface and by constructing theoretical reaction pathways from possible initial structures to the ground state of Si 20 H 20 , we show that there is no driving force towards the global minimum. There exists a huge number of intermediate structures that consist mainly of collapsed cages. Visiting all these intermediate states to find the ground state is not possible on experimentally relevant time scales. In this way the ground state becomes kinetically inaccessible. We contrast the features of the potential energy landscape of Si 20 H 20 with that of C 60 which spontaneously forms by condensation.

Performing high accuracy hybrid functional calculations for condensed matter systems containing a large number of atoms is at present computationally very demanding - when not out of reach - if high quality basis sets are used. We present a highly efficient multiple GPU implementation of the exact exchange operator which allows hybrid functional density-functional theory calculations with systematic basis sets without additional approximations for up to a thousand atoms. This method is implemented in a portable real-space-based algorithm, released as an open-source package. With such a framework hybrid DFT calculations of high quality become accessible on state-of-the-art supercomputers within a time-to-solution of the same order of magnitude as traditional semilocal-GGA functionals.

Póster presentado a la Conferencia bienal Fuerzas y Túnel, celebrada en Jaca (España) del 27 al 29 de junio de 2018., Quantum dot (QD) arrays on surfaces, generated through molecular self-assembly processes, have so far provided researchers with a vast playground to study the electronic properties of new and exotic 2D materials in ultra-high-vacuum (UHV) conditions. By selecting the proper molecular constituents (tectons) and substrate, long-range ordered, periodic and robust nanoporous networks have been achieved, ranging from hydrogen-bonded to metal-organic structures. These are candidates for studying fundamental physical phenomena such as confinement through the scattering of two-dimensional electron gases (2DEGs). Indeed, confinement tunability has already been accomplished by varying the pore (i.e. quantum dot) dimensions, geometrical shape and molecule substrate interactions. In addition, inter-dot coupling has been shown by photoemission that can be modulated through thermodynamics and the condensation of guest elements (Xe atoms). To date, 2DEGs' modification through inter-dot barrier width variations has not been experimentally demonstrated. Herein, sustained upon a combination of local scanning probes (STM/STS/AFM), angle resolved photoemission spectroscopy (ARPES) and extended model calculations, we show that we can precisely engineer the inter-dot barrier width by substitution of a single atom in a haloaromatic compound. As a result, we tune the confinement properties at each nanopore affecting the degree of QD intercoupling both on bulk and thin Ag films alike. Our findings complete the toolbox for tuning surface electronic properties, which started in the 90's with the quantum corrals.

A good hydrogen storage material should adsorb hydrogen in high concentrations and with optimal binding energies. Numerous mixed carbon boron fullerenes which are decorated with metal atoms were previously constructed by hand and proposed as a promising material in this context. We present a fully ab-initio, unbiased structure search in the configurational space of decorated C48B12 and find that most of the hitherto postulated ground state structures are not ground states. We determine the energetically lowest configurations for Be, Ca, Li and Sc decorated C48B12 clusters.

Resumen del trabajo presentado a la QMol Conference: Operating Quantum States in Atoms and Molecular at Surfaces, celebrada en Ascona (Suiza) del 10 al 14 de septiembre de 2017., Quantum dot (QD) arrays on surfaces, generated through molecular self-assembly processes, have so far provided researchers with a vast playground to study the electronic properties of new and exotic 20 materials in ultra-high-vacuum (UHV) conditions. By selecting the proper molecular constituents (tectons) and substrate, long-range ordered, periodic and robust nanoporous networks have been achieved, ranging from hydrogen-bonded to metal-organic structures. Not only do they stand out as ideal templates for nanopatterning, but also as adequate candidates for studying fundamental physical phenomena such as confinement through the scattering of two-dimensional electron gases (2DEGs). lndeed, confinement tunability has already been accomplished by varying the pore (i.e. quantum dot) dimensions, geometrical shape and molecule substrate interactions. In addition, inter-dot coupling has been shown by photoemission through the generation of new dispersive bands that can be modulated through thermodynamics [S) and the condensation of guest elements (Xe atoms). To date, the modification of 2DEGs through inter-dot barrier width variations has not been experimentally demonstrated. Herein, sustained upon a combination of local scanning probes (STM/STS/AFM), angle resolved photoemission spectroscopy (ARPES) and extended model calculations, we show that we can precisely engineer the inter-dot barrier width by substitution of a single atom in a haloaromatic compound. As a result, we tune the confinement properties at each nanopore affecting the degree of QD intercoupling both on bulk and thin Ag films alike. These findings pave the way to reach full control over 2DEGs with the prospect of becoming key for future electronic devices.

