Tuning the electronic and optical properties of small organic acenedithiophene molecular crystals for photovoltaic applications: First principles calculations.

Periodic density functional theory was employed to investigate the impact of chemical modifications on the properties of π-conjugated acenedithiophene molecular crystals. Here, we highlight the importance of the ß-methylthionation effect, the position of the sulfur atoms of the thiacycle group and their size, and the number of central benzene rings in the chemical modification strategy. Our results show that the introduction of the methylthio groups at the ß-positions of the thiophene and the additional benzene ring at the center of the BDT crystal structure are a promising strategy to improve the performance of organic semiconductors, as observed experimentally. We found that ß-MT-ADT exhibits large charge carrier mobility, which is in good agreement with the experimental results and comparable to that of rubrene. In addition, the electronic and optical properties of these ambipolar materials suggest promising performances with ß-MT-ADT > ADT >ß-MT-NDT > NDT > BEDT-BDT >ß-MT-BDT > BDT. Moreover, functionalization with thiacycle-fused sulfur atoms of different sizes and numbers improve the properties of BDT but is still less efficient than the methylthionation effect. Overall, our findings suggest a promising molecular modification strategy for possibly high performance ambipolar organic semiconducting materials.

First-principles study of the structural and electronic properties of tetragonal ZrOX (X = S, Se, and Te) monolayers and their vdW heterostructures for applications in optoelectronics and photocatalysis.

Using first-principles calculations, we have studied the structural and electronic properties of ZrOX (X = S, Se, and Te) monolayers and their van der Waals heterostructures in the tetragonal structure. Our results show that these monolayers are dynamically stable and are semiconductors with electronic bandgaps ranging from 1.98 to 3.16 eV as obtained with the GW approximation. By computing their band edges, we show that ZrOS and ZrOSe are of interest for water splitting applications. In addition, the van der Waals heterostructures formed by these monolayers show a type I band alignment for ZrOTe/ZrOSe and a type II alignment for the other two heterostructures, making them potential candidates for certain optoelectronic applications involving electron/hole separation.

Ab initio molecular dynamics investigation of the co-adsorption of iodine species with CO and H2O in silver-exchanged chabazite.

In the field of nuclear energy, there is particular interest for the trapping of harmful iodine species (I2 and CH3I) that could be released during a nuclear accident, due to their dangereous impact on the human metabolic processes and the environment. Here, the adsorption of these iodine molecules versus several inhibitory compounds (CO, H2O, CH3Cl and Cl2) in the silver exchanged chabazite zeolite is studied in detail using ab initio molecular dynamics simulations at a realistic temperature and composition. Interestingly, we found that the iodine molecules remain attached to the cations even when the number of water molecules inside the structure is greater than two times the number of cations per cell at T = 413 K. For CO, we found that CH3I is more perturbed than I2 by the presence of this inhibitor. Overall, our results indicate that the silver-exchanged chabazite zeolite is a promising candidate to trap iodine species in the case of a severe nuclear accident.

Ab Initio Study of Helicity-Dependent Light-Induced Demagnetization: From the Optical Regime to the Extreme Ultraviolet Regime.

We use ab initio real-time time-dependent density functional theory to investigate the effect of optical and extreme ultraviolet (XUV) circularly polarized femtosecond pulses on the magnetization dynamics of ferromagnetic materials. We demonstrate that the light induces a helicity-dependent reduction of the magnitude of the magnetization. In the XUV regime, where the 3p semicore states are involved, a larger helicity dependence persisting even after the passage of light is exhibited. Finally, we were able to separate the part of the helicity-dependent dynamics due to the absorption from the part due to the inverse Faraday effect. Doing so, we show that the former has, overall, a greater impact on the magnetization than the latter, especially after the pulse and in the XUV regime. This work hints at the yet experimentally unexplored territory of the XUV light-induced helicity-dependent dynamics, which, according to our prediction, could magnify the helicity-dependent dynamics already exhibited in the optical regime.

