Hydrogen bonding characterization in water and small molecules

The prototypical Hydrogen bond in water dimer and Hydrogen bonds in the protonated water dimer, in other small molecules, in water cyclic clusters, and in ice, covering a wide range of bond strengths, are theoretically investigated by first-principles calculations based on the Density Functional Theory, considering a standard Generalized Gradient Approximation functional but also, for the water dimer, hybrid and van-der-Waals corrected functionals. We compute structural, energetic, and electrostatic (induced molecular dipole moments) properties. In particular, Hydrogen bonds are characterized in terms of differential electron densities distributions and profiles, and of the shifts of the centres of Maximally localized Wannier Functions. The information from the latter quantities can be conveyed into a single geometric bonding parameter that appears to be correlated to the Mayer bond order parameter and can be taken as an estimate of the covalent contribution to the Hydrogen bond. By considering the cyclic water hexamer and the hexagonal phase of ice we also elucidate the importance of cooperative/anticooperative effects in Hydrogen-bonding formation.Comment: 11 figure

Van der Waals Interactions in DFT using Wannier Functions without empirical parameters

A new implementation is proposed for including van der Waals (vdW) interactions in Density Functional Theory (DFT) using the Maximally-Localized Wannier functions (MLWFs), which is free from empirical parameters. With respect to the previous DFT/vdW-WF2 method, in the present DFT/vdW-WF2-x approach, the empirical, short-range, damping function is replaced by an estimate of the Pauli exchange repulsion, also obtained by the MLWFs properties. Applications to systems contained in the popular S22 molecular database and to the case of an Ar atom interacting with graphite, and comparison with reference data, indicate that the new method, besides being more physically founded, also leads to a systematic improvement in the description of vdW-bonded systems.Comment: 3 figures. arXiv admin note: text overlap with arXiv:1111.6737, arXiv:1305.7035, arXiv:1603.0866

Cohesive properties of noble metals by van der Waals-corrected Density Functional Theory

The cohesive energy, equilibrium lattice constant, and bulk modulus of noble metals are computed by different van der Waals-corrected Density Functional Theory 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 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.Comment: 3 figure

Hidden by graphene -- towards effective screening of interface van der Waals interactions via monolayer coating

Recent atomic force microscopy (AFM) experiments~[ACS Nano {\bf 2014}, 8, 12410-12417] conducted on graphene-coated SiO$_2$ 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 SiO$_2$, and the tip-substrate attraction was essentially determined only by G. This G vdW {\it 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 {\it 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 {\it opacity} over a broad range of adsorption distances. vdW {\it 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 {\it shield} for modulation and switching of vdW interactions at interfaces and complex nanoscale devices

Transport properties in liquids from first principles: the case of liquid water and liquid Argon

Shear and bulk viscosity of liquid water and Argon are evaluated from first principles in the Density Functional Theory (DFT) framework, by performing Molecular Dynamics simulations in the NVE ensemble and using the Kubo-Greenwood equilibrium approach. Standard DFT functional is corrected in such a way to allow for a reasonable description of van der Waals (vdW) effects. For liquid Argon the thermal conductivity has been also calculated. Concerning liquid water, to our knowledge this is the first estimate of the bulk viscosity and of the shear-viscosity/bulk-viscosity ratio from first principles. By analyzing our results we can conclude that our first-principles simulations, performed at a nominal average temperature of 366 K to guarantee that the systems is liquid-like, actually describe the basic dynamical properties of liquid water at about 330 K. In comparison with liquid water, the normal, monatomic liquid Ar is characterized by a much smaller bulk-viscosity/shear-viscosity ratio (close to unity) and this feature is well reproduced by our first-principles approach which predicts a value of the ratio in better agreement with experimental reference data than that obtained using the empirical Lennard-Jones potential. The computed thermal conductivity of liquid Argon is also in good agreement with the experimental value.Comment: 14 figure

Van der Waals Interactions in Density Functional Theory by combining the Quantum Harmonic Oscillator-model with Localized Wannier Functions

We present a new scheme to include the van der Waals (vdW) interactions in approximated Density Functional Theory (DFT) by combining the Quantum Harmonic Oscillator model with the Maximally Localized Wannier Function technique. With respect to the recently developed DFT/vdW-WF2 method, also based on Wannier Functions, the new approach is more general, being no longer restricted to the case of well separated interacting fragments. Moreover, it includes higher than pairwise energy contributions, coming from the dipole--dipole coupling among quantum oscillators. The method is successfully applied to the popular S22 molecular database, and also to extended systems, namely graphite and H$_2$ adsorbed on the Cu(111) metal surface (in this case metal screening effects are taken into account). The results are also compared with those obtained by other vdW-corrected DFT schemes

Dynamical spin properties of confined Fermi and Bose systems in presence of spin-orbit coupling

Due to the recent experimental progress, tunable spin-orbit (SO) interactions represent ideal candidates for the control of polarization and dynamical spin properties in both quantum wells and cold atomic systems. A detailed understanding of spin properties in SO coupled systems is thus a compelling prerequisite for possible novel applications or improvements in the context of spintronics and quantum computers. Here we analyze the case of equal Rashba and Dresselhaus couplings in both homogeneous and laterally confined two-dimensional systems. Starting from the single-particle picture and subsequently introducing two-body interactions we observe that periodic spin fluctuations can be induced and maintained in the system. Through an analytical derivation we show that the two-body interaction does not involve decoherence effects in the bosonic dimer, and, in the repulsive homogeneous Fermi gas it may be even exploited in combination with the SO coupling to induce and tune standing currents. By further studying the effects of a harmonic lateral confinement --a particularly interesting case for Bose condensates-- we evidence the possible appearance of non-trivial {\it spin textures}, whereas the further application of a small Zeeman-type interaction can be exploited to fine-tune the system polarizability.Comment: 13 pages, 3 figure

Acetylene on Si(100) from first principles: adsorption geometries, equilibrium coverages and thermal decomposition

Adsorption of acetylene on Si(100) is studied from first principles. We find that, among a number of possible adsorption configurations, the lowest-energy structure is a ``bridge'' configuration, where the C$_2$H$_2$ molecule is bonded to two Si atoms. Instead, ``pedestal'' configurations, recently proposed as the lowest-energy structures, are found to be much higher in energy and, therefore, can represent only metastable adsorption sites. We have calculated the surface formation energies for two different saturation coverages, namely 0.5 and 1 monolayer, both observed in experiments. We find that although, in general, the full monolayer coverage is favored, a narrow range of temperatures exists in which the 0.5 monolayer coverage is the most stable one, where the acetylene molecules are adsorbed in a $2\times 2$ structure. This result disagrees with the conclusions of a recent study and represents a possible explanation of apparently controversial experimental findings. The crucial role played by the use of a gradient-corrected density functional is discussed. Finally, we study thermal decomposition of acetylene adsorbed on Si(100) by means of finite-temperature Molecular Dynamics, and we observe an unexpected behavior of dehydrogenated acetylene molecules.Comment: 8 pages, 3 figures (submitted to J. Chem. Phy