Nonlocal Norm-conserving Pseudopotentials Research Articles

The pressure dependences of elastic and lattice dynamic properties of AlAs from ab initio calculations

Ab initio calculations, based on norm-conserving nonlocal pseudopotentials and density functional theory (DFT), are performed to investigate the structural, elastic, dielectric, and vibrational properties of aluminum arsenide (AlAs) with a zinc-blende (B3) structure and a nickel arsenide (B81) structure under hydrostatic pressure. Firstly, the path for the phase transition from B3 to B81 is confirmed by analyzing the energies of different structures, which is in good agreement with previous theoretical results. Secondly, we find that the elastic constants, bulk modulus, static dielectric constants, and the optical phonon frequencies vary in a nearly linear manner under hydrostatic pressure. What is more, the softening mode of the transversal acoustic mode at the X point supports the phase transition in AlAs.

Magnetic Cooperative Effects in Small Ni–Ru Clusters

We report ab initio calculations of the structures, magnetic moments, and electronic properties of Ni(7)-xRu(x) clusters (x = 0-7) using a density-functional method that employs linear combinations of pseudoatomic orbitals as basis sets, nonlocal norm-conserving pseudopotentials, and the generalized gradient approximation to exchange and correlation. The pure clusters Ni(7) and Ru(7) were predicted to have octahedral and cubic structures, respectively, and the binary clusters were found to be either decahedral (Ni(6)Ru, Ni(5)Ru(2), and Ni(4)Ru(3)) or cubic (Ni(3)Ru(4), Ni(2)Ru(5), and NiRu(6)). For Ni(5)Ru(2) and Ni(4)Ru(3) we found a magnetic cooperative phenomenon, which is due to both geometrical effects and electronic contributions through Ni-Ru hybridization.

Electronic, magnetic and structural properties of neutral, cationic and anionic Fe2S2, Fe3S4 and Fe4S4 clusters

This work reports density functional calculations of geometric, electronic and magnetic properties of freestanding iron–sulfur Fe2S2, Fe3S4 and Fe4S4 clusters which are the ones most frequently contained in proteins. We investigate neutral, anionic and cationic clusters using a method that employs linear combinations of atomic orbitals as basis sets, nonlocal norm-conserving pseudopotentials and a generalized gradient approximation to exchange and correlation. The results are discussed in connection with available experimental data. We mainly show that the ground-state geometries of these free clusters are consistent with their structures in core proteins and they are the same in the neutral, anionic and cationic states, but with small distortions. In all cases, an antiferromagnetic order between Fe atoms is always preferred to ferromagnetic and paramagnetic ones. The geometric distortions induced by magnetism decrease with cluster size and the maximum deviation between Fe–Fe distances is 11% in Fe2S2, but only 4% in Fe3S4 and 3% in Fe4S4 clusters.

A density-functional study of the structures and electronic properties of neutral, anionic, and endohedrally doped InxPx clusters

We report extensive ab initio calculations of the structures, binding energies, and magnetic moments of In(x)P(x) and In(x)P(x) (-) clusters (x=1-15) using a density-functional method that employs linear combinations of pseudoatomic orbitals as basis sets, nonlocal norm-conserving pseudopotentials, and the generalized gradient approximation for exchange and correlation. Our results, which are compared with those obtained previously for some of these clusters by means of all-electron calculations, show that hollow cages with alternating In-P bonds are energetically preferred over other structures for both the neutral and anionic species within the range x=6-15. We also consider the endohedrally doped X@In(10)P(10) (X=Cr,Mn,Fe,Co) and Ti@In(x)P(x) (x=7-12) clusters. Our results show that, except for Ti@In(7)P(7) and Ti@In(8)P(8), the transition metal atoms preserve their atomic spin magnetic moments when encapsulated in the InP cages, instead of suffering either a spin crossover or a spin quenching due to hybridization effects. We also show that the stabilities of some empty and doped InP cages can be explained on the basis of the jellium model.

A density-functional study of the vertical ionization potentials of the cluster Mn13

Using the density-functional method SIESTA, nonlocal norm-conserving pseudopotentials and the generalized gradient approximation to exchange and correlation, we show that the two ionization energies of the photoionization spectrum of Mn(13) can be attributed to the presence of the ground state and an excited spin isomer of this species.

Lattice dynamics properties of zinc-blende and Nickel arsenide phases of AlP

Ab initio calculations, based on norm-conserving non-local pseudopotentials and the density functional theory (DFT), have been performed to investigate the behaviour under hydrostatic pressure of the structural, electronic, elastic and dynamical properties of AlP, in both zinc-blende and nickel arsenide phases. Our calculated structural and electronic properties are in good agreement with previous theoretical and experimental results. The phonon dispersion curves, the elastic constants, Born effective charge, etc., were calculated with the local density approximation and the density functional perturbation theory (DFPT). Our results in the pressure behaviour of the elastic and dynamical properties of both phases are in agreement with the experimental data when available, in other case they can be considered as predictions.

