arXiv subjects

T. Thonhauser

Publications and source records attributed to T. Thonhauser.

At least 19 recordsLinked to original sources

arXiv2020

Next-generation non-local van der Waals density functional

The fundamental ideas for a non-local density functional theory -- capable of reliably capturing van der Waals interaction -- were already conceived in the 1990's. In 2004, a seminal paper introduced the first practical non-local exchange-correlation functional called vdW-DF, which has become widely successful and laid the foundation for much further research. However, since then, the functional form of vdW-DF has remained unchanged. Several successful modifications paired the original functional with different (local) exchange functionals to improve performance and the successor vdW-DF2 also updated one internal parameter. Bringing together different insights from almost two decades of development and testing, we present the next-generation non-local correlation functional called vdW-DF3, in which we change the functional form while staying true to the original design philosophy. Although many popular functionals show good performance around the binding separation of van der Waals complexes, they often result in significant errors at larger separations. With vdW-DF3, we address this problem by taking advantage of a recently uncovered and largely unconstrained degree of freedom within the vdW-DF framework that can be constrained through empirical input, making our functional semi-empirical. For two different parameterizations, we benchmark vdW-DF3 against a large set of well-studied test cases and compare our results with the most popular functionals, finding good performance in general for a wide array of systems and a significant improvement in accuracy at larger separations. Finally, we discuss the achievable performance within the current vdW-DF framework, the flexibility in functional design offered by vdW-DF3, as well as possible future directions for non-local van der Waals density functional theory.

cond-mat.mtrl-sci↗

arXiv2019

van der Waals density functional with corrected $C_6$ coefficients

The non-local van der Waals density functional (vdW-DF) has had tremendous success since its inception in 2004 due to its constraint-based formalism that is rigorously derived from a many-body starting point. However, while vdW-DF can describe binding energies and structures for van der Waals complexes and mixed systems with good accuracy, one long-standing criticism---also since its inception---has been that the $C_6$ coefficients that derive from the vdW-DF framework are largely inaccurate and can be wrong by more than a factor of two. It has long been thought that this failure to describe the $C_6$ coefficients is a conceptual flaw of the underlying plasmon framework used to derive vdW-DF. We prove here that this is not the case and that accurate $C_6$ coefficient can be obtained without sacrificing the accuracy at binding separations from a modified framework that is fully consistent with the constraints and design philosophy of the original vdW-DF formulation. Our design exploits a degree of freedom in the plasmon-dispersion model $ω_{\mathbf{q}}$, modifying the strength of the long-range van der Waals interaction and the cross-over from long to short separations, with additional parameters tuned_ to reference systems. Testing the new formulation for a range of different systems, we not only confirm the greatly improved description of $C_6$ coefficients, but we also find excellent performance for molecular dimers and other systems. The importance of this development is not necessarily that particular aspects such as $C_6$ coefficients or binding energies are improved, but rather that our finding opens the door for further conceptual developments of an entirely unexplored direction within the exact same constrained-based non-local framework that made vdW-DF so successful in the first place.

cond-mat.mtrl-sci↗

arXiv2017

Advanced capabilities for materials modelling with Quantum ESPRESSO

Quantum ESPRESSO is an integrated suite of open-source computer codes for quantum simulations of materials using state-of-the art electronic-structure techniques, based on density-functional theory, density-functional perturbation theory, and many-body perturbation theory, within the plane-wave pseudo-potential and projector-augmented-wave approaches. Quantum ESPRESSO owes its popularity to the wide variety of properties and processes it allows to simulate, to its performance on an increasingly broad array of hardware architectures, and to a community of researchers that rely on its capabilities as a core open-source development platform to implement theirs ideas. In this paper we describe recent extensions and improvements, covering new methodologies and property calculators, improved parallelization, code modularization, and extended interoperability both within the distribution and with external software.

cond-mat.mtrl-sci↗

arXiv2017

H4-Alkanes: A new class of hydrogen storage material?

The methane-based material (H$_2$)$_4$CH$_4$, also called H4M for short, is in essence a methane molecule with 4 physisorbed H$_2$ molecules. While H4M has exceptionally high hydrogen storage densities when it forms a molecular solid, unfortunately, this solid is only stable at impractically high pressures and/or low temperatures. To overcome this limitation, we show through simulations that longer alkanes (methane is the shortest alkane) also form stable structures that still physisorb 4 H$_2$ molecules per carbon atom; we call those structures H4-alkanes. We further show via molecular dynamics simulations that the stability field of molecular solids formed from H4-alkanes increases remarkably with chain length compared to H4M, just as it does for regular alkanes. From our simulations of H4-alkanes with lengths 1, 4, 10, and 20, we see that e.g. for the 20-carbon the stability field is doubled at higher pressures. While even longer chains show only insignificant improvements, we discuss various other options to stabilize H4-alkanes more. Our proof-of-principle results lay the groundwork to show that H4-alkanes can become viable hydrogen storage materials.