We report on the ab initio discovery of a novel putative ground state for quasi two-dimensional TiO$_2$ through a structural search using the minima hopping method with an artificial neural network potential. The structure is based on a honeycomb lattice and is energetically lower than the experimentally reported lepidocrocite sheet by 7~meV/atom, and merely 13~meV/atom higher in energy than the ground state rutile bulk structure. According to our calculations, the hexagonal sheet is stable against mechanical stress, it is chemically inert and can be deposited on various substrates without disrupting the structure. Its properties differ significantly from all known TiO$_2$ bulk phases with a large gap of 5.05~eV that can be tuned through strain engineering., Comment: 8 pages, 5 figures

Systematic ab initio structure prediction was applied for the first time to predict low energy surface reconstructions by employing the minima hopping method on the ��-boron (111) surface. Novel reconstruction geometries were identified and carefully characterized in terms of structural and electronic properties. Our calculations predict the formation of a planar, mono-layer sheet at the surface, which is responsible for conductive surface states. Furthermore, the isolated boron sheet is shown to be the ground state 2D-structure in vacuum at a hole density of ��=1/5 and is therefore a potential candidate as a precursor for boron nano-structures.

Through a systematic structural search we found an allotrope of carbon with Cmmm symmetry which we predict to be more stable than graphite for pressures above 10 GPa. This material, which we refer to as Z-carbon, is formed by pure sp3 bonds and is the only carbon allotrope which provides an excellent match to unexplained features in experimental X-ray diffraction and Raman spectra of graphite under pressure. The transition from graphite to Z-carbon can occur through simple sliding and buckling of graphene sheets. Our calculations predict that Z-carbon is a transparent wide band gap semiconductor with a hardness comparable to diamond., Comment: 4 pages, 5 figures

Using the minima hopping global geometry optimization method on the density functional potential energy surface we study medium size and large boron clusters. Even though for isolated medium size clusters the ground state is a cage like structure they are unstable against external perturbations such as contact with other clusters. The energy landscape of larger boron clusters is glass like and has a large number of structures which are lower in energy than the cages. This is in contrast to carbon and boron nitride systems which can be clearly identified as structure seekers in our minima hopping runs. The differences in the potential energy landscape explain why carbon and boron nitride systems are found in nature whereas pure boron fullerenes have not been found., Comment: 5 pages 5 figures

Using a systematic search algorithm we identify several types of point defects in the boron fullerene with 80 atoms. All these point defect leave the cage structure intact. In addition the cage structure is also very stable with respect to elastic deformations and addition or removal of atoms.

The accuracy of the energy landscape of silicon systems obtained from various density functional methods, a tight binding scheme and force fields is studied. Quantum Monte Carlo results serve as quasi exact reference values. In addition to the well known accuracy of DFT methods for geometric ground states and metastable configurations we find that DFT methods give a similar accuracy for transition states and thus a good overall description of the energy landscape. On the other hand, force fields give a very poor description of the landscape that are in most cases too rugged and contain many fake local minima and saddle points or ones that have the wrong height.

The energetic ground state of gold clusters with up to 314 atoms consists of rather complicated geometries that have only a weak resemblance to the perfect icosahedra, decahedra and octahedra that are encountered for some magic numbers. The structure changes in most cases completely by the addition of a single atom. Other low energy structures are so close in energy that their Boltzmann weight is not negligible at room temperature.

We present a fast and accurate method to calculate the electrostatic energy and forces of interacting particles with the boundary conditions appropriate to surfaces, i.e periodic in the two directions parallel to the surface and free in the perpendicular direction. In the spirit of the Ewald method the problem is divided into a short range and long range part. The charge density responsible for the long range part is represented by plane waves in the periodic directions and by finite elements in the non-periodic direction. Our method has computational complexity of O(N_g log(N_g)) with a very small prefactor, where N_g is the number of grid points., Comment: It has 19 pages and 4 figures

An algorithm for fast calculation of the Coulombic forces and energies of point particles with free boundary conditions is proposed. Its calculation time scales as N log N for N particles. This novel method has lower crossover point with the full O(N^2) direct summation than the Fast Multipole Method. The forces obtained by our algorithm are analytical derivatives of the energy which guarantees energy conservation during a molecular dynamics simulation. Our algorithm is very simple. An MPI parallelised version of the code can be downloaded under the GNU General Public License from the website of our group., Comment: 19 pages, 11 figures, submitted to: Journal of Chemical Physics

