Semilocal Density Functional Theory Research Articles

Comparison of density functionals for nitrogen impurities in ZnO

Hybrid functionals and empirical correction schemes are compared to conventional semi-local density functional theory (DFT) calculations in order to assess the predictive power of these methods concerning the formation energy and the charge transfer level of impurities in the wide-gap semiconductor ZnO. While the generalized gradient approximation fails to describe the electronic structure of the N impurity in ZnO correctly, methods that widen the band gap of ZnO by introducing additional non-local potentials yield the formation energy and charge transfer level of the impurity in reasonable agreement with hybrid functional calculations. Summarizing the results obtained with different methods, we corroborate earlier findings that the formation of substitutional N impurities at the oxygen site in ZnO from N atoms is most likely slightly endothermic under oxygen-rich preparation conditions, and introduces a deep level more than 1 eV above the valence band edge of ZnO. Moreover, the comparison of methods elucidates subtle differences in the predicted electronic structure, e.g., concerning the orientation of unoccupied orbitals in the crystal field and the stability of the charged triplet state of the N impurity. Further experimental or theoretical analysis of these features could provide useful tests for validating the performance of DFT methods in their application to defects in wide-gap materials.

Fluorine-induced local magnetic moment in graphene: A hybrid DFT study

Recent experimental evidence that fluorinated graphene creates local magnetic moments around F adatoms has not been supported by semilocal density-functional theory (DFT) calculations where the adsorption of an isolated F adatom induces no magnetic moment in graphene. Here, we show that such an incorrect prediction of the nonmagnetic ground state is due to the self-interaction error inherent in semilocal exchange-correlation functionals. The present hybrid DFT calculation for an isolated F adatom on graphene predicts not only a spin-polarized ground state with a spin moment of 1 bohr magneton, but also a long-range spin polarization caused by the bipartite nature of the graphene lattice as well as the induced spin polarization of the graphene states. The results provide support for the experimental observations of local magnetic moments in fluorinated graphene.

Optical band gap of NpO2 and PuO2 from optical absorbance of epitaxial films

We report a solution based synthesis of epitaxial thin films of neptunium oxide and plutonium oxide. Actinides represent a challenge to first principle calculations due to features that arise from f orbital interactions. Conventional semi-local density functional theory predicts NpO2 and PuO2 to be metallic, when they are well known insulators. Improvements in theory are dependent on comparison with accurate measurements of material properties, which in turn demand high-quality samples. The high melting point of actinide oxides and their inherent radioactivity makes single crystal and epitaxial film formation challenging. We report on the preparation of high quality epitaxial actinide films. The films have been characterized through a combination of X-ray diffraction and X-ray absorption fine structure (XANES and EXAFS) measurements. We report band gaps of 2.80 ± 0.1 eV and 2.85 ± 0.1 eV at room temperature for PuO2 and NpO2, respectively, and compare our measurements with state-of-the-art calculations.

Nature of visible-light responsive fluorinated titanium dioxides

The origin of the visible-light activity of fluorinated TiO2 has been systematically investigated via comprehensive theoretical studies. The 3-coordinated surface F atoms with higher 1s binding energy are identified to be the origin of the visible light activity of the fluorinated TiO2 by systematically analysing 1s core-level shifts of various types of F atoms in fluorinated TiO2. Further analysing the electronic structures of the fluorinated TiO2 using semi-local density functional theory and non-local hybrid density functional theory calculations demonstrates that the introduction of the 3-coordinated surface F atoms leads to the formation of Ti3+ ions in the sub-surface, which is the cause for the bandgap shrinking, increasing the visible-light activity. The results also reveal that the presence of 3-coordinated surface F atoms strengthens the adsorption of hydroxyl groups in their vicinity to enhance the photocatalytic activity. The findings of this work address a long-standing scientific question, providing useful guidance for the design and development of high performance VL photocatalysts.

