Informative Training Data for Efficient Property Prediction in Metal-Organic Frameworks by Active Learning.

In recent data-driven approaches to material discovery, scenarios where target quantities are expensive to compute and measure are often overlooked. In such cases, it becomes imperative to construct a training set that includes the most diverse, representative, and informative samples. Here, a novel regression tree-based active learning algorithm is employed for such a purpose.

Artificial Neural Network-Based Density Functional Approach for Adiabatic Energy Differences in Transition Metal Complexes.

During the past decades, approximate Kohn-Sham density functional theory schemes have garnered many successes in computational chemistry and physics, yet the performance in the prediction of spin state energetics is often unsatisfactory. By means of a machine learning approach, an enhanced exchange and correlation functional is developed to describe adiabatic energy differences in transition metal complexes. The functional is based on the computationally efficient revision of the regularized, strongly constrained, and appropriately normed functional and improved by an artificial neural network correction trained over a small data set of electronic densities, atomization energies, and/or spin state energetics.

CO and CO adsorption mechanism in Fe(pz)[Pt(CN)] probed by neutron scattering and density-functional theory calculations.

We study the binding mechanism of CO and CO in the porous spin-crossover compound Fe(pz)[Pt(CN)] by combining neutron diffraction (ND), inelastic neutron scattering (INS) and density-functional theory (DFT) calculations. Two adsorption sites are identified, above the open-metal site and between the pyrazine rings. For CO adsorption, the guest molecules are parallel to the neighboring gas molecules and perpendicular to the pyrazine planes.

Assessing the Role of the Kohn-Sham Density in the Calculation of the Low-Lying Bethe-Salpeter Excitation Energies.

We adopt the many-body perturbation theory in conjunction with the Bethe-Salpeter equation (BSE) to compute 57 excitation energies of a set of 37 molecules. By using the PBEh global hybrid functional and a self-consistent scheme on the eigenvalues in , we show a strong dependence of the BSE energy on the starting Kohn-Sham (KS) density functional. This arises from both the quasiparticle energies and the spatial localization of the frozen KS orbitals employed to compute the BSE.

Tunable Proton Conductivity and Color in a Nonporous Coordination Polymer via Lattice Accommodation to Small Molecules.

Nonporous coordination polymers (npCPs) able to accommodate molecules through internal lattice reorganization are uncommon materials with applications in sensing and selective gas adsorption. Proton conduction, extensively studied in the analogue metal-organic frameworks under high-humidity conditions, is however largely unexplored in spite of the opportunities provided by the particular sensitivity of npCPs to lattice perturbations. Here, AC admittance spectroscopy is used to unveil the mechanism behind charge transport in the nonporous 1·2CH CN.

Twin boundary migration in an individual platinum nanocrystal during catalytic CO oxidation.

At the nanoscale, elastic strain and crystal defects largely influence the properties and functionalities of materials. The ability to predict the structural evolution of catalytic nanocrystals during the reaction is of primary importance for catalyst design. However, to date, imaging and characterising the structure of defects inside a nanocrystal in three-dimensions and in situ during reaction has remained a challenge.

Electric field-induced oxygen vacancies in YBaCuO.

The microscopic doping mechanism behind the superconductor-to-insulator transition of a thin film of YBaCuO was recently identified as due to the migration of O atoms from the CuO chains of the film. Here, we employ density-functional theory calculations to study the evolution of the electronic structure of a slab of YBaCuO in the presence of oxygen vacancies under the influence of an external electric field. We find that, under massive electric fields, isolated O atoms are pulled out of the surface consisting of CuO chains.

Strongly Bound Excitons in Metal-Organic Framework MOF-5: A Many-Body Perturbation Theory Study.

During the past years, one of the most iconic metal-organic frameworks (MOFs), MOF-5, has been characterized as a semiconductor by theory and experiments. Here we employ the many-body perturbation theory in conjunction with the Bethe-Salpeter equation to compute the electronic structure and optical properties of this MOF. The calculations show that MOF-5 is a wide-band-gap insulator with a fundamental gap of ∼8 eV.

Improved Spin-State Energy Differences of Fe(II) Molecular and Crystalline Complexes the Hubbard -Corrected Density.

