1,721,084 research outputs found
Adsorption of ethanol on Si(100) from first principles calculations
No full textThe chemisorption of ethanol (C2H6O) on Si(100) is studied from first principles calculations, using a slab approach. Ethanol molecule initially interacts with the Si surface through the barrierless formation of a "dative bond"; from this physisorbed state ethanol can proceed to react with the surface via a number of possible pathways, the most relevant ones being characterized by O-H bond cleavage or O-C bond cleavage. We find that, although the product obtained by the O-C bond cleavage is thermodynamically more stable, the O-H bond cleavage process is kinetically favored. Other possible chemisorbed configurations and reaction paths have been investigated together with the effect of different surface coverages. Our results are in agreement with experimental findings; however they exhibit important differences with respect to those obtained in previous calculations based on a single-dimer cluster model
Are There Immobilized Water Molecules around Hydrophobic Groups? Aqueous Solvation of Methanol from First Principles
No full textStructural, dynamical, bonding, and electronic properties of water molecules around a soluted methanol molecule are studied from first principles. The results are compatible with experiments and qualitatively support the conclusions of recent classical molecular dynamics simulations concerning the controversial issue on the presence of "immobilized" water molecules around hydrophobic groups: the hydrophobic solute slightly reduces the mobility of many surrounding water molecules rather than immobilizing just the few ones which are closest to the methyl group. By generating maximally localized Wannier functions, a detailed description of the polarization effects in both solute and solvent molecules is obtained, which better elucidates the solvation process
Improvement in hydrogen bond description using van der Waals-corrected DFT: The case of small water clusters
No full textThe method recently developed to include van der Waals interactions in DFT (Density Functional Theory) using Wannier functions is applied to small water clusters, characterized by hydrogen-bonding interactions. In particular, water hexamer represents a critical test case since it is relevant for the condensed phases of water and has four low-energy isomers with similar binding energies. We are able to achieve results close to those obtained by high-level quantum chemistry methods and we find that inclusion of van der Waals effects is crucial for an appropriate description of the hydrogen bonds and to get accurate dissociation energy estimates. The relevance of these results for DFT simulations of liquid water is finally discussed
van der Waals Interactions in Density Functional Theory Using Wannier Functions
No full textvan der Waals interactions between atoms and molecules are ubiquitous and very important for many molecular and condensed-matter structures. These systems are often studied from first principles using the density functional theory (DFT), because this approach often represents a good compromise between accuracy and efficiency. However, the commonly used DFT functionals are not able to describe properly van der Waals effects. Most attempts to correct for this problem have a basic semiempirical character, although computationally more expensive first principles schemes have been recently developed. Of course, the key issue is finding a way to include van der Waals interactions in DFT without dramatically increasing the computational cost. We here describe in detail the recently developed scheme, based on the use of the maximally localized Wannier functions, that combines the simplicity of the semiempirical formalism with the accuracy of the first principles approaches and appears to be promising., being simple, efficient, accurate, and transferable (for instance, charge polarization effects are naturally included). The results of successful applications to small molecules, bulk Ar, and the interaction of Ar, He, and H(2) with two different Al surfaces are presented. Directions for further improvements of the method are finally suggested
Erratum: van der Waals interactions in density functional theory using Wannier functions: Improved C_{6} and C_{3} coefficients by a different approach [Phys. Rev. B 85, 073101 (2012)]
No full textErratu
Efficient calculation of Madelung constants for cubic crystals
No full textA very simple and efficient numerical method for evaluating the Madelung constants for cubic crystals, using cubes of increasing size, is proposed. The results of successful applications to the NaCl and CsCl ionic crystals, representative perovskite cubic lattices, and to a simple cubic metal are reported
van der Waals interactions in density functional theory using Wannier functions: Improved C_{6} and C_{3} coefficients by a different approach
No full textA new implementation is proposed for including van der Waals interactions
in Density Functional Theory using the Maximally-Localized Wannier
functions. With respect to the previous DFT/vdW-WF method, the
present DFT/vdW-WF2 approach, which is based on the simpler
London expression and takes into account the intrafragment overlap
of the localized Wannier functions, leads to a considerable improvement
in the evaluation of the van der Waals coefficients, as
shown by the application to a set of selected dimers.
Preliminary results on Ar on graphite and Ne on the Cu(111) metal
surface suggest that also the coefficients, characterizing
molecule-surfaces van der Waals interactions are better estimated
with the new scheme
Cohesive properties of noble metals by van der Waals-corrected density functional theory: Au, Ag, and Cu as case studies
No full textThe cohesive energy, equilibrium lattice constant, and bulk modulus of Au, Ag, and Cu noble metals are computed by different van der Waals (vdW)-corrected density functional theory (DFT) methods, including vdW-DF, vdW-DF2, vdW-DF-cx, rVV10, and PBE-D. Two specifically designed methods are also developed in order to effectively include dynamical screening effects: the DFT/vdW-WF2p method, based on the generation of maximally localized Wannier functions, and the RPAp scheme (in two variants), based on a single-oscillator model of the localized electron response. Comparison with results obtained without explicit inclusion of van der Waals effects, such as with the local density approximation (LDA), PBE, PBEsol, or the hybrid PBE0 functional, elucidates the importance of a suitable description of screened van der Waals interactions even in the case of strong metal bonding. Many-body effects are also quantitatively evaluated within the RPAp approach
Hidden by graphene – Towards effective screening of interface van der Waals interactions via monolayer coating
No full textRecent atomic force microscopy (AFM) experiments [ACS Nano 2014, 8, 12410-12417] conducted on graphene-coated SiO2 demonstrated that monolayer graphene (G) can effectively screen dispersion van der Waals (vdW) interactions deriving from the underlying substrate: despite the single-atom thickness of G, the AFM tip was almost insensitive to SiO2, and the tip-substrate attraction was essentially determined only by G. This G vdW opacity has far reaching implications, encompassing stabilization of multilayer heterostructures, micromechanical phenomena or even heterogeneous catalysis. Yet, detailed experimental control and high-end applications of this phenomenon await sound physical understanding of the underlying physical mechanism. By quantum many-body analysis and ab-initio Density Functional Theory, here we address this challenge providing theoretical rationalization of the observed G vdW opacity for weakly interacting substrates. The non-local density response and ultra slow decay of the G vdW interaction ensure compensation between standard attractive terms and many-body repulsive contributions, enabling vdW opacity over a broad range of adsorption distances. vdW opacity appears most efficient in the low frequency limit and extends beyond London dispersion including electrostatic Debye forces. By virtue of combined theoretical/experimental validation, G hence emerges as a promising ultrathin shield for modulation and switching of vdW interactions at interfaces and complex nanoscale devices
Trends in the Change in Graphene Conductivity upon Gas Adsorption: The Relevance of Orbital Distortion
No full textThe experimental ability to alter graphene (G) conductivity by adsorption of a single gas molecule is promoting the development of ultra-high-sensitivity gas detectors and could ultimately provide a novel playground for future nanoelectronics devices. At present, the underpinning effect is broadly attributed to a variation of G carrier concentration, caused by an adsorption-induced Fermi-level shift. By means of first-principle Kubo-Greenwood calculations, here we demonstrate that adsorbate-induced orbital distortion could also lead to small but finite G conductivity changes, even in the absence of Fermi-level shifts. This mechanism enables a sound physical interpretation of the observed variable sensitivity of G devices to different chemical moieties, and it can be strongly enhanced by using a suitable Ni substrate, thereby opening new pathways for the optimal design of operational nanoscale detectors
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