Hybrid localized graph kernel for machine learning energy-related properties of molecules and solids.

Nowadays, the coupling of electronic structure and machine learning techniques serves as a powerful tool to predict chemical and physical properties of a broad range of systems. With the aim of improving the accuracy of predictions, a large number of representations for molecules and solids for machine learning applications has been developed. In this work we propose a novel descriptor based on the notion of molecular graph. While graphs are largely employed in classification problems in cheminformatics or bioinformatics, they are not often used in regression problem, especially of energy-related properties. Our method is based on a local decomposition of atomic environments and on the hybridization of two kernel functions: a graph kernel contribution that describes the chemical pattern and a Coulomb label contribution that encodes finer details of the local geometry. The accuracy of this new kernel method in energy predictions of molecular and condensed phase systems is demonstrated by considering the popular QM7 and BA10 datasets. These examples show that the hybrid localized graph kernel outperforms traditional approaches such as, for example, the smooth overlap of atomic positions and the Coulomb matrices.

Establishing the accuracy of density functional approaches for the description of noncovalent interactions in ionic liquids.

We test a number of dispersion corrected versatile Generalized Gradient Approximation (GGA) and meta-GGA functionals for their ability to predict the interactions of ionic liquids, and show that most can achieve energies within 1 kcal mol-1 of benchmarks. This compares favorably with an accurate dispersion corrected hybrid, ωB97X-V. Our tests also reveal that PBE (Perdew-Burke-Ernzerhof GGA) calculations using the plane-wave projector augmented wave method and Gaussian Type Orbitals (GTOs) differ by less than 0.6 kJ mol-1 for ionic liquids, despite ions being difficult to evaluate in periodic cells - thus revealing that GTO benchmarks may be used also for plane-wave codes. Finally, the relatively high success of explicit van der Waals density functionals, compared to elemental and ionic dispersion models, suggests that improvements are required for low-cost dispersion correction models of ions.

Assessment and prediction of band edge locations of nitrides using a self-consistent hybrid functional.

Due to their optimal bandgap size and large defect tolerance, nitrides are becoming pivotal materials in several optoelectronic devices, photovoltaics, and photocatalysts. A computational method that can accurately predict their electronic structures is indispensable for exploring new nitride materials. However, the relatively small bandgap of nitrides, which stems from the subtle balance between ionic and covalent bond characteristics, makes conventional density functional theory challenging to achieve satisfactory accuracy. Here, we employed a self-consistent hybrid functional where the Hartree-Fock mixing parameter is self-consistently determined and thus the empiricism of the hybrid functional is effectively removed to calculate the bandgaps of various nitride compounds. By comparing the bandgaps from the self-consistent hybrid functional calculations with the available experimental and high-level GW calculation results, we found that the self-consistent hybrid functional can provide a computationally efficient approach for quantitative predictions of nitride electronic structures with an accuracy level comparable to the GW method. Additionally, we aligned the band edge positions of various nitride compounds using self-consistent hybrid functional calculations, providing material design principles for heterostructures of nitride-based optoelectronic devices. We anticipate the wide use of the self-consistent hybrid functional for accelerating explorations and predictions of new nitride-based functional materials in various photoactive applications.

uMBD: A Materials-Ready Dispersion Correction That Uniformly Treats Metallic, Ionic, and van der Waals Bonding.

Materials design increasingly relies on first-principles calculations for screening important candidates and for understanding quantum mechanisms. Density functional theory (DFT) is by far the most popular first-principles approach due to its efficiency and accuracy. However, to accurately predict structures and thermodynamics, DFT must be paired with a van der Waals (vdW) dispersion correction. Therefore, such corrections have been the subject of intense scrutiny in recent years. Despite significant successes in organic molecules, no existing model can adequately cover the full range of common materials, from metals to ionic solids, hampering the applications of DFT for modern problems such as battery design. Here, we introduce a universally optimized vdW-corrected DFT method that demonstrates an unbiased reliability for predicting molecular, layered, ionic, metallic, and hybrid materials without incurring a large computational overhead. We use our method to accurately predict the intercalation potentials of layered electrode materials of a Li-ion battery system, a problem for which the existing state-of-the-art methods fail. Thus, we envisage broad use of our method in the design of chemo-physical processes of new materials.

