Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session A31: Focus Session: van der Waals Bonding in Advanced Materials: Fundamentals and Simple Systems |
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Sponsoring Units: DMP Chair: Giulia Galli, University of California, Davis Room: C145 |
Monday, March 21, 2011 8:00AM - 8:36AM |
A31.00001: Cohesive Properties of Graphitics and the Random Phase Approximation Invited Speaker: The van der Waals-dominated cohesive energetics of graphitic systems is important in the assembly of many graphene-based nanostructures of current technological interest. In 2006 an unusual power law E~=~-~cD$^{-3}$ was predicted [1] for the van der Waals (vdW, dispersion) interaction energy between parallel graphene sheets at large separations D. By contrast, a conventional sum of pairwise R$^{-6}$ contributions yields E~=~.-~kD$^{-4}$. The unexpected D$^{-3}$ result came from the electronic correlation energy within the Random Phase Approximation (RPA), which can be solved analytically in the distant regime D~.$\to $~.$\infty $. In keeping with other unusual properties of graphene, in this distant non-overlapping regime the relevant response function of a graphene sheet is dominated by the gapless electronic transitions near the Dirac points in the Brillouin Zone where the .$\pi _{z}$ and .$\pi _{z}$ Bloch bands touch. The D$^{.-3}$ result corresponds to a severe failure of pairwise additivity of the vdW interaction between local spatial regions of the sheets, and so could have implications for the most-used nanoscale energy functionals (e.g. [2,3]); these embody pairwise additivity at various levels. It has remained unclear what this result might imply for the interaction between graphene sheets at smaller spacings near to the equilibrium separation, where the response is sampled at shorter wavelengths so that analytic results cannot be obtained. Very recently, numerically well-converged exact-exchange and RPA correlation energies have been obtained for stretched graphite at a wide range of inter-layer spacings down to the equilibrium distance. These results and their implications will be discussed. [Preview Abstract] |
Monday, March 21, 2011 8:36AM - 8:48AM |
A31.00002: Self-consistent calculations of correlation energies within the random phase approximation Stefano de Gironcoli, Ngoc Linh Nguyen, Viet Huy Nguyen, Giulia Galli Calculations of correlation energies within the the formally exact Adiabatic Connection Fluctuation-Dissipation (ACFD) formalism, within the Random Phase Approximation (RPA) for the exchange-correlation kernel, have been recently carried out for a number of isolated and condensed systems. The efficiency of such calculations has been greatly improved by exploiting iterative algorithms to diagonalize RPA dielectric matrices [1]. Unfortunately, for several systems, it has been found that RPA correlation energies may significantly depend about the choice of input single particle wavefunctions [2]. In this work, we derive an expression of the RPA self-consistent potential based on Density Functional Perturbation theory and we present self-consistent RPA calculations for weakly bound molecular dimers, including the controversial case of the Beryllium dimer. \\[4pt] [1] H.-V. Nguyen and S. de Gironcoli, Phys. Rev. B 79, 205114 (2009); H. F. Wilson, F. Gygi, and G. Galli, Phys. Rev. B 78, 113303 (2008). \\[0pt] [2] Huy-Viet Nguyen and G.Galli, J. Chem.Phys. 132, 044109 (2010). [Preview Abstract] |
Monday, March 21, 2011 8:48AM - 9:00AM |
A31.00003: Beyond RPA correlation energies: Evaluation of model exchange-correlation kernels Deyu Lu, Giulia Galli The description of van der Waals dispersion interactions using the so called EXX/RPA method has recently attracted a widespread interest. Overall, equilibrium distances and cohesive energies of weakly bound molecular systems exhibit a significant improvement over the the results of semi-local Density Functional Theory calculations [1,2], due to the proper inclusion of long-range correlation effects. However, cohesive energies still result to be underestimated with respect to experiments in several cases. This is mainly due to the neglect of the exchange-correlation kernel in evaluating response functions entering the correlation energy expression. In this work, we study the effect of several model exchange-correlation kernels and evaluate their performance for molecular systems. \\[4pt] [1] D. Lu, Y. Li, D. Rocca and G. Galli, Phys. Rev. Lett. 102, 206411 (2009)\\[0pt] [2] Y. Li, D. Lu, H-V Nguyen and G. Galli, J. Phys. Chem. A, 114, 1944-1952 (2010) and D. Lu, H-V Nguyen, and G. Galli, J. Chem. Phys. 133, 154110 (2010) [Preview Abstract] |
