Elisa Londero

Vanadium pentoxide (V2O5): A van der Waals density functional study

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.

Desorption of n-alkanes from graphene: a van der Waals density functional study

A recent study of temperature-programmed desorption (TPD) measurements of small linear alkane molecules (n-alkanes, with formula C(N)H(2N+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.

Harris-type van der Waals density functional scheme

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.

Ab initio theory of the N2V defect in diamond for quantum memory implementation

N2V defect in diamond is characterized by means of ab initio methods relying on density functional theory calculated parameters of a Hubbard model Hamiltonian. It is shown that this approach appropriately describes the energy levels of correlated excited states induced by this defect. By determining its critical magneto-optical parameters, we propose to realize a long-living quantum memory by N2V defect in diamond.