Ternary Chalcogenides BaMxTe2 (M = Cu, Ag): Syntheses, Modulated Crystal Structures, Optical Properties, and Electronic Calculations.

Standard solid-state methods produced black crystals of the compounds BaCu0.43(3)Te2 and BaAg0.77(1)Te2 at 1173 K; the crystal structures of each were established using single-crystal X-ray diffraction data. Both crystal structures are modulated. The compound BaCu0.43(3)Te2 crystallizes in the monoclinic superspace group P2(αß1/2)0, having cell dimensions of a = 4.6406(5) Å, b = 4.6596(5) Å, c = 10.362(1) Å, ß = 90.000(9)°, and Z = 2 and an incommensurate vector of q = 0.3499(6)b* + 0.5c*. The compound BaAg0.77(1)Te2 crystallizes in the orthorhombic P21212(α00)000 superspace group with cell dimensions of a = 4.6734(1) Å, b = 4.6468(1) Å, c = 11.1376(3) Å, and Z = 2 and an incommensurate vector of q = 0.364(2)a*. The asymmetric unit of the BaCu0.43(3)Te2 structure comprises eight crystallographically independent sites; that for BaAg0.77(1)Te2 comprises four. In these two structures, each of the M (M = Cu, Ag) atoms is connected to four Te atoms to make two-dimensional layers of [MxTe4/4]n- that are separated by layers of Ba atoms and square nets of Te. A Raman spectroscopic study at 298(2) K on a pelletized polycrystalline sample of BaAg0.8Te2 shows the presence of Ag-Te (83, 116, and 139 cm-1) and Ba-Te vibrations (667 and 732 cm-1). A UV-vis-NIR spectroscopic study on a powdered sample of BaAg0.8Te2 shows the semiconducting nature of the compound with a direct band gap of 1.0(2) eV, consistent with its black color. DFT calculations give a pseudo bandgap with a weak value of the DOS at the Fermi level.

Modulated Linear Tellurium Chains in Ba3ScTe5: Synthesis, Crystal Structure, Optical and Resistivity Studies, and Electronic Structure.

A new ternary telluride, Ba3ScTe5, with a pseudo-one-dimensional structure, was synthesized at 1173 K by standard solid-state methods. A single-crystal X-ray diffraction study at 100(2) K shows the structure to be modulated. The structure of the subcell of Ba3ScTe5 crystallizes with two formula units in the hexagonal space group D6h3-P63/mcm with unit cell dimensions of a = b = 10.1190(5) Å and c = 6.8336(3) Å. The asymmetric unit of the subcell structure consists of four crystallographically independent sites: Ba1 (site symmetry: m2m), Sc1 (-3.m), Te1 (m2m), and Te2 (3.2). Its structure is made up of chains of ∞1[ScTe33-] that are separated by Ba2+ cations. The Sc atoms are bonded to six Te1 atoms that form a slightly distorted octahedral geometry. The structure of the subcell also contains linear infinite chains of Te2 with intermediate Te···Te interactions. The superstructure of Ba3ScTe5 is incommensurate and was solved in the hexagonal superspace group P-6(00γ)0 with a = 10.1188(3) Å and c = 6.8332(3) Å and a modulation vector of q = 0.3718(2)c*. The arrangement and coordination geometries of the atoms in the superstructure are very similar to those in the substructure. However, the main difference is that the infinite chains of Te atoms in the superstructure are distorted owing to the formation of long- and short-bonded pairs of Te atoms. The presence of these chains with intermediate Te···Te interactions makes assignment of the formal oxidation states arbitrary. The optical absorption study of a polycrystalline sample of Ba3ScTe5 that was synthesized by the stoichiometric reaction of elements at 1173 K reveals a direct band gap of 1.1(2) eV. The temperature-dependent resistivity study of polycrystalline Ba3ScTe5 shows semiconducting behavior corroborating the optical studies, while density functional theory calculations report a pseudo band gap of 1.3 eV.