cond-mat.mtrl-sci↗

arXiv2016

Chemistry in confined spaces: Reactivity of the Zn-MOF-74 channels

Using infrared spectroscopy combined with ab initio methods we study reactions of H$_2$O and CO inside the confined spaces of Zn-MOF-74 channels. Our results show that, once the water dissociation reaction H$_2$O$\;\rightarrow\;$OH+H takes place at the metal centers, the addition of 40 Torr of CO at 200 $^{\circ}$C starts the production of formic acid via OH+H+CO$\;\rightarrow\;$HCO$_2$H. Our detailed analysis shows that the overall reaction H$_2$O+CO$\;\rightarrow\;$HCO$_2$H takes place in the confinement of MOF-74 without an external catalyst, unlike the same reaction on flat surfaces. This discovery has several important consequences: It opens the door to a new set of catalytic reactions inside the channels of the MOF-74 system, it suggests that a recovery of the MOF's adsorption capacity is possible after it has been exposed to water (which in turn stabilizes its crystal structure), and it produces the important industrial feedstock formic acid.

physics.chem-ph↗

arXiv2016

Cluster Assisted Water Dissociation Mechanism in MOF-74 and Controlling it Using Helium

We show that the water dissociation reaction H$_2$O$\rightarrow$OH+H in the confined environment of MOF-74 channels can be precisely controlled by the addition of the noble gas He. Elucidating the entire reaction process with ab initio methods and infrared (IR) spectroscopy, we prove that the interaction between water molecules is critical to the formation of water clusters, which reduce the dissociation barrier by up to 37% and thus influence the reaction significantly. Our time-resolved IR measurements confirm that the formation of these clusters can be suppressed by introducing He gas, providing unprecedented control over water dissociation rates. Since the water dissociation reaction is the cause of the structural instability of MOF-74 in the presence of water, our finding of the reaction mechanism lays the groundwork for designing water stable versions of MOF-74 as well as understanding water related phenomena in MOFs in general.

cond-mat.mtrl-sci↗

arXiv2016

Understanding and Controlling Water Stability of MOF-74

Metal organic framework (MOF) materials in general, and MOF-74 in particular, have promising properties for many technologically important processes. However, their instability under humid conditions severely restricts practical use. We show that this instability and the accompanying reduction of the CO$_2$ uptake capacity of MOF-74 under humid conditions originate in the water dissociation reaction H$_2$O$\rightarrow$OH+H at the metal centers. After this dissociation, the OH groups coordinate to the metal centers, explaining the reduction in the MOF's CO$_2$ uptake capacity. This reduction thus strongly depends on the catalytic activity of MOF-74 towards the water dissociation reaction. We further show that-while the water molecules themselves only have a negligible effect on the crystal structure of MOF-74-the OH and H products of the dissociation reaction significantly weaken the MOF framework and lead to the observed crystal structure breakdown. With this knowledge, we propose a way to suppress this particular reaction by modifying the MOF-74 structure to increase the water dissociation energy barrier and thus control the stability of the system under humid conditions.

cond-mat.mtrl-sci↗

arXiv2016

Suppressing diborane production during the hydrogen release of metal borohydrides: The example of alloyed Al(BH$_4$)$_3$

Aluminum borohydride (Al(BH$_4$)$_3$) is an example of a promising hydrogen storage material with exceptional hydrogen densities by weight and volume and a low hydrogen desorption temperature. But, unfortunately, its production of diborane (B$_2$H$_6$) gases upon heating to release the hydrogen restricts its practical use. To elucidate this issue, we investigate the properties of a number of metal borohydrides with the same problem and find that the electronegativity of the metal cation is not the best descriptor of diborane production. We show that, instead, the closely related formation enthalpy is a better descriptor and we find that diborane production is an exponential function thereof. We conclude that diborane production is sufficiently suppressed for formation enthalpies of $-$80 kJ/mol BH$_4$ or lower, providing specific design guidelines to tune existing metal borohydrides or synthesize new ones. We then use first-principles methods to study the effects of Sc alloying in Al(BH$_4$)$_3$. Our results for the thermodynamic properties of the Al$_{1-x}$Sc$_x$(BH$_4$)$_3$ alloy clearly show the stabilizing effect of Sc alloying and thus the suppression of diborane production. We conclude that stabilizing Al(BH$_4$)$_3$ and similar borohydrides via alloying or other means is a promising route to suppress diborane production and thus develop viable hydrogen storage materials.