Monday, March 21, 2011 9:00AM - 9:12AM |
A31.00004: Van der Waals interactions in complex materials: Beyond the pairwise approximation Alexandre Tkatchenko, Robert A. DiStasio, Jr., Roberto Car, Matthias Scheffler Despite the well-known fact that van der Waals (vdW) interactions are many-body in nature and the polarizability is a non-local function, popular vdW-DF [1] and DFT+vdW [2] methods are based on (semi)-local approximations for the polarizability and only model the pairwise part of vdW interactions. Here we show how to go beyond the pairwise (semi)-local approximation to vdW interactions by coupling the recently developed TS scheme [2] with the Fluctuating-Coupled-Dipole Model (CFDM) [3]. The TS scheme provides parameter-free input atomic polarizability distributions and the CFDM allows to model both polarizing and depolarizing local fields, and captures the many-body nature of vdW interactions. Results are presented for small and medium-size molecules, as well as solids. We find that the many-body screening plays a major role in modifying the polarizability of large systems. Our results for vdW coefficients in semiconductor clusters and solids are in excellent agreement with TDDFT calculations. [1] M. Dion \textit{et al.}, Phys. Rev. Lett., \textbf{92}, 246401 (2004); [2] A. Tkatchenko and M. Scheffler, Phys. Rev. Lett., \textbf{102}, 073005 (2009); [3] M. W. Cole \textit{et al.}, Mol. Simul. \textbf{35}, 849 (2009). [Preview Abstract] |
Monday, March 21, 2011 9:12AM - 9:24AM |
A31.00005: Van der Waals materials: what is the origin of the disagreement between ab initio calculations and experiments? Loredana Valenzano, Warren Perger, Jackson Criswell, William Slough The robust prediction of accurate physical properties for molecular solids from first-principles calculations continues to present a significant challenge across a wide variety of scientific disciplines. Comparison between computed and experimental values for physical properties derived from differences between states is often promising (such as bulk modulus), however the result is disappointing for absolute values (such as density). Accurate ab initio calculations describe physics occurring at zero Kelvin; but, properties evaluated experimentally are mostly reported at room temperature. Therefore it should hardly be surprising that ab initio results differ dramatically from experimentally measured values. We show how the results from a calculation at zero Kelvin may be compared to experimental values at higher temperatures, helping to foster a stronger linkage between computational and experimental work on systems such as energetic and pharmaceutical materials and metal-organic frameworks in interaction with guest molecules. Among others, investigated behavior comprises mechanical (elastic constants) and vibrational (infrared and Raman spectra) properties. The computational approach adopted, takes into account van der Waals long-range dispersion interaction through an empirical ``a posteriori'' approach, appropriately fitted to investigate solid materials. [Preview Abstract] |
Monday, March 21, 2011 9:24AM - 9:36AM |
A31.00006: Van der Waals interactions in semiconductor solids Guo-Xu Zhang, Alexandre Tkatchenko, Joachim Paier, Heiko Appel, Matthias Scheffler The binding in semiconductor solids arises mainly from the covalent hybridization of atomic orbitals. Hence, it is typically assumed that van der Waals (vdW) interactions play a minor role for their cohesion. In order to probe this conventional wisdom we develop a method to calculate accurate long-range vdW coefficients for ions and atoms in crystals. We first assess the validity of the Clausius-Mossotti relation between the polarizability and dielectric function for bulk semiconductors by comparing periodic TDDFT calculations to