| 1. |
- Londero, Elisa, 1982, et al.
(författare)
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Desorption of n-alkanes from graphene: a van der Waals density functional study
- 2012
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Ingår i: Journal of Physics Condensed Matter. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 24:42, s. 424212-
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Tidskriftsartikel (refereegranskat)abstract
- A recent study of temperature-programmed desorption (TPD) measurements of small linear alkane molecules (n-alkanes, with formula CNH2N+2) from C(0001) deposited on Pt(111) shows a linear relationship of the desorption energy with increasing n-alkane chain length N. We here present a van der Waals density functional study of the desorption barrier energy of the ten smallest n-alkanes (of carbon chain length N = 1–10) from graphene. We find linear scaling with N, including a non-zero intercept with the energy axis, i.e. an offset at the extrapolation to N = 0. This calculated offset is quantitatively similar to the results of the TPD measurements. From further calculations of the polyethylene polymer we offer a suggestion for the origin of the offset.
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| 2. |
- Berland, Kristian, 1983, et al.
(författare)
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Harris-type van der Waals density functional scheme
- 2013
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Ingår i: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969. ; 88:4
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Tidskriftsartikel (refereegranskat)abstract
- Biomolecular systems that involve thousands of atoms are difficult to address with standard density functional theory (DFT) calculations. With the development of sparse-matter methods such as the van der Waals density functional (vdW-DF) method [M. Dion et al., Phys. Rev. Lett. 92, 246401 (2004)], it is now possible to include the dispersive forces in DFT which are necessary to describe the cohesion and behavior of these systems. vdW-DF implementations can be as efficient as those for traditional DFT. Yet, the computational costs of self-consistently determining the electron wave functions and hence the kinetic-energy repulsion still limit the scope of sparse-matter DFT. We propose to speed up sparse-matter calculations by using the Harris scheme [J. Harris, Phys. Rev. B 31, 1770 (1985)]; that is, we propose to perform electronic relaxations only for separated fragments (molecules) and use a superposition of fragment densities as a starting point to obtain the total energy non-self-consistently. We evaluate the feasibility of this approach for an adaption of the Harris scheme for non-self-consistent vdW-DF (sfd-vdW-DF). We study four molecular dimers with varying degrees of polarity and find that the sfd scheme accurately reproduces standard non-self-consistent vdW-DF for van der Waals dominated systems but is less accurate for those dominated by polar interactions. Results for the S22 set of typical organic molecular dimers are promising.
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| 3. |
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| 4. |
- Londero, Elisa, 1982, et al.
(författare)
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Role of van der Waals bonding in the layered oxide V2O5: First-principles density-functional calculations
- 2010
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Ingår i: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969. ; 82:5, s. 054116-
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Tidskriftsartikel (refereegranskat)abstract
- Sparse matter is characterized by regions with low electron density and its understanding calls for methods to accurately calculate both the van der Waals (vdW) interactions and other bonding. Here we present a first-principles density-functional theory (DFT) study of a layered oxide (V2O5) bulk structure which shows charge voids in between the layers and we highlight the role of the vdW forces in building up material cohesion. The result of previous first-principles studies involving semilocal approximations to the exchange-correlation functional in DFT gave results in good agreement with experiments for the two in-plane lattice parameters of the unit cell but overestimated the parameter for the stacking direction. To recover the third parameter we include the nonlocal (dispersive) vdW interactions through the vdW-DF method [M. Dion, H. Rydberg, E. Schröder, D. C. Langreth, and B. I. Lundqvist, Phys. Rev. Lett. 92, 246401 (2004)] testing also various choices of exchange forms. We find that the transferable first-principles vdW-DF calculations stabilizes the bulk structure. The vdW-DF method gives results in fairly good agreement with experiments for all three lattice parameters.
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| 5. |
- Londero, Elisa, 1982
(författare)
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Theory of van der Waals bonding: from bulk materials to biomolecules
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Sparse matter is abundant in Nature.It encompasses systems characterized by an intrinsic low density of electrons in sizeable regions, where the van der Waals forces contribute considerably to cohesion.Given the length scale of the problem, a prediction of these materials requires appropriate tools within a quantum-mechanical framework.Density Functional Theory (DFT) has proven to provide a powerful approach to a non-empirical characterization of condensed-matter properties.However, in spite of the successes achieved by its local (LDA) and semilocal (GGA) approximations, the description of the van der Waals bonding was until recently far from being satisfactory.This is related to the incapability of the local and semilocal exchange-correlation energy functionals to capture the effect of charge fluctuations that arise spontaneously in matter and that are coupled by the electrodynamic field.The problem has been successfully addressed by a density functional (vdW-DF) that accounts for the dispersive interactions by introducing a nonlocality in the correlation term.The work presented in this Thesis contributes to shed light on the van der Waals bonding in soft matter by means of the vdW-DF functional.In particular, it investigates three structurally and electronically different systems (namely belonging to the class of bulks, surfaces and biomolecules) in order to test structural characteristics, cohesive energies, physisorption-related properties and corrugation.A major issue when treating large sparse matter systems coincides with the limit introduced by the selfconsistent calculation of the kinetic term which increases the computing time and memory needed.An attempt to speed-up the computations is presented by taking advantage of the Harris scheme, a nonselfconsistent DFT formulation valid for weakly interacting systems.The account of dispersion forces has a direct impact on the structure resulting in an undoubtedly better description ofthe atom configuration and the morphology of sparse matter.In addition, it is documented that their fingerprint can also be detected in subtle changes of the band structure and the density of states.
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| 6. |
- Londero, Elisa, 1982, et al.
(författare)
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Vanadium pentoxide (V2O5): a van der Waals density functional study
- 2011
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Ingår i: Computer Physics Communications. - : Elsevier BV. - 0010-4655. ; 182:9, s. 1805-1809
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Tidskriftsartikel (refereegranskat)abstract
- The past few years have brought renewed focus on the physics behind the class of materials characterized by long-range interactions and wide regions of low electron density, sparse matter. There is now much work on developing the appropriate algorithms and codes able to correctly describe this class of materials within a parameter-free quantum physical description. In particular, van der Waals (vdW) forces play a major role in building up material cohesion in sparse matter. This work presents an application to the vanadium pentoxide (V2O5) bulk structure of two versions of the vdW-DF method, a first-principles procedure for the inclusion of vdW interactions in the context of density functional theory (DFT). In addition to showing improvement compared to traditional semilocal calculations of DFT, we discuss the choice of various exchange functionals and point out issues that may arise when treating systems with large amounts of vacuum.
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