cond-mat.mtrl-sci↗

arXiv2015

Spin signature of nonlocal-correlation binding in metal organic frameworks

We develop a proper nonempirical spin-density formalism for the van der Waals density functional (vdW-DF) method. We show that this generalization, termed svdW-DF, is firmly rooted in the single-particle nature of exchange and we test it on a range of spin systems. We investigate in detail the role of spin in the nonlocal-correlation driven adsorption of H$_2$ and CO$_2$ in the linear magnets Mn-MOF74, Fe-MOF74, Co-MOF74, and Ni-MOF74. In all cases, we find that spin plays a significant role during the adsorption process despite the general weakness of the molecular-magnetic responses. The case of CO$_2$ adsorption in Ni-MOF74 is particularly interesting, as the inclusion of spin effects results in an increased attraction, opposite to what the diamagnetic nature of CO$_2$ would suggest. We explain this counter-intuitive result, tracking the behavior to a coincidental hybridization of the O $p$ states with the Ni $d$ states in the down-spin channel. More generally, by providing insight on nonlocal correlation in concert with spin effects, our nonempirical svdW-DF method opens the door for a deeper understanding of weak nonlocal magnetic interactions.

cond-mat.mtrl-sci↗

arXiv2014

van der Waals forces in density functional theory: The vdW-DF method

A density functional theory (DFT) that accounts for van der Waals (vdW) interactions in condensed matter, materials physics, chemistry, and biology is reviewed. The insights that led to the construction of the Rutgers-Chalmers van der Waals Density Functional (vdW-DF) are presented with the aim of giving a historical perspective, while also emphasising more recent efforts which have sought to improve its accuracy. In addition to technical details, we discuss a range of recent applications that illustrate the necessity of including dispersion interactions in DFT. This review highlights the value of the vdW-DF method as a general-purpose method, not only for dispersion bound systems, but also in densely packed systems where these types of interactions are traditionally thought to be negligible.

cond-mat.mtrl-sci↗

arXiv2014

Tuning the hydrogen desorption of Mg(BH$_4$)$_2$ through Zn alloying

We study the effect of Zn alloying on the hydrogen desorption properties of Mg(BH$_4$)$_2$ using $\it{ab\ initio}$ simulations. In particular, we investigate formation/reaction enthalpies/entropies for a number of compounds and reactions at a wide range of temperatures and Zn concentrations in Mg$_{1-x}$Zn$_x$(BH$_4$)$_2$. Our results show that the thermodynamic stability of the resulting material can be significantly lowered through Zn alloying. We find that e.g. the solid solution Mg$_{2/3}$Zn$_{1/3}$(BH$_4$)$_2$ has a reaction enthalpy for the complete hydrogen desorption of only 25.3 kJ/mol H$_2$$-$a lowering of 15 kJ/mol H$_2$ compared to the pure phase and a corresponding lowering in critical temperature of 123 K. In addition, we find that the enthalpy of mixing is rather small and show that the decrease in reaction enthalpy with Zn concentration is approximately linear.

cond-mat.mtrl-sci↗

arXiv2014

Structural, elastic, thermal, and electronic response of small-molecule-loaded metal organic framework materials

We combine infrared spectroscopy, nano-indentation measurements, and \emph{ab initio} simulations to study the evolution of structural, elastic, thermal, and electronic responses of the metal organic framework MOF-74-Zn when loaded with H$_2$, CO$_2$, CH$_4$, and H$_2$O. We find that the molecular adsorption in this MOF triggers remarkable responses in all of these properties of the host material, with specific signatures for each of the guest molecules. With this comprehensive study we are able to clarify and correlate the underlying mechanisms regulating these responses with changes of the physical and chemical environment. Our findings suggest that metal organic framework materials in general, and MOF-74-Zn in particular, can be very promising materials for novel transducers and sensor applications, including highly selective small-molecule detection in gas mixtures.

cond-mat.mtrl-sci↗

arXiv2014

Positional disorder in ammonia borane at ambient conditions

We solve a long-standing experimental discrepancy of NH$_3$BH$_3$, which---as a molecule---has a threefold rotational axis, but in its crystallized form at room temperature shows a fourfold symmetry about the same axis, creating a geometric incompatibility. To explain this peculiar experimental result, we study the dynamics of this system with ab initio Car-Parrinello molecular dynamics and nudged-elastic-band simulations. We find that rotations, rather than spatial static disorder, at angular velocities of 2 rev/ps---a time scale too small to be resolved by standard experimental techniques---are responsible for the fourfold symmetry.

cond-mat.mtrl-sci↗

arXiv2014

van der Waals density functionals built upon the electron-gas tradition: Facing the challenge of competing interactions

