Density Functional Theory Ones Research Articles

Deuterium uptake, desorption and sputtering from W(110) surface covered with oxygen

Rate equation modelling is performed to simulate D2 and D2+D2+ exposure of the W(110) surface with varying coverage of oxygen atoms (O) from the clean surface up to 0.75 monolayer of O. Density Functional Theory (DFT) calculated energetics are used as inputs for the surface processes and desorption energies are optimized to best reproduce the Thermal Desorption Spectrometry (TDS) experiments obtained for D2 exposure. For the clean surface, the optimized desorption energies (1.10 eV–1.40 eV) are below the DFT ones (1.30 eV–1.50 eV). For the O covered surface, the main desorption peak is reproduced with desorption energies of 1.10 eV and 1.00 eV for 0.50 and 0.75 monolayer of O respectively. This is slightly higher than the DFT predicted desorption energies. In order to simulate satisfactorily the total retention obtained experimentally for D2+D2+ exposure, a sputtering process needs to be added to the model, describing the sputtering of adsorbed species (D atoms) by the incident D ions. The impact of the sputtering process on the shape of the TDS spectra, on the total retention and on the recycling of D from the wall is discussed. In order to better characterize the sputtering process, especially its products and yields, atomistic calculations such as molecular dynamics are suggested as a next step for this study.

Excitonic Effects in Fe/As K‐Edge Absorption for Iron Based Superconductors: A Combined DFT and BSE Analysis

AbstractThis work provides a detailed theoretical calculations of Fe/As K‐edge X‐ray absorption spectra of six different iron based superconducting compounds FeSe, LiFeAs, NaFeAs, , , and , by using three distinct levels of calculations to evaluate the electron–hole interaction properly. First, density functional theory (DFT) in absence of core–hole effect which completely neglects the electron–hole interaction; second, supercell based core–hole calculations where the static electron–hole Coulomb interaction is taken into account; and finally, by solving the Bethe–Salpeter equation (BSE) which takes care of the dynamical screening of photo‐electron and core–hole. The characteristic peaked features and spectral weight distribution in Fe K‐edge absorption spectrum matches well in both the theoretical methods, especially when “core–hole” effect are explicitly considered for iron chalcogenides. Hence, the electron–hole screening effect is almost equivalent in both the methods for these compounds. Occurrence of an additional higher energy peak and bifurcation of some peaks in the BSE spectrum may be attributed to excitonic effects. BSE based As K‐edge absorption spectrum differs significantly with the DFT ones in some of the studied compounds, indicating more paramount excitonic effects for As edges. Nonresonant inelastic X‐ray scattering is also investigated at various momentum transfers and converges to corresponding X‐ray absorption spectrum at low momentum transfers.

Dipole Polarizability of C28 and its Counterparts Nb4B18 and Ta4B18. Insights from a Density Functional Theory (DFT) Endeavour

We report on a preliminary investigation of the non linear optical (NLO) properties and in particular dipole polarizability. The target species are two perfect tetrahedral nanoclusters Nb4B18 and Ta4B18, along with their nanofullerene counterpart that is C28. Our study based on density functionals (DFs) that have gained popularity among the scientific community. In addition we performed Hartree-Fock calculations known for not including dynamic electron correlation. The DF obtained values are characterized by some dispersion, with maximal differences to be around 5 %, in all three cases. Given that the DFT introduces a fuzzy percentage of electron correlation sets the observed convergence of HF values to DFT ones is at least surprising. Furthermore, it should be said that though the values can be characterized as accurate their reliability should not be taken for granted. Last, we note the smooth convergence of LC-BLYP, LC-BP86, LC-BPW91 to LC-whPBE.

Modeling microsolvation clusters with electronic-structure calculations guided by analytical potentials and predictive machine learning techniques.