We recently showed that the DFT+U approach with a linear-response yields adiabatic energy differences biased toward high spin [Mariano . , 6755-6762]. Such bias is removed here by employing a density-corrected DFT approach where the PBE functional is evaluated on the Hubbard -corrected density.

Biased Spin-State Energetics of Fe(II) Molecular Complexes within Density-Functional Theory and the Linear-Response Hubbard Correction.

The spin-state energetics of six Fe(II) molecular complexes are computed using the linear-response Hubbard approach within DFT. The adiabatic energy differences, Δ, between the high-spin ( = 2) and the low-spin ( = 0) states are computed and compared with accurate-coupled cluster-corrected CASPT2 results. We show that DFT+U fails in correctly capturing the ground state for strong-field ligands yielding Δ that are almost constant throughout the molecular series.

Divergent Adsorption-Dependent Luminescence of Amino-Functionalized Lanthanide Metal-Organic Frameworks for Highly Sensitive NO Sensors.

A novel gas sensing mechanism exploiting lanthanide luminescence modulation upon NO adsorption is demonstrated here. Two isostructural lanthanide-based metal-organic frameworks (MOFs) are used, including an amino group as the sensitive recognition center for NO molecules. The transfer of energy from the organic ligands to Ln is strongly dependent on the presence of NO, resulting in an unprecedented photoluminescent sensing scheme.

Accurate Prediction of the S Excitation Energy in Solvated Azobenzene Derivatives via Embedded Orbital-Tuned Bethe-Salpeter Calculations.

By employing the Bethe-Salpeter formalism coupled with a nonequilibrium embedding scheme, we demonstrate that the paradigmatic case of S band separation between and in azobenzene derivatives can be computed with excellent accuracy compared to experimental optical spectra. Besides embedding, we show that the choice of the Kohn-Sham exchange correlation functional for DFT is critical, despite the iterative convergence of quasiparticle energies. We address this by adopting an orbital-tuning approach via the global hybrid functional, PBEh, yielding an environment-consistent ionization potential.

A switchable iron-based coordination polymer toward reversible acetonitrile electro-optical readout.

Efficient and low cost detection of harmful volatile organic compounds (VOCs) is a major health and environmental need in industrialized societies. For this, tailor-made porous coordination polymers are emerging as promising molecular sensing materials thanks to their responsivity to a wide variety of external stimuli and could be used to complement conventional sensors. Here, a non-porous crystalline 1D Fe(ii) coordination polymer acting as a porous acetonitrile host is presented.

Probing the electric field-induced doping mechanism in YBaCuO using computed Cu K-edge x-ray absorption spectra.

We recently demonstrated that the superconductor-to-insulator transition induced by ionic liquid gating of the high temperature superconductor YBaCuO (YBCO) is accompanied by a deoxygenation of the sample [A. M. Perez-Munoz , Proc.

Tuning Gas Adsorption by Metal Node Blocking in Photoresponsive Metal-Organic Frameworks.

By combining first-principles calculations and classical molecular simulations, an atomistic-level of understanding was provided towards the notable change in CO adsorption upon light treatment in two recently reported photoactive metal-organic frameworks, PCN-123 and Cu (AzoBPDC) (AzoBiPyB). It was demonstrated that the reversible decrease in gas adsorption upon isomerization can be primarily attributed to the blocking of the strong adsorbing sites at the metal nodes by azobenzene molecules in a cis configuration. The same mechanism was found to apply also to other molecules, for example, alkanes and toxic gases.

In operando evidence of deoxygenation in ionic liquid gating of YBa2Cu3O7-X.

Field-effect experiments on cuprates using ionic liquids have enabled the exploration of their rich phase diagrams [Leng X, et al. (2011) Phys Rev Lett 107(2):027001]. Conventional understanding of the electrostatic doping is in terms of modifications of the charge density to screen the electric field generated at the double layer.

Probing gas adsorption in MOFs using an efficient ab initio widom insertion Monte Carlo method.

We propose a novel biased Widom insertion method that can efficiently compute the Henry coefficient, K , of gas molecules inside porous materials exhibiting strong adsorption sites by employing purely DFT calculations. This is achieved by partitioning the simulation volume into strongly and weakly adsorbing regions and selectively biasing the Widom insertion moves into the former region. We show that only few thousands of single point energy calculations are necessary to achieve accurate statistics compared to many hundreds of thousands or millions of such calculations in conventional random insertions.