direct extrapolation of the frequency-dependent TDDFT polarizability for finite clusters. We find a good agreement between these two approaches for computing vdW $C_6(V)$ coefficients for a broad variation in the unit cell volume $V$ for diamond, Si, and Ge crystals. When using TDDFT@HSE with the Nanoquanta kernel, the volume-dependent dielectric constant of Si and Ge is in excellent agreement with experimental data. The crystal-field screening reduces the vdW coefficients by a factor of two compared to corresponding free-atom and effective hybridized $C_6[n(r)]$ values [1]. The use of accurate $C_6(V)$ coefficients in the PBE+vdW method [1] improves cohesive properties of Si and Ge in comparison to experimental data. [1] A. Tkatchenko and M. Scheffler, Phys. Rev. Lett. \textbf{102}, 073005 (2009). [Preview Abstract] |
Monday, March 21, 2011 9:36AM - 9:48AM |
A31.00007: Van der Waals density functionals applied to solids Jiri Klimes, David Bowler, Angelos Michaelides Dispersion interactions are ubiquitous in nature and contribute to the binding in biomolecules or to the adsorption of molecules on surfaces. However, due to their non-local nature they are difficult to describe accurately with electronic structure methods. It is now well established that standard density functional theory functionals give misleading results for systems where dispersion is important. The van der Waals density functional (vdW-DF) of Dion et al. [Dion et al., Phys. Rev. Lett. 92, 246401 (2004)] is one of several promising approaches for accounting for dispersion. We have shown that with an improved treatment of the exchange part it can offer much better than chemical accuracy for a range of weakly interacting molecular systems [Klime\v{s} et al., J. Phys.: Cond. Matt. 22, 022201 (2010)]. Here we extend this work beyond the weakly bonded regime and report results for lattice constants of solids (metals, semiconductors, ionic solids) and geometries and atomization energies of molecules. This extensive and rigorous test of vdW-DF shows how to a large extent such properties are dependent on its underlying exchange functional. We use this new insight to discuss prospects for further improvement of the method. [Preview Abstract] |
Monday, March 21, 2011 9:48AM - 10:00AM |
A31.00008: Application of van der Waals Density Functionals to Extended Systems Kyuho Lee, David C. Langreth Recently we proposed\footnote{K. Lee, \'{E}. D. Murray, L. Kong, B. I. Lundqvist, and D. C. Langreth, Phys.\ Rev.\ B\ \textbf{82}, 081101(R) (2010).} a second version of a van der Waals density functional\footnote{M. Dion, H. Rydberg, E. Schr\"{o}der, D. C. Langreth, and B. I. Lundqvist, Phys.\ Rev.\ Lett. \textbf{92}, 246401 (2004); T. Thonhauser, V. R. Cooper, S. Li, A. Puzder, P. Hyldgaard, and D. C. Langreth, Phys.\ Rev.\ B \textbf{76}, 125112 (2007).} and showed its accuracy for small molecular duplexes as well as a few extended systems. As further applications to extended systems, we present results for molecular adsorptions on surfaces, molecular crystals, and organic ferroelectrics. A comparison with experiments is also given for different functionals. [Preview Abstract] |
Monday, March 21, 2011 10:00AM - 10:12AM |
A31.00009: An Efficient Real-Space Implementation of the van der Waals Energy and Analytical Forces in Plane-Wave Ab Initio Molecular Dynamics Robert DiStasio, Zhaofeng Li, Ulrich Aschauer, Xifan Wu, Roberto Car In this work, we present an efficient algorithmic implementation of the energy and analytical forces of the recent density functional based van der Waals (vdW) correction proposed by Tkatchenko and Scheffler (PRL 102, 073005 (2009)) within the framework of plane-wave based ab initio molecular dynamics. The algorithm presented herein is a highly parallelizable, order (N) formulation that allows for accurate treatment of large molecular systems with a computational cost that is negligible with respect to the underlying evaluation of the exchange-correlation functional. The computational resources and performance of our algorithm, which utilizes a real-space implementation of the molecular pro-density, will be analyzed and compared against a reciprocal-space formulation of the Hirshfeld volume based on a spherical wave expansion of the underlying plane-wave basis. The effects of this vdW correction are demonstrated within the context of the oxygen-oxygen and oxygen-hydrogen radial distribution functions obtained via highly accurate PBE0-based liquid water simulations. [Preview Abstract] |