We propose a new methodology to study, at the density functional theory (DFT) level, the clusters resulting from the microsolvation of alkali-metal ions with rare-gas atoms. The workflow begins with a global optimization search to generate a pool of low-energy minimum structures for different cluster sizes. This is achieved by employing an analytical potential energy surface (PES) and an evolutionary algorithm (EA). The next main stage of the methodology is devoted to establish an adequate DFT approach to treat the microsolvation system, through a systematic benchmark study involving several combinations of functionals and basis sets, in order to characterize the global minimum structures of the smaller clusters. In the next stage, we apply machine learning (ML) classification algorithms to predict how the low-energy minima of the analytical PES map to the DFT ones. An early and accurate detection of likely DFT local minima is extremely important to guide the choice of the most promising low-energy minima of large clusters to be re-optimized at the DFT level of theory. In this work, the methodology was applied to the Li+Krn (n = 2-14 and 16) microsolvation clusters for which the most competitive DFT approach was found to be the B3LYP-D3/aug-pcseg-1. Additionally, the ML classifier was able to accurately predict most of the solutions to be re-optimized at the DFT level of theory, thereby greatly enhancing the efficiency of the process and allowing its applicability to larger clusters.

The open-framework structure of KSbGe3O9 flux-grown crystals investigated by X-ray diffraction, vibrational spectroscopy, and DFT calculations

We report on the preparation and the X-ray crystal structure of colorless KSbGe3O9, its vibrational properties (Raman and infrared studies), and density functional theory (DFT) calculations. KSbGe3O9, grown by the high-temperature flux method from K2Mo4O13 flux, is thermally stable at least up to 1200 ​°C and is isostructural to the benitoite BaTiSi3O9 (space group P6¯c2 (no. 188)). The hexagonal unit cell contains two formula units and the structure was refined to R1 ​= ​0.0324 from single-crystal X-ray diffraction data. KSbGe3O9 is characterized with only one crystallographically independent Ge atom involved in three-member units [Ge3O9]6- of regular germanate tetrahedra. The K+ ions are located in channels and, like SbV, are octahedrally surrounded by oxygen atoms. The KSbGe3O9 local structure and the planarity of Ge3O3 rings are also studied by a room-temperature vibrational investigation using non-polarized infrared and Raman spectroscopy. Both the infrared and Raman phonon modes have been assigned from the agreement observed between our experimental data and the corresponding DFT ones. In particular, two E’ (TO) modes (both IR and Raman active) characterize the planarity of the Ge3O3 ring in the ab plane.

Strain-induced effects on the electronic properties of 2D materials

Thanks to the ultrahigh flexibility of 2D materials and to their extreme sensitivity to applied strain, there is currently a strong interest in studying and understanding how their electronic properties can be modulated by applying a uniform or nonuniform strain. In this work, using density functional theory (DFT) calculations, we discuss how uniform biaxial strain affects the electronic properties, such as ionization potential, electron affinity, electronic gap, and work function, of different classes of 2D materials from X-enes to nitrides and transition metal dichalcogenides. The analysis of the states in terms of atomic orbitals allows to explain the observed trends and to highlight similarities and differences among the various materials. Moreover, the role of many-body effects on the predicted electronic properties is discussed in one of the studied systems. We show that the trends with strain, calculated at the GW level of approximation, are qualitatively similar to the DFT ones solely when there is no change in the character of the valence and conduction states near the gap.

Optimizing the tight-binding parametrization of the quasi-one-dimensional superconductor K2Cr3As3

We study the tight-binding dispersion of the recently discovered superconductor K2Cr3As3, obtained from Wannier projection of Density Functional Theory (DFT) results. In order to establish quantitatively the actual degree of quasi-one-dimensionality of this compound, we analyze the electronic band structure for two reduced sets of hopping parameters: one restricted to the Cr-As tubes and another one retaining a minimal number of in-plane hoppings. The corresponding total and local density of states of the compound are also computed with the aim of assessing the tight-binding results with respect to the DFT ones. We find a quite good agreement with the DFT results for the more extended set of hopping parameters, especially for what concerns the orbitals that dominate at the Fermi level. Therefore, we conclude that one cannot avoid taking into account in-plane hoppings up to the next-nearest-neighbors cells even only to describe correctly the Fermi surface cuts and the populations along the kz direction. Such a choice of a minimal number of hopping parameters directly reflects in the possibility of correctly describing correlations and magnetic interactions.