Accuracy of density functional theory in the prediction of carbon dioxide adsorbent materials

Density functional theory (DFT) has become the computational method of choice for modeling and characterization of carbon dioxide adsorbents, a broad family of materials which at present are urgently sought after for environmental applications. The description of polar carbon dioxide (CO(2)) molecules in low-coordinated environments like surfaces and porous materials, however, may be challenging for local and semi-local DFT approximations. Here, we present a thorough computational study in which the accuracy of DFT methods in describing the interactions of CO(2) with model alkali-earth-metal (AEM, Ca and Li) decorated carbon structures, namely anthracene (C(14)H(10)) molecules, is assessed. We find that gas-adsorption energies and equilibrium structures obtained with standard (i.e. LDA and GGA), hybrid (i.e. PBE0 and B3LYP) and van der Waals exchange-correlation functionals of DFT dramatically differ from the results obtained with second-order Møller-Plesset perturbation theory (MP2), an accurate computational quantum chemistry method. The major disagreements found can be mostly rationalized in terms of electron correlation errors that lead to wrong charge-transfer and electrostatic Coulomb interactions between CO(2) and AEM-decorated anthracene molecules. Nevertheless, we show that when the concentration of AEM atoms in anthracene is tuned to resemble as closely as possible the electronic structure of AEM-decorated graphene (i.e. an extended two-dimensional material), hybrid exchange-correlation DFT and MP2 methods quantitatively provide similar results.

Assessment of a Nonlocal Correction Scheme to Semilocal Density Functional Theory Methods.

A nonlocal correction method to (semi)local density-functional theory (DFT) methods is derived that is based on a partitioning of the correlation-energy density into atom-atom contributions. Nonlocal interaction contributions, which are absent in standard DFT methods, are introduced in this method by using atomic weight functions that do not vanish exponentially as the atomic densities but with the inverse sixth power of the atomic distances. The parameters contained in these weight functions were fitted to reproduce intermolecular interaction energies for a range a small dimer systems. The new functional has then been tested both for intermolecular interactions, using the S22, S66 × 8, and IonHB databases from Hobza et al., and for other thermodynamical properties using a subset of 14 databases of the GMTKN30 database of Grimme et al. It is found that for intermolecular interactions the accuracy of the method is often higher than with standard DFT+D methods while for other properties, such as reaction energies or relative conformation energies of medium sized organic molecules, the accuracy is similar to hybrid-DFT+D methods. The nonlocal correction method has been tested also to predict the interaction energy of the water-graphene system yielding an estimated interaction energy of -2.87 kcal/mol, which is in line with previous theoretical investigations.

Electronic structure of assembled graphene nanoribbons: Substrate and many-body effects

Experimentally measured electronic band gaps of atomically sharp straight and chevronlike armchair graphene nanoribbons (GNRs) adsorbed on a gold substrate are smaller than theoretically predicted quasiparticle band gaps of their free-standing counterparts [Linden et al., Phys. Rev. Lett. 108, 216801 (2012)]. The influence of the substrate on electronic properties of both straight and chevronlike GNRs is here investigated including many-body effects beyond semilocal density-functional theory. The predicted small electron transfer from a straight or chevronlike GNR to the gold surface is found to lead to a surface polarization at the GNR-metal interface responsible for a significant reduction of the quasiparticle band gap of the GNR. This reduction is quantified using a semiclassical image charge model. By considering both quasiparticle and surface polarization corrections, we obtain theoretical band gaps that are consistent with experimental ones for gold-supported GNRs.

