arXiv subjects

Stefano de Gironcoli

Publications and source records attributed to Stefano de Gironcoli.

At least 37 records · Page 2Linked to original sources

arXiv2016

Molecular bonding with the RPAx: from weak dispersion forces to strong correlation

In a recent article [Phys. Rev. B 90, 125102 (2014)] we showed that the random phase approximation with exchange (RPAx) gives accurate total energies for a diverse set of systems including the high and low density regime of the homogeneous electron gas, the N$_2$ molecule, and the H$_2$ molecule at dissociation. In this work, we present results for the van der Waals bonded Ar$_2$ and Kr$_2$ dimers and demonstrate that the RPAx gives superior dispersion forces as compared to the RPA. We then show that this improved description is crucial for the bond formation of the Mg$_2$ molecule. In addition, the RPAx performs better for the Be$_2$ dissociation curve at large nuclear separation but, similar to the RPA, fails around equilibrium due to the build up of a large repulsion hump. For the strongly correlated LiH molecule at dissociation we have also calculated the RPAx potential and find that the correlation peak at the bond mid-point is overestimated as compared to the RPA and the exact result. The step feature is missing and hence the delocalisation error is comparable to the RPA. This is further illustrated by a smooth energy versus fractional charge curve and a poor description of the LiH dipole moment at dissociation.

cond-mat.str-el↗

arXiv2014

Correlation energy within exact-exchange ACFD theory: systematic development and simple approximations

We have calculated the correlation energy of the homogeneous electron gas (HEG) and the dissociation energy curves of molecules with covalent bonds from a novel implementation of the adiabatic connection fluctuation dissipation (ACFD) expression including the exact exchange (EXX) kernel. The EXX kernel is defined from first order perturbation theory and used in the Dyson equation of time-dependent density functional theory. Within this approximation (RPAx), the correlation energies of the HEG are significantly improved with respect to the RPA up to densities of the order of $r_s \approx 10$. However, beyond this value, the RPAx response function exhibits an unphysical divergence and the approximation breaks down. Total energies of molecules at equilibrium are also highly accurate but we find a similar instability at stretched geometries. Staying within an exact first order approximation to the response function we use an alternative resummation of the higher order terms. This slight redefinition of RPAx fixes the instability in total energy calculations without compromising the overall accuracy of the approach.

cond-mat.str-el↗

arXiv2013

Hubbard-corrected DFT energy functionals: the LDA+U description of correlated systems

The aim of this review article is to assess the descriptive capabilities of the Hubbard-rooted LDA+U method and to clarify the conditions under which it can be expected to be most predictive. The paper illustrates the theoretical foundation of LDA+U and prototypical applications to the study of correlated materials, discusses the most relevant approximations used in its formulation, and makes a comparison with other approaches also developed for similar purposes. Open "issues" of the method are also discussed, including the calculation of the electronic couplings (the Hubbard U), the precise expression of the corrective functional and the possibility to use LDA+U for other classes of materials. The second part of the article presents recent extensions to the method and illustrates the significant improvements they have obtained in the description of several classes of different systems. The conclusive section finally discusses possible future developments of LDA+U to further enlarge its predictive power and its range of applicability.

cond-mat.mtrl-sci↗

arXiv2011

{\it Ab initio} $^{27}Al$ NMR chemical shifts and quadrupolar parameters for $Al_2O_3$ phases and their precursors

The Gauge-Including Projector Augmented Wave (GIPAW) method, within the Density Functional Theory (DFT) Generalized Gradient Approximation (GGA) framework, is applied to compute solid state NMR parameters for $^{27}Al$ in the $α$, $θ$, and $κ$ aluminium oxide phases and their gibbsite and boehmite precursors. The results for well-established crystalline phases compare very well with available experimental data and provide confidence in the accuracy of the method. For $γ$-alumina, four structural models proposed in the literature are discussed in terms of their ability to reproduce the experimental spectra also reported in the literature. Among the considered models, the $Fd\bar{3}m$ structure proposed by Paglia {\it et al.} [Phys. Rev. B {\bf 71}, 224115 (2005)] shows the best agreement. We attempt to link the theoretical NMR parameters to the local geometry. Chemical shifts depend on coordination number but no further correlation is found with geometrical parameters. Instead our calculations reveal that, within a given coordination number, a linear correlation exists between chemical shifts and Born effective charges.