Predictions of Electronic, Transport, and Structural Properties of Magnesium Sulfide (MgS) in the Rocksalt Structure

We report results from ab-initio, self-consistent density functional theory (DFT) calculations of electronic, transport and bulk properties of rock salt magnesium sulfide (MgS). In the absence of experimental data on these properties, except for the bulk modulus, these results are predictions. Our calculations utilized the Ceperley and Alder local density approximation (LDA) potential and the linear combination of Gaussian orbitals (LCGO). The key difference between our computations and other previous ab-initio DFT ones stems from our use of successively larger basis sets, in consecutive, self-consistent calculations, to attain the ground state of the material. We predicted an indirect (Γ-X) band gap of 3.278 eV for a room temperature lattice constant of 5.200A. We obtained a predicted low temperature indirect (Γ-X) band gap of 3.512 eV, using the equilibrium lattice constant of 5.183A. We found a theoretical value of 79.76 GPa for the bulk modulus; it agrees very well with the experimental finding of 78 ± 3.7 GPa.

Tuning Electronic Properties and Band Alignments of Phosphorene Combined With MoSe2 and WSe2

Stacks of two-dimensional crystals in van der Waals heterostructures pave the way to novel applications in electronics and optoelectronics. Based on first-principles calculations, we study heterobilayers constructed with phosphorene on MoSe2 and WSe2. Both combinations are stable upon contact, while van der Waals interaction leads to a long-range structural bending, affecting electronic properties from phosphorene. Including quasiparticle effects, strong orbital overlaps are observed in the heterobilayers, influencing band offsets and, hence, emphasizing the importance of quasiparticle calculations over standard density functional theory ones. Interface effects also change the heterostructure type, modify local band gaps, and favor indirect transitions. To tailor the electronic properties of the heterostructures, interface interactions and external perturbations are taken into account through vertical pressures and electric fields. Uniaxial pressure strongly affects local direct gaps, and small electric f...

Lowest ionisation and excitation energies of biologically important heterocyclic planar molecules

ABSTRACTWe calculate the lowest ionisation and excitation energies in a variety of biologically important molecules, i.e. π-conjugated systems like DNA and RNA bases and isomers plus related heterocyclic molecules. For approximately half of these molecules, there are no experimental and theoretical/numerical data in the literature, as far as we know. These electronic transitions are mainly but not exclusively of π and π–π* character, respectively. We perform symmetry-constrained density functional theory (DFT) geometry optimisation at the B3LYP/6-311++G** level of theory. At the DFT-obtained ground-state geometries, we calculate vertical ionisation energies with ionisation potential coupled cluster with singles and doubles (IP-EOMCCSD) and vertical excitation energies with the completely renormalised equation of motion coupled cluster with singles, doubles, and non-iterative triples (CR-EOMCCSD(T)) method. We also investigate whether a simple semi-empirical Hückel-type model approach with novel parametrisation could provide reasonable estimates of the lowest π ionisation and π–π* excitation energies. Our coupled cluster (CC) results are in very good agreement with experimental data, while the Hückel-type model predictions generally follow the trends with some deviation. Finally, we investigate the effect of basis set in IP-EOMCCSD energies and we compare our CR-EOMCCSD(T) results with time-dependent DFT (TDDFT) ones.

Vibrational spectroscopy and molecular dynamics of water monomers and dimers adsorbed on polycyclic aromatic hydrocarbons

This paper reports structures, energetics, dynamics and spectroscopy of H2O and (H2O)2 systems adsorbed on coronene (C24H12), a compact polycyclic aromatic hydrocarbon (PAH). On-the-fly Born-Oppenheimer molecular dynamics simulations are performed for temperatures T varying from 10 to 300 K, on a potential energy surface obtained within the self-consistent-charge density-functional based tight-binding (SCC-DFTB) approach. Anharmonic infrared (IR) spectra are extracted from these simulations. We first benchmark the SCC-DFTB semi-empirical hamiltonian vs. DFT (Density Functional Theory) calculations that include dispersion, on (C6H6)(H2O)1,2 small complexes. We find that charge corrections and inclusion of dispersion contributions in DFTB are necessary to obtain consistent structures, energetics and IR spectra. Using this Hamiltonian, the structures, energetics and IR features of the low-energy isomers of (C24H12)(H2O)1,2 are found to be similar to the DFT ones, with evidence for a stabilizing edge-coordination. The temperature dependence of the motions of H2O and (H2O)2 on the surface of C24H12 is analysed, revealing ultra-fast periodic motion. The water dimer starts diffusing at a higher temperature than the water monomer (150 K vs. 10 K respectively), which appears to be consistent with the binding energies. Qualitative and quantitative analyses of the effects of T on the IR spectra are performed. Anharmonic factors in particular are derived and it is shown that they can be used as signatures for the presence of PAH-water complexes. Finally, this paper lays the foundations for the studies of larger (PAH)m(H2O)n clusters, that can be treated with the efficient computational approach benchmarked in this paper.