Generalized reconstruction model ofSiC(001)−(n×2)surfaces and Si ad-dimer strings

We report ab initio calculations on the atomic and electronic structure of the $7\ifmmode\times\else\texttimes\fi{}2$ and $8\ifmmode\times\else\texttimes\fi{}2$ surfaces of cubic $\mathrm{SiC}(001)$ carried out within local density approximation of the density functional theory. Supercells, nonlocal norm-conserving pseudopotentials and Gaussian orbital basis sets are used to describe the surface systems. Our results for the surface structure are in good accord with a lot of experimental data. These investigations complement our previous ab initio studies on $c(4\ifmmode\times\else\texttimes\fi{}2)$, $3\ifmmode\times\else\texttimes\fi{}2$ and $5\ifmmode\times\else\texttimes\fi{}2$ reconstructions of the $\mathrm{SiC}(001)$ surface. On the basis of all our results, we suggest a general two adlayer asymmetric dimer model (TAADM) for $n\ifmmode\times\else\texttimes\fi{}2$ reconstructions of $\mathrm{SiC}(001)$. The model is based on two structural building blocks. The first contains an asymmetric dimer in one partial adlayer while the second contains an asymmetric dimer configuration of six Si adatoms in two partial adlayers. These are characteristic for the $c(4\ifmmode\times\else\texttimes\fi{}2)$ and $3\ifmmode\times\else\texttimes\fi{}2$ reconstructions, respectively. Employing the model for higher $n$ values allows us to suggest optimal structures for a large host of $n\ifmmode\times\else\texttimes\fi{}2$ reconstructions. Comparing the formation energies of the $c(4\ifmmode\times\else\texttimes\fi{}2)$ and all $n\ifmmode\times\else\texttimes\fi{}2$ structures considered, we find in agreement with experiment that the $3\ifmmode\times\else\texttimes\fi{}2$ and $c(4\ifmmode\times\else\texttimes\fi{}2)$ reconstructions are most favorable for silicon-rich and silicon-poor growth conditions, respectively. The formation energies of all other reconstructions fall in between these two limits. In particular, the general TAADM allows us to rationalize the occurrence of periodic and nonperiodic Si ad-dimer-string arrangements at Si-terminated $3\mathrm{C}\text{\ensuremath{-}}\mathrm{SiC}(001)$ surfaces in very good accord with experimental data.

First-principles effective-mass Hamiltonian for semiconductor nanostructures in a magneticfield

A multi-band effective-mass Hamiltonian is derived for lattice-matched semiconductornanostructures in a slowly varying external magnetic field. The theory is derived from thefirst-principles magnetic-field coupling Hamiltonian of Pickard and Mauri, which isapplicable to nonlocal norm-conserving pseudopotentials in the local density approximationto density functional theory. The pseudopotential of the nanostructure is treated as aperturbation of a bulk reference crystal, with linear and quadratic response terms includedin perturbation theory. The resulting Hamiltonian contains several interface terms that havenot been included in previous envelope-function theories for nanostructures in a magneticfield. The derivation provides the first direct analytical expressions showing how thecoupling of the nonlocal potential to the magnetic field influences the effective magneticdipole moment of the electron.

Metallization of the3C−SiC(001)−(3×2)surface induced by hydrogen adsorption: A first-principles investigation

We report ab initio calculations on the atomic and electronic structure, formation energies and surface vibrational modes of a variety of hydrogenated $3C\text{\ensuremath{-}}\mathrm{Si}\mathrm{C}(001)\text{\ensuremath{-}}(3\ifmmode\times\else\texttimes\fi{}2)$ surfaces to shed more light on the recently discovered hydrogen-induced metallization of this semiconductor surface. The calculations are carried out employing the local density approximation of density functional theory. The adsorption systems are described using supercells, nonlocal norm-conserving pseudopotentials and Gaussian orbital basis sets. First, we show in agreement with other most recent ab initio calculations that Si dangling bonds at the third layer, originally suggested for explaining the experimentally observed surface metallization, are not stable. Instead, for low H exposure, we find H atoms to form monohydrides at the top layer and to become adsorbed in angular Si-H-Si bonds on the third layer. Due to these bonds the surface becomes metallic. We scrutinize further possibilities of surface metallization investigating numerous conceivable other H configurations on the first three layers of the $\mathrm{Si}\mathrm{C}(001)\text{\ensuremath{-}}(3\ifmmode\times\else\texttimes\fi{}2)$ surface. It turns out that H atoms can also occupy bridge positions in angular Si-H-Si bonds on the second layer yielding a locally stable metallic surface. For larger H exposure, we find H atoms to form dihydrides at the top layer and to become adsorbed on the second and/or third layer in angular Si-H-Si bridge bonds. These structures are metallic, as well, and they are energetically even more favorable. In all cases considered, the angular Si-H-Si bonds are the origin of the surface metallization giving rise to partially occupied surface state bands which close the gap and overlap in parts with the lower projected conduction bands. Finally we address surface phonons. Some of the calculated frequencies are in very good accord with experiment. Others, particularly characteristic for the Si-H-Si bridge bonds, have not been resolved in experiment, to date.

