Optimized local modes for lattice dynamical applications

We present a new scheme for the construction of highly localized lattice Wannier functions. The approach is based on a heuristic criterion for localization and takes the symmetry constraints into account from the start. We compare the local modes thus obtained with those generated by other schemes and find that they also provide a better description of the relevant vibrational subspace.Comment: 6 pages, ReVTeX, plus four postscript files for figure

Maximally-localized Wannier functions for entangled energy bands

We present a method for obtaining well-localized Wannier-like functions (WFs) for energy bands that are attached to or mixed with other bands. The present scheme removes the limitation of the usual maximally-localized WFs method (N. Marzari and D. Vanderbilt, Phys. Rev. B 56, 12847 (1997)) that the bands of interest should form an isolated group, separated by gaps from higher and lower bands everywhere in the Brillouin zone. An energy window encompassing N bands of interest is specified by the user, and the algorithm then proceeds to disentangle these from the remaining bands inside the window by filtering out an optimally connected N-dimensional subspace. This is achieved by minimizing a functional that measures the subspace dispersion across the Brillouin zone. The maximally-localized WFs for the optimal subspace are then obtained via the algorithm of Marzari and Vanderbilt. The method, which functions as a postprocessing step using the output of conventional electronic-structure codes, is applied to the s and d bands of copper, and to the valence and low-lying conduction bands of silicon. For the low-lying nearly-free-electron bands of copper we find WFs which are centered at the tetrahedral interstitial sites, suggesting an alternative tight-binding parametrization.Comment: 13 pages, with 9 postscript figures embedded. Uses REVTEX and epsf macro

Systematic Study of Electron Localization in an Amorphous Semiconductor

We investigate the electronic structure of gap and band tail states in amorphous silicon. Starting with two 216-atom models of amorphous silicon with defect concentration close to the experiments, we systematically study the dependence of electron localization on basis set, density functional and spin polarization using the first principles density functional code Siesta. We briefly compare three different schemes for characterizing localization: information entropy, inverse participation ratio and spatial variance. Our results show that to accurately describe defect structures within self consistent density functional theory, a rich basis set is necessary. Our study revealed that the localization of the wave function associated with the defect states decreases with larger basis sets and there is some enhancement of localization from GGA relative to LDA. Spin localization results obtained via LSDA calculations, are in reasonable agreement with experiment and with previous LSDA calculations on a-Si:H models.Comment: 16 pages, 11 Postscript figures, To appear in Phys. Rev.

Intra-molecular coupling as a mechanism for a liquid-liquid phase transition

We study a model for water with a tunable intra-molecular interaction $J_\sigma$, using mean field theory and off-lattice Monte Carlo simulations. For all $J_\sigma\geq 0$, the model displays a temperature of maximum density.For a finite intra-molecular interaction $J_\sigma > 0$,our calculations support the presence of a liquid-liquid phase transition with a possible liquid-liquid critical point for water, likely pre-empted by inevitable freezing. For J=0 the liquid-liquid critical point disappears at T=0.Comment: 8 pages, 4 figure

Magazynowanie wodoru na arkuszu grafenu: analiza ścieżek fizykosorpcji, dyfuzji i chemisorpcji metodą obliczeń ab initio