Adsorption energies of molecular oxygen on Au clusters.

The adsorption properties of O(2) molecules on anionic, cationic, and neutral Au(n) clusters (n=1-6) are studied using the density functional theory (DFT) with the generalized gradient approximation (GGA), and with the hybrid functional. The results show that the GGA calculations with the PW91 functional systemically overestimate the adsorption energy by 0.2-0.4 eV than the DFT ones with the hybrid functional, resulting in the failure of GGA with the PW91 functional for predicting the adsorption behavior of molecular oxygen on Au clusters. Our DFT calculations with the hybrid functional give the same adsorption behavior of molecular oxygen on Au cluster anions and cations as the experimental measurements. For the neutral Au clusters, the hybrid DFT predicts that only Au(3) and Au(5) clusters can adsorb one O(2) molecule.

Theoretical study on the stereochemistry of intramolecular hetero Diels–Alder cycloaddition reactions of azoalkenes

AbstractThe molecular mechanism for the intramolecular hetero Diels–Alder cycloaddition reactions of azoalkenes has been studied by using quantum mechanical calculations at the density functional theory (DFT) Becke's three‐parameter exchange functional and the gradient‐corrected functional of Lee, Yang, and Paar (B3LYP) and second‐order Møller–Plesset (MP2) theory levels with the 6‐31G* basis set. The molecular mechanism corresponds to a concerted mechanism with a slightly asynchronous bond‐formation process where the formed internal bond (N4–C5) obtained with DFT is longer than the peripheral bond (C1–C6) formed in the transition state. Otherwise, the (C1–C6) bond obtained with MP2 is longer than the (N4–C5) bond formed in the transition state. Solvent (dichloromethane) effects have been considered with the polarizable continuum model (PCM). The MP2 calculations predicted an endo kinetic preference, whereas the DFT ones rendered endo/exo transition states that are practically isoenergetic. According to our analyses, the dispersion interaction (which stabilizes the endo orientation through π–π secondary contacts) and the solvent effects (which favor the exo addition) are the factors governing the stereoselectivity of the process. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 95: 133–136, 2003

The performance of density-functional theory in challenging cases: Halogen oxides

Halogen dioxides (FOO, ClOO, BrOO, OClO, OBrO), their cationic and anionic derivatives and two isomers of ClO3 have been studied by means of density-functional theory (DFT) and the results compared with those from high level ab initio molecular orbital calculations. Three different density functionals (SVWN, B3LYP, and G96LYP) combined with a 6-311+G(2df ) basis set were used to obtain geometries and vibrational frequencies, which were then compared with MP2 (second-order Moller–Plesset), QCISD, and CCSD(T) (coupled-cluster single double triple) results. The B3LYP/6-311+G(2df ) calculations generally give geometries and frequencies in excellent agreement with those calculated from high level ab initio calculations such as CCSD(T). Exceptions, such as ClOO and BrOO, arise when high spin contamination at B3LYP level produces spurious results. Atomisation enthalpies evaluated at B3LYP/6-311+G(3df ) level of theory are observed to be in good agreement with the experimental values. In some particular cases this agreement is better than that obtained at CCSD(T)/6-311+G(3df ) level. For ionization enthalpies the CCSD(T) calculations seem to be superior to the DFT ones. Wave function instabilities [with respect to the UHF (unrestricted Hartree–Fock) transformation in the case of the cations and internal symmetry breaking in the case of the OXO (X=Cl, Br) compounds and the C3v isomer of ClO3] are observed less frequently when DFT methods are used.