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Sede di Cagliari, Italy

Resta R.,University of Trieste | Resta R.,CNR Institute of Materials
Physical Review Letters | Year: 2010

Flexoelectricity is the linear response of polarization to a strain gradient. Here we address the simplest class of dielectrics, namely, elemental cubic crystals, and we prove that therein there is no extrinsic (i.e., surface) contribution to flexoelectricity in the thermodynamic limit. The flexoelectric tensor is expressed as a bulk response of the solid, manifestly independent of surface configurations. Furthermore, we prove that the flexoelectric responses induced by a long-wavelength phonon and by a uniform strain gradient are identical. © 2010 The American Physical Society. Source

Resta R.,University of Trieste | Resta R.,CNR Institute of Materials
Physical Review Letters | Year: 2011

In a linear magnetoelectric the lattice is coupled to electric and magnetic fields; both affect the longitudinal-transverse splitting of zone-center optical phonons on equal footing. A response matrix relates the macroscopic fields (D, B) to (E, H) at infrared frequencies. It is shown that the response matrices at frequencies 0 and fulfill a generalized Lyddane-Sachs-Teller relationship. The right-hand side member of such relationship is expressed in terms of weighted averages over the longitudinal and transverse excitations of the medium, and assumes a simple form for a harmonic crystal. © 2011 American Physical Society. Source

Vargiu A.V.,CNR Institute of Materials | Magistrato A.,International School for Advanced Studies
Inorganic Chemistry | Year: 2012

Molecules that selectively recognize DNA mismatches (MMs) play a key role as nucleic acids probes and as chemotherapeutic agents. Metalloinsertors bind to the minor groove (mG) of double strand (ds) DNA, expelling the mismatched base pairs and acting as their p-stacking replacement. In contrast, metallo-intercalators bind to the major groove (MG) of ds DNA and p-stack to adjacent base pairs. In this study we focused on structural and energetic properties of Δ-[Rh(bpy) 2(chrysi)] 3+ (1), Δ-[Ru(bpy) 2(ddpz)] 2+ (2), and Δ-[Ru(bpy) 2(eilatin)] 2+ (3) as prototypical examples of metallo-insertors and intercalators. For all molecules we characterized both insertion and intercalation into a DNA dodecamer via force field based molecular dynamics (MD) and hybrid quantum-classical (QM/MM) MD simulations. A structural analysis of the 1-3/DNA noncovalent adducts reveals that the insertion provokes an untwist of the DNA, an opening of the mG and of the phosphate backbone in proximity of the mismatch, while the intercalation induces smaller changes of these structural parameters. This behavior appears to be correlated with the size of the inserting/intercalating ligand in proximity of the metal coordination site. Moreover, our simulations show that the different selectivity of 1 toward distinct MM types may be correlated with the thermodynamic stability of the MMs in the free DNA and with that of the corresponding insertion adduct. Understanding the factors which tune a specific insertion is of crucial importance for designing specific luminescent probes that selectively recognize MMs, as well as for developing more effective anticancer drugs active in MM repair of deficient cells lines. © 2012 American Chemical Society. Source

Piccinin S.,CNR Institute of Materials | Stamatakis M.,University College London
ACS Catalysis | Year: 2014

CO oxidation on O-precovered Pd(111) surfaces exhibits remarkably different reactivities at different temperatures, which correlate with structural changes in the atomic O overlayer. Stoichiometric titration experiments by Nakai et al. (J. Chem. Phys. 2006, 124, 224712) show that although the p(2 × 2) ordered phase is inert, the (√3 × √3) and p(2 × 1) phases that form at 320 and 190 K, respectively, have different apparent activation energies and reaction orders with respect to O coverage. In this work, we perform first-principles-based kinetic Monte Carlo (kMC) simulations to understand the behavior of this catalytic system and shed light on the origin of the changes in reactivity. Accounting explicitly for lateral interactions among adsorbates and for their impact on the activation energies of the elementary processes, our simulations reproduce quantitatively the main features of the experimental measurements, and we show that the relative rates of CO adsorption and surface reaction are different as the temperature changes. We find that ordering of the adsorbate layer strongly depends on the strength of the lateral interactions but does not have a significant role on the catalytic properties of the system. © 2014 American Chemical Society. Source

Silvestrelli P.L.,University of Padua | Ambrosetti A.,CNR Institute of Materials
Physical Review B - Condensed Matter and Materials Physics | Year: 2013

The DFT/vdW-WF2 method, recently developed to include the van der Waals (vdW) interactions in density functional theory (DFT) using the maximally localized Wannier functions, is improved by taking into account screening effects and applied to the study of adsorption of rare gases and small molecules, H2, CH4, and H2O on the Cu(111) metal surface, and of H2 on Al(111), and Xe on Pb(111), which are all cases where screening effects are expected to be important. Screening is included in DFT/vdW-WF2 by following different recipes, also considering the single-layer approximation adopted to mimic a screened metal substrate. Comparison of the computed equilibrium binding energies and distances, and the C3 coefficients characterizing the adparticle-surface van der Waals interactions, with available experimental and theoretical reference data show that the improvement with respect to the original unscreened approach is remarkable. The results are also compared with those obtained by other vdW-corrected DFT schemes. © 2013 American Physical Society. Source

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