The structure of an Fe monolayer on the Ni (111) surface. A density-functional study using the generalized gradient approximation

Using a density-functional method that employs linear combinations of atomic orbitals as basis sets, nonlocal norm-conserving pseudopotentials and the generalized gradient approximation for exchange and correlation, we found that at 0 K the atoms of an Fe monolayer on the Ni (111) surface occupy hcp rather than fcc sites, in keeping with previous predictions made using the ab initio all-electron full-potential linearized augmented plane wave method with the local spin density approximation.

Theoretical investigation of metal-molecule interface with terminal groups

The effects of terminal groups on the electron transport between metal electrodes and molecule are investigated through metal-molecule-metal systems using the first principles method, which is based on the density functional theory, with norm-conserving nonlocal pseudopotentials and nonequilibrium Green's functions. Eight Au-molecule-Au open systems are constructed and numerically examined, where gold atoms are used as electrode, benzene and borazine as core molecules, and sulphur (S), oxygen (O), selenium (Se), and cyano-group (CN) as terminal groups. Gold electrodes are described through a three-dimensional atomic model. The current-voltage (I-V) characteristics, density of states, and transmission functions of constructed systems are calculated and analyzed. Results show that the transmission properties of the systems are affected greatly by the terminal groups and are dependent on the core molecule as well. Se is demonstrated as the best terminal group to couple borazine to Au electrodes and CN is the best one to couple benzene to Au electrodes.

Noncollinear magnetic order in the six-atom Mn cluster

We report ab initio calculations of the structures, binding energies, and magnetic moments of the lowest-energy isomers of the cluster Mn6 that were performed using SIESTA, a density-functional method that employs linear combinations of pseudoatomic orbitals as basis sets, nonlocal norm-conserving pseudopotentials, and the local spin-density approximation for exchange and correlation. Our results predict that ground-state Mn6 has a noncollinear magnetic configuration.

A density-functional study of the structures, binding energies and total spins of Ni–Fe clusters using nonlocal norm-conserving pseudopotentials and the generalized gradient approximation

We report ab initio calculations of the structures, binding energies, and total spins of the clusters Ni(13), Ni(19), Ni(23), Ni(26), Ni(12)Fe, Ni(11)Fe(2), Ni(18)Fe, Ni(17)Fe(2), Ni(22)Fe, Ni(20)Fe(3), and Ni(25)Fe using a density-functional method that employs linear combination of atomic orbitals as basis sets, nonlocal norm-conserving pseudopotentials, and the generalized gradient approximation for exchange and correlation. Our results show that the Fe-doped Ni clusters, which have icosahedral or polyicosahedral ground-state structures similar to those of the corresponding pure Ni clusters, are most stable with the Fe atoms occupying internal positions, as has also been inferred from experimental results on the adsorption of molecular nitrogen on the cluster surfaces. We also rule out the possibility that the experimentally observed difference between the (nonpolyicosahedral) configurations of N(2)-saturated Ni(26) and N(2)-saturated Ni(25)Fe be due to the influence of the Fe atom on the energy of the underlying metal cluster.

Ab initio calculations of quantum transport through Al atomic wire mixed with various atoms

We present an efficient self-consistent method for the quantum transport of atomic-scale structures bridged between two metallic electrodes. It is based on the density-functional theory using the recursion-transfer-matrix (RTM) method with a separable form of norm-conserving nonlocal pseudopotentials. We did calculations with this method on the electric conductance of Al atomic wires with various kinds of a single atom mixed at the contact to one electrode. We found that the bonding nature of the atom at the contact significantly affects the transport properties. The bias drop in the atomic wire is strongly influenced by the local polarization of the atom at the contact.

Ab initiocalculations for quantum transport through atomic bridges by the recursion transfer-matrix method

We present an efficient self-consistent method for the quantum transport of atomic-scale electronic conductors bridged between two metallic electrodes. It is based on density-functional theory using the recursion-transfer-matrix method with separable norm-conserving nonlocal pseudopotentials. We performed calculations with this method on the electric conductance of Al atomic wires with several kinds of substitutional atoms connecting to one of the electrodes. We found that a local mixing of atoms at the contact significantly affects the transport properties. The conductance is strongly dependent on the bonding nature of the atom at the contact and does not change much as a length of the atomic wire increases. The bias drop in the wire is influenced by the local polarization of the atom at the contact.