The theoretical description of sparse matter attracts much interest, in particular for those ground-state properties that can be described by density functional theory (DFT). One proposed approach, the van der Waals density functional (vdW-DF) method, rests on strong physical foundations and offers simple yet accurate and robust functionals. A very recent functional within this method called vdW-DF-cx [K. Berland and P. Hyldgaard, Phys. Rev. B 89, 035412] stands out in its attempt to use an exchange energy derived from the same plasmon-based theory from which the nonlocal correlation energy was derived. Encouraged by its good performance for solids, layered materials, and aromatic molecules, we apply it to several systems that are characterized by competing interactions. These include the ferroelectric response in PbTiO$_3$, the adsorption of small molecules within metal-organic frameworks (MOFs), the graphite/diamond phase transition, and the adsorption of an aromatic-molecule on the Ag(111) surface. Our results indicate that vdW-DF-cx is overall well suited to tackle these challenging systems. In addition to being a competitive density functional for sparse matter, the vdW-DF-cx construction presents a more robust general purpose functional that could be applied to a range of materials problems with a variety of competing interactions.

cond-mat.mtrl-sci↗

arXiv2013

High-throughput screening of small-molecule adsorption in MOF

Using high-throughput screening coupled with state-of-the-art van der Waals density functional theory, we investigate the adsorption properties of four important molecules, H_2, CO_2, CH_4, and H_2O in MOF-74-M with M = Be, Mg, Al, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Sr, Zr, Nb, Ru, Rh, Pd, La, W, Os, Ir, and Pt. We show that high-throughput techniques can aid in speeding up the development and refinement of effective materials for hydrogen storage, carbon capture, and gas separation. The exploration of the configurational adsorption space allows us to extract crucial information concerning, for example, the competition of water with CO_2 for the adsorption "pockets." We find that only a few noble metals---Rh, Pd, Os, Ir, and Pt---favor the adsorption of CO_2 and hence are potential candidates for effective carbon-capture materials. Our findings further reveal significant differences in the binding characteristics of H_2, CO_2, CH_4, and H_2O within the MOF structure, indicating that molecular blends can be successfully separated by these nano-porous materials.

cond-mat.mtrl-sci↗

arXiv2013

NMR study of small molecule adsorption in MOF-74-Mg

We calculate the carbon nuclear magnetic resonance (NMR) shielding for CO_2 and the hydrogen shieldings for both H_2 and H_2O inside the metal organic framework MOF-74-Mg. Our ab initio calculations are at the density functional theory level using the van der Waals including density functional vdW-DF. The shieldings are obtained while placing the small molecules throughout the structure, including the calculated adsorption site for various loading scenarios. We then explore relationships between loading, rotational and positional characteristics, and the NMR shieldings for each adsorbate. Our NMR calculations show a change in the shielding depending on adsorbate, position, and loading in a range that is experimentally observable. We further provide a simple model for the energy and the NMR shieldings throughout the cavity of the MOF. By providing this mapping of shielding to position and loading for these adsorbates, we argue that NMR probes could be used to provide additional information about the position at which these small molecules bind within the MOF, as well as the loading of the adsorbed molecule.

cond-mat.mtrl-sci↗

arXiv2013

When metal organic frameworks turn into linear magnets

We investigate the existence of linear magnetism in the metal organic framework materials MOF-74-Fe, MOF-74-Co, and MOF-74-Ni, using first-principles density functional theory. MOF-74 displays regular quasi-linear chains of open-shell transition metal atoms, which are well separated. Our results show that within these chains-for all three materials-ferromagnetic coupling of significant strength occurs. In addition, the coupling in-between chains is at least one order of magnitude smaller, making these materials almost perfect 1D magnets at low temperature. The inter-chain coupling is found to be anti-ferromagnetic, in agreement with experiments. While some quasi-1D materials exist that exhibit linear magnetism-mostly complex oxides, polymers, and a few other rare material-they are typically very difficult to synthesize. The significance of our finding is that MOF-74 is very easy to synthesize and it is likely the simplest realization of the 1D Ising model in nature. MOF-74 could thus be used in future experiments to study 1D magnetism at low temperature.

cond-mat.mtrl-sci↗

arXiv2012

Diffusion of Small Molecules in Metal Organic Framework Materials

Ab initio simulations are combined with in situ infrared spectroscopy to unveil the molecular transport of H$_2$, CO$_2$, and H$_2$O in the metal organic framework MOF-74-Mg. Our study uncovers---at the atomistic level---the major factors governing the transport mechanism of these small molecules. In particular, we identify four key diffusion mechanisms and calculate the corresponding diffusion barriers, which are nicely confirmed by time-resolved infrared experiments. We also answer a long-standing question about the existence of secondary adsorption sites for the guest molecules, and we show how those sites affect the macroscopic diffusion properties. Our findings are important to gain a fundamental understanding of the diffusion processes in these nano-porous materials, with direct implications for the usability of MOFs in gas sequestration and storage applications.

cond-mat.mtrl-sci↗