cond-mat.mtrl-sci↗

arXiv2009

A new look on the nature of high-spin to low-spin transition in Fe2O3

Iron sesquioxide (Fe2O3) displays pressure and temperature induced spin and structural transitions. Our calculations show that, density functional theory (DFT), in the generalized gradient approximation (GGA) scheme, is capable of capturing both the transitions. The ambient pressure corundum type phase (hematite or alpha-Fe2O3), having R_3c symmetry, gets distorted by the application of pressure and transforms to a distorted corundum type or Rh2O3(II) phase with Pbcn symmetry, in agreement with recent experiments. GGA + U calculations show the same trend but shift the transition pressures to higher values. Experimentally, the onset of the structural transition begins in the vicinity of the spin transition pressure and whether the system undergoes spin transition in the corundum type (HP1) or in the Rh2O3(II) type (HP2) phase, is still a controversial issue. With a relatively simple, but general, octahedral structural parameter, Voct (the octahedral volume around iron ions), we show that in order to acquire a low spin (LS) state from a high spin (HS) one, the system does not necessarily need to change the crystal structure. Rather, the spin transition is a phenomenon that concerns the cation octahedra and the spin state of the system depends mainly on the value of Voct, which is governed by two distinct equations of state, separated by a well defined volume gap, for the HS and LS states respectively. Analysis of the results on the basis of octahedral volume allows to sum up and bridge the gap between two experimental results and thus provides a better description of the system in the region of interest.

cond-mat.mtrl-sci↗

arXiv2009

Efficient and accurate calculation of exact exchange and RPA correlation energies in the Adiabatic-Connection Fluctuation-Dissipation theory

Recently there has been a renewed interest in the calculation of exact-exchange and RPA correlation energies for realistic systems. These quantities are main ingredients of the so-called EXX/RPA+ scheme which has been shown to be a promising alternative approach to the standard LDA/GGA DFT for weakly bound systems where LDA and GGA perform poorly. In this paper, we present an efficient approach to compute the RPA correlation energy in the framework of the Adiabatic-Connection Fluctuation-Dissipation formalism. The method is based on the calculation of a relatively small number of eigenmodes of RPA dielectric matrix, efficiently computed by iterative density response calculations in the framework of Density Functional Perturbation Theory. We will also discuss a careful treatment of the integrable divergence in the exact-exchange energy calculation which alleviates the problem of its slow convergence with respect to Brillouin zone sampling. As an illustration of the method, we show the results of applications to bulk Si, Be dimer and atomic systems.

cond-mat.mtrl-sci↗

arXiv2009

Structural and spin transitions in Fe$_{2}$O$_{3}$

First principles density functional calculations for Fe$_{2}$O$_{3}$ has been performed over a wide range of pressures. The ground state is corundum-type hematite and is an antiferromagnetic insulator. This is in good agreement with experiment and other theoretical studies. On increasing pressure, the ground-state high-spin magnetic phase transforms to low spin via the closure of the charge transfer gap. The system also evolves to a new orthorhombic structure. Distorted corundum or Rh$_{2}$O$_{3}$(II) type structure with Pbcn symmetry and Pbnm type perovskite structure are two known competitive candidates for this structural phase, based on {\it single}--cationic type and {\it two}--cationic type picture, respectively. In our calculations, at about 38 GPa, Rh$_{2}$O$_{3}$(II) type structure becomes more stable with respect to the ground state hematite. Relative stability of Pbnm type perovskite is ruled out by our calculations in this pressure regime. The Rh$_{2}$O$_{3}$(II) type structure remains in its low spin state, with 1 $μ_{B}/$Fe atom, up to about 120 GPa. At this pressure the nonmagnetic solution in Rh$_{2}$O$_{3}$(II) type structure becomes more favorable with respect to the low spin one. By further increasing the pressure at about 330 GPa, the system evolves to yet another new structural phase. This new orthorhombic structural phase is nonmagnetic and has Pmc2$_{1}$ symmetry, a subgroup of Cmcm. Surprisingly, on furthering rising the pressure, a Pbnm type nonmagnetic solution becomes competitive with the Pmc2$_{1}$ type structure and finally becomes stable at about 880 GPa.