Establishing the accuracy of density functional approaches for the description of noncovalent interactions in biomolecules.

Biomolecules have complex structures, and noncovalent interactions are crucial to determine their conformations and functionalities. It is therefore critical to be able to describe them in an accurate but efficient manner in these systems. In this context density functional theory (DFT) could provide a powerful tool to simulate biological matter either directly for relatively simple systems or coupled with classical simulations like the QM/MM (quantum mechanics/molecular mechanics) approach. Additionally, DFT could play a fundamental role to fit the parameters of classical force fields or to train machine learning potentials to perform large scale molecular dynamics simulations of biological systems. Yet, local or semi-local approximations used in DFT cannot describe van der Waals (vdW) interactions, one of the essential noncovalent interactions in biomolecules, since they lack a proper description of long range correlation effects. However, many efficient and reasonably accurate methods are now available for the description of van der Waals interactions within DFT. In this work, we establish the accuracy of several state-of-the-art vdW-aware functionals by considering 275 biomolecules including interacting DNA and RNA bases, peptides and biological inhibitors and compare our results for the energy with highly accurate wavefunction based calculations. Most methods considered here can achieve close to predictive accuracy. In particular, the non-local vdW-DF2 functional is revealed to be the best performer for biomolecules, while among the vdW-corrected DFT methods, uMBD is also recommended as a less accurate but faster alternative.

Grafting of iron on amorphous silica surfaces from ab initio calculations.

Iron over silica catalytic systems have attracted considerable attention due to their activity and selectivity in different reactions, for instance, in the hydrodeoxygenation process. Here, the grafting mechanisms of iron under various forms (one atom, two atoms, or a cluster) on silica surfaces are studied using ab initio calculations. Various geometries with different locations of iron on the silica structure have been investigated, and it is found that a strong interaction between iron and the silanol groups takes place, mostly driven by the formation of Fe-O-Si bonds, and in few cases by nearby surface OH groups, creating Fe-OH-Si bonds. For the cluster, we show that the most favorable adsorption mode induces a strong effect on the silica surface accompanied with a large charge transfer, making it very stable and promising for a large panel of applications.

Ag5U(PS4)3: A Transition-Metal Actinide Phosphochalcogenide.

The structure of Ag5U(PS4)3 is unique, as in the literature there are no other structures of the type MAnPQ (M = transition metal, An = actinide, Q = S, Se, or Te). The compound has been synthesized at 1123 K by standard solid-state methods, and its single-crystal X-ray structure has been determined at 100(2) K. Ag5U(PS4)3 crystallizes in a remarkable new structure type in space group P3221 of the trigonal system with three formula units in a hexagonal cell of dimensions a = b = 9.6635(2) Å, c = 17.1834(4) Å, and γ = 120°. In the structure, each U atom is coordinated to eight S atoms in a bicapped trigonal prismatic manner. Each P atom is tetrahedrally coordinated to four S atoms. Two of the three unique Ag atoms are connected to four S atoms in a distorted tetrahedral manner, whereas the third unique Ag atom forms an Ag2S6 species. The overall structure consists of U polyhedra connected to each other via PS4 tetrahedra through edge-sharing in a zigzag fashion along the c axis to form infinite layers. PS4 groups and the Ag atoms pack these layers. From density functional theory calculations, the total density of states of Ag5U(PS4)3 is asymmetric with respect to spin and has finite spin polarization in the crystal cell: the magnetic moments of two of the U atoms are parallel, whereas the magnetic moment of the third U atom is antiparallel.

Synthesis and Characterization of Ba2Ag2Se2(Se2).

