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Raji A.T.,Abdus Salam International Center For Theoretical Physics | Raji A.T.,University of Cape Town | Mazzarello R.,RWTH Aachen | Scandolo S.,Abdus Salam International Center For Theoretical Physics | And 4 more authors.
Solid State Communications | Year: 2011

The electronic structures of hexagonal closed-packed (h.c.p) titanium containing a vacancy and krypton impurity atoms at various insertion sites are calculated by first-principles methods in the framework of the density-functional theory (DFT). The density of states (DOS) for titanium containing a vacancy defect shows resonance-like features. Also, the bulk electron density decreases from ∼0.15 3 to ∼0.05 3 at the vacancy centre. Electronic structure calculations have been performed to investigate what underlies the krypton site preference in titanium. The DOS of the nearest-neighbour (NN) titanium atoms to the octahedral krypton appears to be less distorted (relative to pure titanium) when compared to the NN titanium atoms to the tetrahedral krypton. The electronic density deformation maps show that polarization of the titanium atoms is stronger when the krypton atom is located at the tetrahedral site. Since krypton is a closed-shell atom, thus precluding any bonding with the titanium atoms, we may conclude that the polarization of the electrons in the vicinity of the inserted krypton atoms and the distortion of the DOS of the NN titanium atoms to the krypton serve to indicate which defect site is preferred when a krypton atom is inserted into titanium. Based on these considerations, we conclude that the substitutional site is the most favourable one, and the octahedral is the preferred interstitial site, in agreement with recent DFT calculations of the energetics of krypton impurity sites© 2011 Elsevier B.V. All rights reserved.

Zen A.,London Center for Nanotechnology | Zen A.,University College London | Sorella S.,International School for Advanced Studies | Sorella S.,Democritos National Simulation Center | And 6 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2016

Diffusion Monte Carlo (DMC) simulations for fermions are becoming the standard for providing high-quality reference data in systems that are too large to be investigated via quantum chemical approaches. DMC with the fixed-node approximation relies on modifications of the Green's function to avoid singularities near the nodal surface of the trial wave function. Here we show that these modifications affect the DMC energies in a way that is not size consistent, resulting in large time-step errors. Building on the modifications of Umrigar et al. and DePasquale et al. we propose a simple Green's function modification that restores size consistency to large values of the time step, which substantially reduces time-step errors. This algorithm also yields remarkable speedups of up to two orders of magnitude in the calculation of molecule-molecule binding energies and crystal cohesive energies, thus extending the horizons of what is possible with DMC. © 2016 American Physical Society.

Rossi M.,University of Milan | Vitali E.,University of Milan | Vitali E.,Democritos National Simulation Center | Galli D.E.,University of Milan | Reatto L.,University of Milan
Journal of Physics Condensed Matter | Year: 2010

Defects are believed to play a fundamental role in the supersolid state of 4He. We have studied solid 4He in two dimensions (2D) as a function of the number of vacancies nv, up to 30, inserted in the initial configuration at ρ = 0.0765 Å-2, close to the melting density, with the exact zero-temperature shadow path integral ground state method. The crystalline order is found to be stable also in the presence of many vacancies and we observe two completely different regimes. For small nv, up to about 6, vacancies form a bound state and cause a decrease of the crystalline order. At larger nv, the formation energy of an extra vacancy at fixed density decreases by one order of magnitude to about 0.6K. It is no longer possible to recognize vacancies in the equilibrated state because they mainly transform into quantum dislocations and crystalline order is found almost independently of how many vacancies have been inserted in the initial configuration. The one-body density matrix in this latter regime shows a non-decaying large distance tail: dislocations, that in 2D are point defects, turn out to be mobile, their number is fluctuating, and they are able to induce exchanges of particles across the system mainly triggered by the dislocation cores. These results indicate that the notion of the incommensurate versus the commensurate state loses meaning for solid 4He in 2D, because the number of lattice sites becomes ill defined when the system is not commensurate. Crystalline order is found to be stable also in 3D in the presence of up to 100 vacancies. © 2010 IOP Publishing Ltd.

Dupuy N.,CNRS Institute of Mineralogy, Materials Physics and Cosmochemistry | Bouaouli S.,University Pierre and Marie Curie | Mauri F.,CNRS Institute of Mineralogy, Materials Physics and Cosmochemistry | Sorella S.,International School for Advanced Studies | And 2 more authors.
Journal of Chemical Physics | Year: 2015

We study the ionization energy, electron affinity, and the π → π- (1La) excitation energy of the anthracene molecule, by means of variational quantum Monte Carlo (QMC) methods based on a Jastrow correlated antisymmetrized geminal power (JAGP) wave function, developed on molecular orbitals (MOs). The MO-based JAGP ansatz allows one to rigorously treat electron transitions, such as the HOMO → LUMO one, which underlies the 1La excited state. We present a QMC optimization scheme able to preserve the rank of the antisymmetrized geminal power matrix, thanks to a constrained minimization with projectors built upon symmetry selected MOs. We show that this approach leads to stable energy minimization and geometry relaxation of both ground and excited states, performed consistently within the correlated QMC framework. Geometry optimization of excited states is needed to make a reliable and direct comparison with experimental adiabatic excitation energies. This is particularly important in π-conjugated and polycyclic aromatic hydrocarbons, where there is a strong interplay between low-lying energy excitations and structural modifications, playing a functional role in many photochemical processes. Anthracene is an ideal benchmark to test these effects. Its geometry relaxation energies upon electron excitation are of up to 0.3 eV in the neutral 1La excited state, while they are of the order of 0.1 eV in electron addition and removal processes. Significant modifications of the ground state bond length alternation are revealed in the QMC excited state geometry optimizations. Our QMC study yields benchmark results for both geometries and energies, with values below chemical accuracy if compared to experiments, once zero point energy effects are taken into account. © 2015 AIP Publishing LLC.

Raji A.T.,University of Cape Town | Mazzarello R.,RWTH Aachen | Scandolo S.,Abdus Salam International Center For Theoretical Physics | Scandolo S.,Democritos National Simulation Center | And 3 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

We report on the migration of monovacancies, divacancies, and substitutional krypton atoms in hcp titanium using the nudged elastic band method, in the framework of ab initio density functional theory. The divacancy migration energy barrier is found to be lower than that of a monovacancy. The migration of substitutional krypton in titanium is governed by the vacancy mechanism, if there is an excess of nonequilibrium vacancies after the implantation. However, if thermal vacancies dominate, krypton atom migration is expected to proceed via the dissociation mechanism. We also present ab initio calculations on the formation of clusters of multiple substitutional impurity krypton atoms and the interactions between krypton impurities and vacancies in titanium. We have found that, analogous to multiple interstitial krypton clusters, clusters of substitutional krypton atoms are energetically unstable, but are stabilized by vacancies. However, the efficiency of stabilization by vacancies depends strongly on the spatial distribution of the vacancies within the clusters. This study indicates the possibility of inert gases to nucleate in voids created by vacancies in ion implantation processes. © 2011 American Physical Society.

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