Resolution-of-identity approach to Hartree–Fock, hybrid density functionals, RPA, MP2 and GW with numeric atom-centered orbital basis functions

The efficient implementation of electronic structure methods is essential for first principles modeling of molecules and solids. We present here a particularly efficient common framework for methods beyond semilocal density-functional theory (DFT), including Hartree–Fock (HF), hybrid density functionals, random-phase approximation (RPA), second-order Møller–Plesset perturbation theory (MP2) and the GW method. This computational framework allows us to use compact and accurate numeric atom-centered orbitals (NAOs), popular in many implementations of semilocal DFT, as basis functions. The essence of our framework is to employ the ‘resolution of identity (RI)’ technique to facilitate the treatment of both the two-electron Coulomb repulsion integrals (required in all these approaches) and the linear density-response function (required for RPA and GW). This is possible because these quantities can be expressed in terms of the products of single-particle basis functions, which can in turn be expanded in a set of auxiliary basis functions (ABFs). The construction of ABFs lies at the heart of the RI technique, and we propose here a simple prescription for constructing ABFs which can be applied regardless of whether the underlying radial functions have a specific analytical shape (e.g. Gaussian) or are numerically tabulated. We demonstrate the accuracy of our RI implementation for Gaussian and NAO basis functions, as well as the convergence behavior of our NAO basis sets for the above-mentioned methods. Benchmark results are presented for the ionization energies of 50 selected atoms and molecules from the G2 ion test set obtained with the GW and MP2 self-energy methods, and the G2-I atomization energies as well as the S22 molecular interaction energies obtained with the RPA method.

Electronic structure of copper phthalocyanine fromG0W0calculations

We present all-electron G0W0 calculations for the electronic structure of the organic semiconductor copper phthalocyanine, based on semilocal and hybrid density-functional theory (DFT) starting points. We show that G0W0 calculations improve the quantitative agreement with high resolution photoemission and inverse photoemission experiments. However, the extent of the improvement provided by G0W0 depends significantly on the choice of the underlying DFT functional, with the hybrid functional serving as a much better starting point than the semilocal one. In particular, strong starting-point dependence is observed in the energy positions of highly localized molecular orbitals. This is attributed to self-interaction errors (SIE), due to which the orbitals obtained from semilocal DFT do not approximate the quasi-particle (QP) orbitals as well as those obtained from hybrid DFT. Our findings establish the viability of the G0W0 approach for describing the electronic structure of metal-organic systems, given a judiciously chosen DFT-based starting point.

A Parameter-Free Density Functional That Works for Noncovalent Interactions

The failure of semilocal density functional theory for medium- and long-range noncovalent molecular interactions is a long-standing challenge for computational chemistry. Here, we assess the perfor...

Unexpected Deformations Induced by Surface Interaction and Chiral Self‐Assembly of CoII‐Tetraphenylporphyrin (Co‐TPP) Adsorbed on Cu(110): A Combined STM and Periodic DFT Study

In a combined scanning tunnelling microscopy (STM) and periodic density functional theory (DFT) study, we present the first comprehensive picture of the energy costs and gains that drive the adsorption and chiral self-assembly of highly distorted Co(II)-tetraphenylporphyrin (Co-TPP) conformers on the Cu(110) surface. Periodic, semi-local DFT calculations reveal a strong energetic preference for Co-TPP molecules to adsorb at the short-bridge site when organised within a domain. At this adsorption site, a substantial chemical interaction between the molecular core and the surface causes the porphyrin macrocycle to accommodate close to the surface and in a flat geometry, which induces considerable tilting distortions in the phenyl groups. Experimental STM images can be explained in terms of these conformational changes and adsorption-induced electronic effects. For the ordered structure we unambiguously show that the substantial energy gain from the molecule–surface interaction recuperates the high cost of the induced molecular and surface deformations as compared with gas phase molecules. Conversely, singly adsorbed molecules prefer a long-bridge adsorption site and adopt a non-planar, saddle-shape conformation. By using a van der Waals density functional correction scheme, we found that the intermolecular π–π interactions make the distorted conformer more stable than the saddle conformer within the organic assembly. These interactions drive supramolecular assembly and also generate chiral expression in the system, pinning individual molecules in a propeller-like conformation and directing their assembly along non-symmetric directions that lead to the coexistence of mirror-image chiral domains. Our observations reveal that a strong macrocycle–surface interaction can trigger and stabilise highly unexpected deformations of the molecular structure and thus substantially extend the range of chemistries possible within these systems.