cond-mat.mtrl-sci↗

arXiv2009

Van der Waals Coefficients of Atoms and Molecules from a Simple Approximation for the Polarizability

A simple and computationally efficient scheme to calculate approximate imaginary-frequency dependent polarizability, hence asymptotic van der Waals coefficient, within density functional theory is proposed. The dynamical dipolar polarizabilities of atoms and molecules are calculated starting from the Thomas-Fermi-von Weizsäcker (TFvW) approximation for the independent-electron kinetic energy functional. The van der Waals coefficients for a number of closed-shell ions and a few molecules are hence calculated and compared with available values obtained by fully first-principles calculations. The success in these test cases shows the potential of the proposed TFvW approximate response function in capturing the essence of long range correlations and may give useful information for constructing a functional which naturally includes van der Waals interactions.

cond-mat.mtrl-sci↗

arXiv2008

Lowering Effective Coordination Promotes Adsorption of NO on Rh(100) and Rh/MgO(100)surfaces

We have studied the adsorption of NO, and the coadsorption of N and O, on four physical and hypothetical systems: unstrained and strained Rh(100) surfaces and monolayers of Rh atoms on strained and unstrained MgO(100) surfaces. We find that as we go from Rh(100) to Rh/Mg0(100), via the other two hypothetical systems, the effective coordination progressively decreases, the $d$-band narrows and its center shifts closer to the Fermi level, and the strength of adsorption and co-adsorption increases. Both strain and the presence of the oxide substrate contribute significantly to this. However, charge-transfer is found to play a negligible role, due to a cancelling out between donation and back-donation processes. Our results suggest that lowering effective coordination of Rh catalysts by strain, roughening or the use of inert substrates might improve reaction rates for the reduction of NO to N$_2$.

cond-mat.mtrl-sci↗

arXiv2007

Interplay between Bonding and Magnetism in the Adsorption of NO on Rh Clusters

We have studied the adsorption of NO on small Rh clusters, containing one to five atoms, using density functional theory in both spin-polarized and non-spin-polarized forms. We find that NO bonds more strongly to Rh clusters than it does to Rh(100) or Rh(111); however, it also quenches the magnetism of the clusters. This (local) effect results in reducing the magnitude of the adsorption energy, and also washes out the clear size-dependent trend observed in the non-magnetic case. Our results illustrate the competition present between the tendencies to bond and to magnetize, in small clusters.

cond-mat.mtrl-sci↗

arXiv2006

Origins of low- and high-pressure discontinuities of $T_{c}$ in niobium

The discontinuities of $T_{c}$ in Niobium under pressure are examined by means of the pseudopotential plane-wave implementation of the electron-phonon coupling calculated from density-functional perturbation theory. Both low- and high-pressure discontinuities of $T_{c}$ have their origin in the Kohn anomalies and are caused by the low-frequency phonons, but the mechanism leading to the discontinuities is different in the two cases. The low-pressure anomaly is associated with a global decrease of the nesting factor in the whole Brillouin Zone and not to a visible change in the band structure. The high-pressure anomaly is instead connected with a well-pronounced change in the band structure.

cond-mat.supr-con↗

arXiv2004

A linear response approach to the calculation of the effective interaction parameters in the LDA+U method

In this work we reexamine the LDA+U method of Anisimov and coworkers in the framework of a plane-wave pseudopotential approach. A simplified rotational-invariant formulation is adopted. The calculation of the Hubbard U entering the expression of the functional is discussed and a linear response approach is proposed that is internally consistent with the chosen definition for the occupation matrix of the relevant localized orbitals. In this way we obtain a scheme whose functionality should not depend strongly on the particular implementation of the model in ab-initio calculations. We demonstrate the accuracy of the method, computing structural and electronic properties of a few systems including transition and rare-earth correlated metals, transition metal monoxides and iron-silicate.

cond-mat.mtrl-sci↗

arXiv2003

Electron localization in pure and defective ceria by a unified LDA+U approach

The electronic and structural properties of pure and defective cerium oxide are investigated using a unified LDA+U approach which allows to treat the different valence states of Ce occurring for different stoichiometries on a same ground, without any {\it a priori} assumptions on the defect chemistry. The method correctly predicts the atomistic and electronic structures of the \4ce and \3ce bulk phases, as well as the subtle defect chemistry in reduced materials \xce, which controls the oxygen storage functionality of cerium-based oxides. The analysis of the energetics highlights the limits of the LDA+U method and possible extensions are proposed.

