Quantum-ESPRESSO

In this work, three new pharmaceutical salts of fenbendazole (FNB), a benzimidazole-based anthelmintic drug, with sulfonic acids have been obtained and thoroughly investigated by different analytical techniques, including thermal methods, infrared/Raman spectroscopy, and theoretical methods (periodic DFT computations and Bader analyses of the crystalline electronic density). Single-crystal and high-resolution synchrotron powder X-ray diffraction data for the first time made it possible to determine the crystal structures of mesylate and tosylate salts of the drug, which were further validated by dispersion-corrected density functional theory calculations. All the solid forms were stabilized by a robust R 2 2 (8) supramolecular motif formed by relatively strong N−H•••O hydrogen bonds. In the monohydrate of FNB tosylate, a considerable gain in the stabilization energy was due to the intermolecular interactions generated by the water molecules. A careful examination of the solubility−pH profile of the FNB salts revealed that, despite being thermodynamically unstable within the physiologically relevant pH range, the new solid forms demonstrated superior dissolution performance in terms of both the apparent solubility and the release rate in comparison to the parent drug. Since FNB has also been reported to possess anticancer activity, improving the drug's poor physicochemical properties through salt formation with the selected sulfonic acids is expected to promote further investigations toward repurposing of this potent compound.

The shielding tensor in 13C nuclear magnetic resonance (NMR) offers important information about the structural aspects of carbon materials from a local point of view. Not only the symmetry of the carbon site but also the presence of local structural distortions can affect the values of the isotropic shielding constant, the shielding anisotropy, and the deviation from axial symmetry. In this report, the 13C shielding in a single graphene sheet was calculated using density functional theory (DFT) via the gauge-including projector augmented plane wave (GIPAW) method. After performing convergence tests involving changes of k sampling and supercell size, the calculations were extended to graphene-based systems, including graphene bilayer and stacked graphene sheets, finally leading to hexagonal graphite. The calculated results showed good agreement with experimental values obtained by 13C NMR measurements in different types of carbon materials, evidencing the power of the DFT calculations for predicting NMR parameters in graphene-based nanocarbons.

In this paper we present first-principles calculations, based on both density functional theory and maximally localized Wannier functions, to study the electronic properties and interlayer coupling of twisted MoS 2 /NbSe 2 heterobilayers. We accurately investigate different stacking configurations and commensurate twist angles by including an in-depth analysis of the interlayer van der Waals interaction. The metallic character of the investigated heterostructures is dominated, at the Fermi energy, by the NbSe 2 atomic orbitals and shows a strong dependence on the twist angle. Notably, at the smallest considered twist angle, band structure flattening at the Fermi energy shows up, which should result in a lower conductivity of the metallic heterobilayer. The electrostatic potential analysis reveals no significant modification of the potential pattern with respect to the potentials of the isolated layers, with the exception of the interface region. A moderate electronic charge redistribution, compatible with electronically weakly coupled layers, is set up following the formation of the interface. The dependence of the electronic structure on the twist angle acts as a new degree of freedom for tuning properties relevant in electronic device applications.

The crystal structure, energy band structure and optical absorption of the N,V-doped and N,Na-doped anatase are studied by means of the first-principle pseudo-potential plane wave and linear muffin-tin orbitals methods. We show that the nitrogen and vanadium atoms have a tendency to form covalently bonded pairs. The crystal lattice distortions associated with doping essentially affect the optical absorption. With doping the impurity bands emerge in the band gap of the host anatase, however, a noticeable increase of optical absorption takes place at the energy only above 3 eV. Possible impact of this effect on the photocatalytic activity of the doped anatase is outlined.

Carbon monofluoride (CF)n and graphane are two very different materials from the practical point of view, but the basic chemical motifs of these materials are closely related: both can be described as two-dimensional polycyclic (fluoro-/hydro-)carbons. However, the actual experimental data on the structure of these materials is ambiguous ((CF)n) or scarce (graphane). Herein, we report a detailed computational study of structure of (CF)n and graphane, both in a monolayer configuration and in three-dimensional stacked arrangements. A crucial point in achieving a proper description of layer interactions is the use of a nonlocal density functional to describe long-range dispersion attraction from first principles. We find strong qualitative and quantitative similarities between the two materials in both conformational energetics (including a “gauche-chair” conformational motif not considered in previous studies) and layer stacking arrangements. A molecular mechanics force field is derived for (CF)n that performs exceptionally well at reproducing our quantum chemical results and fits into a very general OPLS/AA molecular mechanics framework. The combined results of quantum chemical calculations and classical molecular dynamics simulations using the new force field suggest a pathway to explain the too-small experimental in-plane lattice constant values observed in these materials, as well as the variation of interlayer distance in (CF)n, on the common basis of conformational disorder.

In this paper we present an implementation of the modified Becke-Johnson meta-GGA functional (TB09) in the PWSCF program of Quantum ESPRESSO package via the Libxc library. Using the functionals of TB09 we calculated the band gaps of some semiconductors and compared the results with previous calculations and experiments. We showed that combining GGA pseudo-potential with TB09 functionals would improve greatly the band gaps compared with the GGA calculations. The details of our implementation and code samples are also given.