The connection between self-interaction and static correlation: a random phase approximation perspective

Semi-local density functional theory suggests a connection between static correlation and self-interaction. It is difficult to make such a connection from the wave function theory perspective, since few wave function methods permit self-interaction error. However, the random phase approximation for ground-state correlation, which has a wave function derivation, does include self-interaction in its direct (Hartree) variant. This variant also describes left–right correlation. The self-interaction can be removed by means of second-order screened exchange; however, this also has negative consequences for the description of static correlation. This paper discusses the connection between the two concepts (static correlation and self-interaction) from the perspective provided by the random phase approximation.

Hybrid density functional calculations of redox potentials and formation energies of transition metal compounds

We compare the accuracy of conventional semilocal density functional theory (DFT), the DFT+U method, and the Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional for structural parameters, redox react ...

Structural and Electronic Properties of the Methyl-Terminated Si(111) Surface

We have investigated the structural and electronic properties of the methyl-terminated Si(111) surface using first-principles calculations at the semilocal density functional theory level, inclusive of semiempirical dispersion forces between the methyl groups. In agreement with previous ab initio studies and consistent with low-energy electron diffraction measurements, we find that the most stable geometry corresponds to a (1 × 1), symmetric pattern, with methyl groups vertically bound to the Si substrate. Therefore, our computed scanning tunneling microscopy (STM) images exhibit three-fold symmetry at variance from those reported by recent experiments. The main contribution to the tunneling current comes from surface resonant states close to the Fermi level that are very weakly affected by self-energy corrections within many body perturbation theory. Our computed current−voltage profiles are in qualitative agreement with experiment. We suggest that the differences between computed and measured STM patter...

Mixed-Valency Signature in Vibrational Inelastic Electron Tunneling Spectroscopy

Density functional theory simulations of the vibrational inelastic electron tunneling spectroscopy (IETS) of O2 on Ag(110) permits us to solve its unexplained IETS data [Hahn, Phys. Rev. Lett. 85, 1914 (2000)]. When semilocal density functional theory is corrected by including static intra-atomic correlations, the IETS simulations are in excellent agreement with the experiment. The unforeseen consequence of our calculations is that when adsorbed along the [001] direction, molecular O2 on Ag(110) is a mixed-valent system. This analysis of IETS unambiguously reveals the paramagnetic nature of O2 on Ag(110).

The Crucial Role of Dispersion in the Cohesion of Nonbridged Binuclear Os → Cr and Os → W Adducts

The concept of a dative metal-metal bond is generally used to designate the donor-acceptor (DA) interaction of an electron-saturated metal center with another electron-deficient--or unsaturated--metal center. This type of DA bonding extended to the field of coordination complexes constitutes a borderline case of weak metal-metal interaction, among which the so-called metallophilic interactions occurring with 4d, 5d, and other late-transition-metal complexes are the most documented and representative examples. From a general standpoint, the peculiar position of the so-called dative metal-metal bond in chemical bonding stems from its presumed covalent character, which contrasts with the situation encountered with metallophilic interactions, which are essentially supported by dispersion and electrostatic forces and somewhat sustained by relativistic effects. In this study, the nature of the metal-metal bond in nonbridged 5d-3d Os-Cr and 5d-5d Os-W adducts, i.e., (Me(3)P)(CO)(4)Os-M(CO)(5) (M = Cr, W) and (CO)(5)Os-Cr(CO)(5), was addressed by resorting to state-of-the-art quantum-chemical methods. Semilocal density functional theory (DFT) approximations like Becke-Perdew or TPSS, the double-hybrid B2PLYP functional, as well as the corresponding dispersion, including TPSS-D and B2PLYP-D functionals and the wave-function-based spin-component-scaled second-order perturbative Moller-Plesset theory (SCS-MP2), were used. Energy decomposition analysis combined with the analysis of pairwise interfragment correlation energies from Pipek-Mezey localized molecular orbitals in combination with SCS-MP2 led to a clear demonstration of the significant role of dispersion (London) forces in the stabilization of the title adducts, wherein the Os-metal DA bond bears a rather low covalent character. These results plead in favor of a systematic recourse to dispersion including DFT approximations when addressing organometallic and coordination complexes.