Probing the mechanism of CO₂ capture in diamine-appended metal-organic frameworks using measured and simulated X-ray spectroscopy.

Diamine-appended metal-organic frameworks display great promise for carbon capture applications, due to unusual step-shaped adsorption behavior that was recently attributed to a cooperative mechanism in which the adsorbed CO2 molecules insert into the metal-nitrogen bonds to form ordered ammonium carbamate chains [McDonald et al., Nature, 2015, 519, 303]. We present a detailed study of this mechanism by in situ X-ray absorption spectroscopy and density functional theory calculations.

Cooperative insertion of CO2 in diamine-appended metal-organic frameworks.

The process of carbon capture and sequestration has been proposed as a method of mitigating the build-up of greenhouse gases in the atmosphere. If implemented, the cost of electricity generated by a fossil fuel-burning power plant would rise substantially, owing to the expense of removing CO2 from the effluent stream. There is therefore an urgent need for more efficient gas separation technologies, such as those potentially offered by advanced solid adsorbents.

Understanding Trends in CO2 Adsorption in Metal-Organic Frameworks with Open-Metal Sites.

Using van der Waals-corrected density functional theory and a local chemical bond analysis, we study and explain trends in the binding between CO2 and open-metal coordination sites within a series of two metal-organic frameworks (MOFs), BTT, and MOF-74 for Ca, Mg, and nine divalent transition-metal cations. We find that Ti and V result in the largest CO2 binding energies and show that for these cations the CO2 binding energies for both structure types are twice the value expected based on pure electrostatics. We associate this behavior with the specific electronic configuration of the divalent cations and symmetry of the metal coordination site upon CO2 binding, which result in empty antibonding orbitals between CO2 and the metal cation.

Probing adsorption interactions in metal-organic frameworks using X-ray spectroscopy.

We explore the local electronic signatures of molecular adsorption at coordinatively unsaturated binding sites in the metal-organic framework Mg-MOF-74 using X-ray spectroscopy and first-principles calculations. In situ measurements at the Mg K-edge reveal distinct pre-edge absorption features associated with the unique, open coordination of the Mg sites which are suppressed upon adsorption of CO2 and N,N'-dimethylformamide. Density functional theory shows that these spectral changes arise from modifications of local symmetry around the Mg sites upon gas uptake and are strongly dependent on the metal-adsorbate binding strength.

The mechanism of carbon dioxide adsorption in an alkylamine-functionalized metal-organic framework.

The mechanism of CO2 adsorption in the amine-functionalized metal-organic framework mmen-Mg2(dobpdc) (dobpdc(4-) = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate; mmen = N,N'-dimethylethylenediamine) was characterized by quantum-chemical calculations. The material was calculated to demonstrate 2:2 amine:CO2 stoichiometry with a higher capacity and weaker CO2 binding energy than for the 2:1 stoichiometry observed in most amine-functionalized adsorbents. We explain this behavior in the form of a hydrogen-bonded complex involving two carbamic acid moieties resulting from the adsorption of CO2 onto the secondary amines.

Ab initio carbon capture in open-site metal-organic frameworks.

During the formation of metal-organic frameworks (MOFs), metal centres can coordinate with the intended organic linkers, but also with solvent molecules. In this case, subsequent activation by removal of the solvent molecules creates unsaturated 'open' metal sites known to have a strong affinity for CO(2) molecules, but their interactions are still poorly understood. Common force fields typically underestimate by as much as two orders of magnitude the adsorption of CO(2) in open-site Mg-MOF-74, which has emerged as a promising MOF for CO(2) capture.

CO2 capture by metal-organic frameworks with van der Waals density functionals.

We use density functional theory calculations with van der Waals corrections to study the role of dispersive interactions on the structure and binding of CO(2) within two distinct metal-organic frameworks (MOFs): Mg-MOF74 and Ca-BTT. For both classes of MOFs, we report calculations with standard gradient-corrected (PBE) and five van der Waals density functionals (vdW-DFs), also comparing with semiempirical pairwise corrections. The vdW-DFs explored here yield a large spread in CO(2)-MOF binding energies, about 50% (around 20 kJ/mol), depending on the choice of exchange functional, which is significantly larger than our computed zero-point energies and thermal contributions (around 5 kJ/mol).