cond-mat.mtrl-sci↗

arXiv2001

Ab initio calculation of the thermal properties of Cu: Performance of the LDA and GGA

The thermal properties of bulk copper are investigated by performing ab initio DFT and DFPT calculations and using the quasiharmonic approximation for the free energy. Using both the LDA and the GGA for the exchange-correlation potential, we compute the temperature dependence of the lattice constant, coefficient of thermal expansion, bulk modulus, pressure derivative of the bulk modulus, phonon frequencies, Grueneisen parameters, and the electronic and phonon contributions to the specific heats at constant volume and constant pressure. We obtain answers in closer agreement with experiment than those obtained from more approximate earlier treatments. The LDA/GGA errors in computing anharmonic quantities are significantly smaller than those in harmonic quantities. We argue that this should be a general feature, and also argue that LDA/GGA errors should increase with temperature.

cond-mat.mtrl-sci↗

arXiv2001

Role of defects in the electronic properties of amorphous/crystalline Si interface

The mechanism determining the band alignment of the amorphous/crystalline Si heterostructures is addressed with direct atomistic simulations of the interface performed using a hierarchical combination of various computational schemes ranging from classical model-potential molecular dynamics to ab-initio methods. We found that in coordination defect-free samples the band alignment is almost vanishing and independent on interface details. In defect-rich samples, instead, the band alignment is sizeably different with respect to the defect-free case, but, remarkably, almost independent on the concentration of defects. We rationalize these findings within the theory of semiconductor interfaces.

cond-mat.mtrl-sci↗

arXiv1999

Dipole-quadrupole interactions and the nature of phase III of compressed hydrogen

A new class of strongly infrared active structures is identified for phase III of compressed molecular H2 by constant-pressure ab initio molecular dynamics and density-functional perturbation calculations. These are planar quadrupolar structures obtained as a distortion of low-pressure quadrupolar phases, after they become unstable at about 150 GPa due to a zone-boundary soft phonon. The nature of the II-III transition and the origin of the IR activity are rationalized by means of simple electrostatics, as the onset of a stabilizing dipole-quadrupole interaction.

cond-mat.mtrl-sci↗

arXiv1998

The reconstruction of Ni and Rh (001) surfaces upon Carbon, Nitrogen, or Oxygen adsorption

Nickel and Rhodium (001) surfaces display a similar - as from STM images - clock reconstruction when half a monolayer of C/Ni, N/Ni or O/Rh is adsorbed; no reconstruction is observed instead for O/Ni. Adsorbate atoms sit at the center of the black squares of a chess-board, $c(2\times 2)$, pattern and two different reconstructions are actually compatible with the observed STM images - showing a $(2\times 2)p4g$ pattern - according to whether a rotation of the black or white squares occurs. We report on a first - principles study of the structure of X/Ni(001) and X/Rh(001) surfaces (X=C,N,O) at half a monolayer coverage, performed using density-functional theory. Our findings are in agreement with all available experimental information and shed new light on the mechanisms responsible for the reconstructions. We show that the same substrate may display different reconstructions - or no reconstruction - upon adsorption of different atomic species, depending on the relative importance of the chemical and steric factors which determine the reconstruction.

cond-mat.mtrl-sci↗

arXiv1998

Effects of disorder on the optical properties of the (Zn,Mg)(S,Se) quaternary alloy

The electronic and optical properties of (Zn,Mg)(S,Se) wide-gap solid solutions are studied using ab initio techniques and starting from the previously determined atomistic structure of the alloy. Compositional disorder is shown to close substantially the gap with respect to the predictions of the virtual-crystal approximation. The bowing of the fundamental gap vs. composition predicted by our calculations is in very good agreement with experiments available for the Zn(S,Se) pseudo-binary alloy. At temperatures typical for MBE growth, the quaternary alloy displays a rather large amount of short-range order whose effect is to slightly but unmistakably open the gap. Our results agree well with recent experimental data for the quaternary alloy.

cond-mat.mtrl-sci↗