Single crystals and a polycrystalline sample of Ba2Ag2Se2(Se2) were synthesized by standard solid-state chemistry methods at 1173 and 973 K, respectively. The crystal structure of this ternary compound was established by single-crystal X-ray diffraction studies at 100(2) K. The superstructure of this compound is commensurate and crystallizes in the space group P21/ c of a monoclinic system with cell constants of a = 6.1766(2) Å, b = 6.1788(2) Å, c = 21.5784(8) Å, and ß = 90.02(1)° ( Z = 4). The asymmetric unit of the superstructure comprises eight atoms occupying general positions: two Ba atoms, two Ag atoms, and four Se atoms. In this structure, each Ag atom is tetrahedrally coordinated with four adjacent Se atoms to form distorted AgSe4 units that share edges with the neighboring tetrahedra to form a two-dimensional [AgSe4/4]- layer. These layers are separated by Ba2+ and Se22- units. The presence of the Se22- unit is also supported by an intense band at around 247 cm-1 in the Raman spectrum of Ba2Ag2Se2(Se2). A density functional theory study shows that the compound is a semiconductor with a calculated band gap of 1.1 eV. As determined by UV-visible spectroscopy, the direct and indirect band gaps are 1.23(2) and 1.10(2) eV, respectively, in good agreement with the theory and consistent with the black color of the compound. A temperature-dependent resistivity study also confirms the semiconducting nature of Ba2Ag2Se2(Se2).

Benchmarking the performance of approximate van der Waals methods for the structural and energetic properties of SiO2 and AlPO4 frameworks.

Density functional theory (DFT) calculations using sixteen different approaches, fourteen of which were designed to include dispersion interactions [DFT + D and van der Waals (vdW)-DF methods], were performed for a set of sixteen framework compounds with either SiO2 or AlPO4 composition. The compounds include four dense structures (α-quartz, α-cristobalite, and their AlPO4 analogues), eight all-silica zeolites, and four aluminophosphate zeotypes (AlPOs). We analyzed the performance in reproducing the equilibrium structure for all systems, and computed bulk moduli and relative stabilities were compared to experiments for those compounds where experimental data are available. We found that the results obtained with functionals that take into account dispersive interactions are closer to experiments than those obtained with a bare generalized gradient functional. However, the variation among individual methods is considerable, and functionals that perform well for one quantity may give rather large deviations for another. Taking together the whole body of results, it appears that the Perdew-Burke-Ernzerhof functional including a many-body dispersion correction and the rev-vdW-DF2 methods present the best performance for the description of SiO2 and AlPO4 materials.

Benchmarking several van der Waals dispersion approaches for the description of intermolecular interactions.

Seven methods, including three van der Waals density functionals (vdW-DFs) and four different variants of the Tkatchenko-Scheffler (TS) methods, are tested on the A24, L7, and Taylor et al.'s "blind" test sets. It is found that for these systems, the vdW-DFs perform better that the TS methods. In particular, the vdW-DF-cx functional gives binding energies that are the closest to the reference values, while the many-body correction of TS does not always lead to an improvement in the description of molecular systems. In light of these results, several directions for further improvements to describe van der Waals interactions are discussed.

Performance of CuII -, PbII -, and HgII -Exchanged Mordenite in the Adsorption of I2 , ICH3 , H2 O, CO, ClCH3 , and Cl2 : A Density Functional Study.

Periodic dispersion-corrected DFT is used to investigate the adsorption of I2 and ICH3 , which may be released during a severe nuclear accident, for three divalent cation (Cu2+ , Pb2+ and Hg2+ )-exchanged mordenites with an Si/Al ratio of 23. Gases such as H2 O, CO, ClCH3 , and Cl2 present in the containment atmosphere can inhibit the selective adsorption of iodine species. To identify the most promising adsorbents, a systematic study is performed in which all the possible cationic sites in the main channel of the mordenite structure are considered. For the energetically most stable sites, the divalent cation is located in the small rings (five- or six-membered) containing two Al atoms, while in the energetically less stable configurations, the two Al atoms are far apart (>7 Å) and the cation is close to only one Al atom. Upon adsorption of the various molecules, the coordination number of the cation decreases with increasing interaction energy, as the molecules can attract the divalent cations from the framework. Finally, the computed interaction energies show that Hg-mordenite (MOR) could be a suitable material for selective adsorption of volatile iodine species, contrary to Cu-MOR and Pb-MOR.