Structure and Energetics of Azobenzene on Ag(111): Benchmarking Semiempirical Dispersion Correction Approaches

We employ normal-incidence x-ray standing wave and temperature programed desorption spectroscopy to derive the adsorption geometry and energetics of the prototypical molecular switch azobenzene at Ag(111). This allows us to assess the accuracy of semiempirical correction schemes as a computationally efficient means to overcome the deficiency of semilocal density-functional theory with respect to long-range van der Waals (vdW) interactions. The obtained agreement underscores the significant improvement provided by the account of vdW interactions, with remaining differences mainly attributed to the neglect of electronic screening at the metallic surface.

Hybrid functional study of proper and improper multiferroics

We present a detailed study of the structural, electronic, magnetic and ferroelectric properties of prototypical proper and improper multiferroic (MF) systems such as BiFeO(3) and orthorhombic HoMnO(3), respectively, within density functional theory (DFT) and using the Heyd-Scuseria-Ernzerhof hybrid functional (HSE). By comparing our results with available experimental data as well as with state-of-the-art GW calculations, we show that the HSE formalism is able to account well for the relevant properties of these compounds and it emerges as an accurate tool for predictive first-principles investigations on multiferroic systems. We show that effects beyond local and semilocal DFT approaches (as provided by HSE) are necessary for a realistic description of MFs. For the electric polarization, a decrease is found for MFs with magnetically-induced ferroelectricity, such as HoMnO(3), where the calculated polarization changes from approximately 6 muC cm(-2) using Perdew-Burke-Ernzerhof (PBE) to approximately 2 muC cm(-2) using HSE. However, for proper MFs, such as BiFeO(3), the polarization slightly increases upon introduction of exact exchange. Our findings therefore suggest that a general trend for the HSE correction to bare density functional cannot be extracted; rather, a specific investigation has to be carried out on each compound.

Azobenzene versus 3,3′,5,5′-tetra-tert-butyl-azobenzene (TBA) at Au(111): characterizing the role of spacer groups

We present large-scale density-functional theory (DFT) calculations and temperature programmed desorption measurements to characterize the structural, energetic and vibrational properties of the functionalized molecular switch 3,3',5,5'-tetra-tert-butyl-azobenzene (TBA) adsorbed at Au(111). Particular emphasis is placed on exploring the accuracy of the semi-empirical dispersion correction approach to semi-local DFT (DFT-D) in accounting for the substantial van der Waals component in the surface bonding. In line with previous findings for benzene and pure azobenzene at coinage metal surfaces, DFT-D significantly overbinds the molecule, but seems to yield an accurate adsorption geometry as far as can be judged from the experimental data. Comparing the trans adsorption geometry of TBA and azobenzene at Au(111) reveals a remarkable insensitivity of the structural and vibrational properties of the -N[double bond, length as m-dash]N- moiety. This questions the established view of the role of the bulky tert-butyl-spacer groups for the switching of TBA in terms of a mere geometric decoupling of the photochemically active diazo-bridge from the gold substrate.

Theoretical analysis of the momentum-dependent loss function of bulk Ag

We analyze the momentum-dependent loss function of bulk silver, calculated via the random-phase approximation and based on the band structure from semi-local density functional theory calculations. In the energy range of 0 – 55 eV the spectrum reveals the existence of seven well-pronounced peaks, in accordance with experiment. The two lowest of these are collective plasmon excitations, while the higher ones originate from inter-band transitions. Due to the different nature of those peaks, they display a different dependence on the momentum transfer q. While the plasmon peaks show an approximately quadratic q-dependence, the peaks caused by inter-band transitions are characterized by a much less pronounced dispersion.