Density-functional calculations of the structures, binding energies, and spin multiplicities of Fe–C clusters

We report ab initio calculations of the structures, binding energies and spin multiplicities of the clusters Fe2, C2, FeCn (n=1–4) and Fe2Cn (n=1–3) using a density-functional method that employs linear combinations of atomic orbitals as basis sets, nonlocal norm-conserving pseudopotentials, and the generalized gradient approximation for exchange and correlation. Our results for the pure dimers and the monometallic carbide clusters are in good general agreement with those obtained in previous theoretical studies and with available experimental data. All the dimetallic carbide clusters are predicted to have cyclic planar geometries that are stabilized (except, of course, in Fe2C) by transannular bonds. In particular, the pentagonal geometry of Fe2C3, with transannular Fe–Fe and Fe–C bonds and an FeC2 unit that is almost identical to free FeC2, parallels that of Ti2C3. However, this Fe2C3 structure is almost isoenergetic with another in which the C atoms aggregate to form a quasilinear C3 substructure, as in Co2C3. This is consistent with the position of Fe in the 3d metal series, intermediate between met-car formers (Ti, V, Cr) and nonformers (Co, Ni), and with the fact that mass spectra show Fe8C12 not to be significantly more stable than FemCn clusters of several other stoichiometries.

Development and implementation of the exact exchange method for semiconductors using a localized basis set

One of the major deficiencies of density functional theory is presented in the approximation of the exchange energy term. An important advance in solving this problem has been the development of orbital-dependent exchange functionals. The exact exchange method is one of the best defined releases of such functionals. Up to now it has been applied in solid systems only using a plane wave representation basis set. In this paper we present a development and implementation of the exact exchange formalism for solid semiconductors using a basis set of localized numerical functions. The implementation of the exact exchange scheme has been carried out in the SIESTA code, as a new path to get the exchange part in the Kohn–Sham energy and potential. This program is an ab initio periodic fully self-consistent density functional code which uses norm-conserving non-local pseudopotentials. Linear combination of confined numerical pseudoatomic orbitals have been used to represent the Kohn–Sham orbitals. The calculation results of the electronic properties of several semiconductor systems using different qualities of the basis set are compared with experimental results and presented in this paper.

Investigation of lithium insertion in anode material CuSn for lithium-ion batteries

An ab initio method with mixed-basis norm-conserving non-local pseudopotentials has been employed to investigate the non-carbon-baring anode material-CuSn for lithium-ion batteries. The lithium intercalation energies and their electronic structures have been calculated. The changes of volume, band structures, electronic density of states and charge density contour plots for lithium intercalation in CuSn are also presented. The characteristics of CuSn as an anode material for lithium ion batteries are also discussed. By calculation, we have found that the insertion formation energy of Li insertion CuSn electrode with zinc-blende-type structures as the host framework is about 3.5eV.

First-principles calculation on the formation energies oflithium insertion in In Sb

InSb as an anode material in lithium batteries has attracted considerable attention in recent investigations. An ab initio method with norm-conserving non-local pseudopotentials based on the local density functional theory has been used to investigate the non-carbon-bearing anode material InSb for lithium batteries. The formation energies of lithium intercalation and their electronic structures have been calculated. The changes of volume, band structures, electronic density of states and charge density contour plots for lithium intercalation in InSb are also discussed. We found that the formation energies of Li insertion in InSb are all about 2.2 eV per Li atom.

Density-functional method for nonequilibrium electron transport

We describe an ab initio method for calculating the electronic structure, electronic transport, and forces acting on the atoms, for atomic scale systems connected to semi-infinite electrodes and with an applied voltage bias. Our method is based on the density functional theory (DFT) as implemented in the well tested Siesta approach (which uses non-local norm-conserving pseudopotentials to describe the effect of the core electrons, and linear combination of finite-range numerical atomic orbitals to describe the valence states). We fully deal with the atomistic structure of the whole system, treating both the contact and the electrodes on the same footing. The effect of the finite bias (including selfconsistency and the solution of the electrostatic problem) is taken into account using nonequilibrium Green's functions. We relate the nonequilibrium Green's function expressions to the more transparent scheme involving the scattering states. As an illustration, the method is applied to three systems where we are able to compare our results to earlier ab initio DFT calculations or experiments, and we point out differences between this method and existing schemes. The systems considered are: (1) single atom carbon wires connected to aluminum electrodes with extended or finite cross section, (2) single atom gold wires, and finally (3) large carbon nanotube systems with point defects.