Communication: A novel implementation to compute MP2 correlation energies without basis set superposition errors and complete basis set extrapolation.

By using a formulation based on the dynamical polarizability, we propose a novel implementation of second-order Møller-Plesset perturbation (MP2) theory within a plane wave (PW) basis set. Because of the intrinsic properties of PWs, this method is not affected by basis set superposition errors. Additionally, results are converged without relying on complete basis set extrapolation techniques; this is achieved by using the eigenvectors of the static polarizability as an auxiliary basis set to compactly and accurately represent the response functions involved in the MP2 equations. Summations over the large number of virtual states are avoided by using a formalism inspired by density functional perturbation theory, and the Lanczos algorithm is used to include dynamical effects. To demonstrate this method, applications to three weakly interacting dimers are presented.

Synthesis, Crystal Structure, Theoretical, and Resistivity Study of BaUSe3.

The black-colored compound BaUSe3 has been synthesized at 1173 K by a stoichiometric reaction of the elements in a CsCl flux. BaUSe3 crystallizes in the GdFeO3 structure type. There is no change in structure between 100 and 298 K. The U atoms in this structure are octahedrally connected to six Se atoms. Each octahedral unit shares all six corners with neighboring octahedra, forming a three-dimensional network. BaUSe3 can be charge balanced as Ba(2+)U(4+)(Se(2-))3. DFT electronic structure calculations found BaUSe3 to be antiferromagnetic in its ground state and to be a semiconductor with a band gap of 2.5 eV. The band gap is inconsistent with the black color of the material and with the small activation energy of 0.12(1) eV obtained from resistivity measurements. A UV-vis spectrum indicated that there was no band gap above 1 eV. It is possible that, for BaUSe3, intrinsic and extrinsic impurities from the flux create midgap states that lead to the experimentally measured narrow optical gap. More likely, BaUSe3 presents a challenge to DFT calculations as applied to 5f materials.

Impact of the Si/Al ratio on the selective capture of iodine compounds in silver-mordenite: a periodic DFT study.

Silver modified zeolites with a mordenite structure can capture volatile iodine compounds (I2 and ICH3) which can be released during a severe nuclear accident. However under these particular conditions, molecules such as CO and H2O present in the containment atmosphere are expected to inhibit the adsorption of iodine compounds. In the present work, periodic density functional theory calculations have been carried out to investigate the interaction of I2, ICH3, H2O and CO molecules in silver-exchanged mordenite with various Si/Al ratios with the aim of finding values that favor a selective adsorption of I2 and ICH3. Computational results show that the interaction energies of CO and H2O remain of the same order of magnitude (from -120 to -140 kJ mol-1 for CO and from -90 to -120 kJ mol-1 for H2O) for all the investigated Si/Al ratios. In contrast, ICH3 is increasingly strongly adsorbed as the Si/Al ratio decreases, from around -145 kJ mol-1 when Si/Al = 47 to -190 kJ mol-1 for Si/Al = 5. The same trend is observed for I2 with a larger amplitude: from -135 kJ mol-1 for Si/Al = 47 to -300 kJ mol-1 for Si/Al = 5. Therefore, the use of silver-exchanged mordenite with Si/Al ratios of 5 or 11 would drastically limit the inhibiting effect of contaminants on the adsorption of volatile iodine species. Also for the same ratios, a spontaneous dissociation of I2 during its adsorption is observed, leading to the formation of AgI complexes which are prerequisite for the immobilization of iodine in the long term.