Hydrogen is frequently touted as the "fuel of the future" because of its huge potential as clean energy source, although the large-scale adoption of this technology has yet to be realized. One of the remaining barriers to the utilization of hydrogen energy is an efficient and inexpensive means of hydrogen storage. In this work we investigate the nature of this process by first principle calculation. In particular, we study the way in which the H2 molecule can interact with graphene sheet through physisorption and chemisorption mechanism. The first mechanism involves the condensation of the hydrogen molecule on the graphene as a result of weak van der Waals forces, while the chemisorption mechanism involves the preliminary dissociation of the H2 molecule and the subsequent reaction of hydrogen atoms with the unsatured C-C bonds to form C-H bonds. To study carefully the possible physisorbed configurations on the graphene sheet, we take in to account van der Waals (vdW) interactions in DFT using the new method (DFT/vdW-WF) recently developed in our group and based on the concept of maximally localized Wannier functions. There are three possible way in which the H2 molecule can adapt to the structure of graphene: the hollow, the bridge and the top site called H, B and T configurations, respectively. We find the hollow site to be most stable physisorbed state with a binding energy of -50 meV. This value, in agreement with experimental results, is also compared with other vdW-correction methods as described in the following paper. Diffusion of the physisorbed configurations on the graphene sheet and activated reaction pathways in which the molecule starts from a physisorbed configuration to end up in a chemisorbed configurations have also been studied.Ze względu na możliwość magazynowania wodoru na arkuszu grafenu, badamy nature tego procesu metoda obliczeń ab initio. Cząsteczka H2 może oddziaływać z grafenem na dwa sposoby: fizykosorpcje i chemisorpcje. Mechanizm fizykosorpcji polega na kondensacji cząsteczki wodoru na grafenie na skutek słabych oddziaływań van der Waalsa, podczas gdy mechanizm chemisorpcji polega na wstępnej dysocjacji cząsteczki H2 i następnie reakcji atomów wodoru z niewysyconymi wiązaniami C-C co umożliwia tworzenie wiązań C-H. W celu zobrazowania mechanizmu fizykosorpcji cząsteczek H2 na arkuszu grafenu, analizujemy oddziaływania van der Waalsa (vdW) stosujac teorie funkcjonału gestości (DFT) za pomocą nowej metody (DFT/vdW-WF) niedawno opracowanej w naszej grupie na podstawie koncepcji maksymalnie zlokalizowanych funkcji Wanniera. Analizujemy możliwość absorpcji w różnych miejscach siatki grafenu i orientacje cząsteczki H2 w stosunku do płaszczyzny siatki grafenu. Najbardziej stabilnym miejscem fizykosorbcji jest wnętrze pierścienia grafenu z energia wiązania -50 meV. Wartość ta, zgodna z wynikami doświadczeń, jest także porównywana wartości uzyskanych innymi metodami korekcji vdW. Typowe bariery energetyczne charakteryzujące ścieżkę dyfuzji, są rzędu ∼10 meV. Sytuacja jest inna, gdy cząsteczki wodoru pokonują barierę energetyczna i chemisorbują jako jeden atom wodoru na powierzchni grafenu. Bariery energetyczne w tym przypadku wynoszą około 2-3 eV, w zależności od wybranej ścieżki reakcji

On the polarity of buckminsterfullerene with a water molecule inside

Since the recent achievement of Kurotobi and Murata to capture a water molecule in a C60 fullerene (Science2011, 333, 613), there has been a debate about the properties of this H2O@C60 complex. In particular, the polarity of the complex, which is thought to be underlying the easy separation of H2O@C60 from the empty fullerene by HPLC, was calculated and found to be almost equal to that of an isolated water molecule. Here we present our detailed analysis of the charge distribution of the water-encapsulated C60 complex, which shows that the polarity of the complex is, with 0.5 ± 0.1 D, indeed substantial, but significantly smaller than that of H2O. This may have important implications for the aim to design water-soluble and biocompatible fullerenes

Chemisorption of Barrelene (C8H8) on Si(100) surface from first principles.

The chemisorption of C8H8 bicyclo[2.2.2]-2,5,7-octatriene (barrelene) on the Si(100) surface is studied from first principles calculations. We find that, in the most stable configuration, barrelene is bonded to Si(100) through four Si 12C bonds, with the C 12C bonds which are orthogonal to the underlying Si dimers. The chemisorption reaction responsible for this structure is driven by the biradical nature of the Si 12Si dimer bond. Two others, slightly less stable configurations, exist which are also characterized by four Si 12C bonds but have a different orientation or location with respect to the Si(100) surface. The properties of these and other, less stable configurations have been investigated. For the most stable structures, the effect of different surface coverages has been also studied, showing a tendency to easily form complete monolayers of barrelene on the Si surface. On the basis of energetic and kinetic considerations, we expect that chemisorption of barrelene monolayers on the Si(100) surface will be characterized however by a certain amount of disorder. Finally, several possible reaction pathways, leading from one stable structure to another of lower energy or from a molecule in the gas phase to a chemisorbed configuration, have been investigated in detail and estimates of the relative energy barriers are given

Hot electrons and the approach to metallic behavior in Kx(KCl)1-x

The approach to the metallic phase of molten Kx(KCl)1-x mixtures is studied using ab initio molecular dynamics based on finite-temperature density functional theory. The finite electronic temperature is found to result in new and unexpected effects. In particular, we observe a thermally induced lowering of the predicted DC conductivity, which greatly improves the agreement with the experiment, and a widening of the HOMO-LUMO energy gap. We expect that these are genuinely new physical effects which could be observed also in other systems