Methods exhibiting linear scaling with respect to the size of the system, so called O(N) methods, are an essential tool for the calculation of the electronic structure of large systems containing many atoms. They are based on algorithms which take advantage of the decay properties of the density matrix. In this article the physical decay properties of the density matrix will therefore first be studied for both metals and insulators. Several approaches to construct O(N) algorithms will then be presented and critically examined under various aspects. Finally some issues which are relevant only for self-consistent O(N) methods such as the calculation of the Hartree potential and mixing issues will be discussed., Comment: Enlarged and corrected version, 73 pages

We study the thermodynamic stability at low temperatures of a series of alkali-metal-zinc double-cation borohydrides, including LiZn(BH4)(3), LiZn2(BH4)(5), NaZn(BH4)(3), NaZn2(BH4)(5), KZn(BH4)(3), and KZn2(BH4)(5). While LiZn2(BH4)(5), NaZn(BH4)(3), NaZn2(BH4)(5), and KZn(BH4)(3) were recently synthesized, LiZn(BH4)(3) and KZn2(BH4)(5) are hypothetical compounds. Using the minima-hopping method, we discover two new lowest-energy structures for NaZn(BH4)(3) and KZn2(BH4)(5) which belong to the C2/c and P2 space groups, respectively. These structures are predicted to be both thermodynamically stable and dynamically stable, implying that their existence may be possible. On the other hand, we predict that the lowest-energy P1 structure of LiZn(BH4)(3) is unstable, suggesting a possible reason elucidating why this compound has not been experimentally identified. In exploring the low-energy structures of these compounds, we find that their energetic ordering is sensitive to the inclusion of the van der Waals interactions. We also find that a proper treatment of these interactions, e.g., as given by a nonlocal density functional such as vdW-DF2, is necessary to address the stability of the low-energy structures of these compounds.

In recent years, machine learned potentials (MLPs) have seen tremendous progress and rapid adoption by the materials science community. Due to their high speed and accuracy, MLPs are well suited for sampling complex potential energy surfaces (PESs) with molecular dynamics and Monte Carlo (MC) simulations. Nonetheless, many open challenges remain. Despite the outstanding performance of MLPs, it has become clear, that the builtin assumption of locality limits their applicability for systems where long-ranged effects due to charge transfer are present. But even for systems, for which local MLPs provide an adequate description, complete sampling of the PES can still be hindered by high energy barriers and advanced algorithms are needed to take full advantage of the MLPs capabilities. In this thesis, both above-mentioned challenges are addressed. In the first part, the fourth generation high-dimensional neural network potential (4G-HDNNP) is introduced. While previous generations of MLPs rely on atomic energies and charges, that only depend on the local atomic environment, the 4G-HDNNP is also able to describe long-ranged interactions caused by charge transfer effects. A charge equilibration scheme based on environment dependent electronegativities allows for the prediction of accurate atomic charges, that depend on the global state of a system, including the total charge. These charges are then not only used to compute electrostatic energies and forces, but also fed into the neural networks describing the short ranged interactions. This allows for an accurate description of changes in local bond-lengths and reactivity due to far-away changes in the electronic structure. The method’s performance is demonstrated on multiple test systems which are incorrectly described by previous methods. The second part of the thesis, focuses on the Funnel Hopping Monte Carlo (FHMC) method. FHMC introduces a new, global, MC move to directly circumvent high energy barriers that prevent complete sampling of the configuration space during MC simulations. Gaussian mixtures, fit to the Boltzmann distribution of low energy regions, are used to propose the FHMC moves without violating the detailed balance condition. FHMC therefore allows for direct sampling of the complete Boltzmann distribution without resorting to any approximate expansion of the potential energy. Anharmonic effects are therefore fully included. The method is first tested on two prototypical multi-funnel systems, namely the 38 and 75 atom Lennard-Jones clusters. We then used FHMC to study a material called methylammonium lead iodide (MaPbI3), for which we constructed a highly accurate MLP. In a recent structure search study, two non-perovskite phases of MaPbI3 were discovered, that, despite being lower in energy than the known perovskite phases, are absent in experiments. Our FHMC simulations, for which we extended the original algorithm to periodic boundary conditions, show, that above 200 K, the experimentally observed phases are thermodynamically preferred. This explains, the absence of the non-perovskite phases in experiments, since at room temperature the perovskite phases are readily obtained. The high energetic barriers then lead to kinetic trapping of the perovskite phases upon cooling.