Monday, March 21, 2011 10:12AM - 10:24AM |
A31.00010: Comparison of methods for inclusion of van der Waals interactions: the case of physisorption of nucleobases on graphene Duy Le, Abdelkader Kara, Talat S. Rahman The physisorption of the nucleobases adenine (A), cytosine (C), guanine (G), thymine (T), and uracil (U) on graphene is studied using many flavors of density functional theory (DFT): the generalized gradient approximation (GGA) with the inclusion of van der Waals (vdW) interaction based on the TS approach [A. Tkatchenko and M. Scheffler, \textit{PRL} \textbf{102}, 073005 (2009)], our simplified version of this approach, the vdW density functional (vdW-DF) [M. Dion \textit{et al.}, \textit{PRL} \textbf{92}, 246401 (2004)], and the vdW-DF2 [K. Lee \textit{et al., PRB} \textbf{82}, 081101 (2010)] methods. The binding energies of nucleobases on graphene lie in the range of 496{\-}962 meV and are found to be in the following order G$>$A$>$T$>$C$>$U within vdW-DF, vdW-DF2 and our method and G$>$A$>$T$\sim $C$>$U in the TS approach. The binding separations lie between 3.29{\-}3.53 {\AA} and are found to be about 0.1{\-} 0.2 {\AA} shorter in DFT-D, as compared to vdW-DF approaches. We comment on the efficiency of combining the DFT-D and vdW-DF methods to study vdW interactions in molecular adsorption. [Preview Abstract] |
Monday, March 21, 2011 10:24AM - 10:36AM |
A31.00011: Spinodal de-wetting of thin films in the presence of oscillatory Casimir forces Leonardo Golubovic, Adi Constantinescu Long range de-wetting forces, e.g., van der Waals interactions, may drive the formation of large clusters in thin films of polymeric materials, and in liquid and solid metals films. We elucidate film de-wetting in the presence of spatially oscillatory Casimir forces, such as the fermionic Casimir forces mediated by conducting electrons in metal films. What happens with interfaces of a liquid metal film in the presence of the spatially oscillating forces? Is the film going to exhibit spinodal de-wetting instability yielding the formation of clusters ? We find that, at low temperatures, the film interface pins to the minima of the oscillatory Casimir force potential. This suppresses the spinodal de-wetting. However, at elevated temperatures, the interface efficiently hops between the minima of the oscillatory potential, and the film quickly de-wets and structures into clusters. The spinodal de-wetting is governed by an effective non-oscillatory de-wetting potential that entropically emerges from a coarse-graining of the oscillatory Casimir force potential. [Preview Abstract] |
Monday, March 21, 2011 10:36AM - 10:48AM |
A31.00012: Dynamic Precision Measurement of the Casimir Force Using Gold Surface Chia-Cheng Chang, Umar Mohideen High precision dynamic Casimir force measurements between a gold coated sphere and plate are performed in UHV with short coherence length light source interferometer will be presented. A comparison to the theory using generalized Plasma and Drude model at room temperature will be discussed. [Preview Abstract] |
Monday, March 21, 2011 10:48AM - 11:00AM |
A31.00013: Inelastic Helium Atom Scattering from the Commensurate Monolayer Solid H$_2$/NaCl(001) L.W. Bruch, F.Y. Hansen A calculation of inelastic low energy helium atomic scattering by a monolayer with one-phonon creation\footnote{F. Y. Hansen and L. W. Bruch, J. Chem. Phys. {\bf 127}, 204708 (2007)} is reported for the dilated quantum monolayer solid H$_2$/NaCl(001). The shear horizontal phonon mode again is accessed for small misalignment of the scattering plane relative to the monolayer axes. Qualitative agreement for the systematic trends in the inelastic scattering experiments\footnote{F. Traeger and J. P. Toennies, J. Phys. Chem. B {\bf 108}, 14710 (2004)} is achieved. Two monolayer phonon branches are identified. The role of the Debye-Waller attenuation in diffraction intensities is discussed. [Preview Abstract] |
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