Linear Combinations Of Pseudoatomic Orbitals Research Articles

First-principles Study of Spiral Spin Density Waves in Monolayer MnCl2 Using Generalized Bloch Theorem

We investigated the spiral spin density waves in the monolayer 1T-MnCl$_2$ for a set of spiral vectors based on first-principles calculations. The magnetic ground states were evaluated by means of the generalized Bloch theorem within the linear combination of pseudo-atomic orbitals. To reach our purpose, a flat spiral configuration was constructed for the Mn magnetic atom by fixing the direction of its magnetic moment. We confirmed that the ground state was a spiral ground state. We also clarified that a phase transition from a spiral ground state to the other ground states, such as the ferromagnetic state or the antiferromagnetic state, appears when introducing the hole-electron doping. Therefore, we justify that introducing the hole-electron doping tunes the phase transition in the monolayer 1T-MnCl$_2$.

First-principles Study of Spin-wave Excitations of 3d Transition Metals with Linear Combination of Pseudo-atomic Orbitals

We have employed the generalized Bloch theorem to evaluate the spin stiffness constants of 3$d$ transition metals (bcc-Fe, fcc-Co, and fcc-Ni) within the linear combination of pseudo-atomic orbitals (LCPAO). The spin stiffness constants were obtained by fitting the spin-wave energy curve, which relates to the total energy difference and the spiral vectors. In order to convince the reliable spin stiffness constants, we also provided the convergences of spin stiffness constants in terms of the cutoff radius and the number of orbitals. After observing the specific cutoff radius and the basis orbital, at which the spin stiffness constant converges, we used those two parameters to compute the Curie temperature by using the mean field approximation and the random phase approximation. For the latter approximation, we applied the so-called Debye approximation, which is intended to reduce very significantly many required wavevectors to evaluate the Curie temperature. We claimed that our results are in good agreement with both other calculations and experiments.

Implementation of Generalized Bloch Theorem Using Linear Combination of Pseudo-Atomic Orbitals

We have implemented the generalized Bloch theorem based on first-principles calculations using a linear combination of pseudo-atomic orbitals (LCPAO) as the basis sets. In order to test our implementation in a code, we examined three systems that have been reported in experiments or other calculations, namely, the carrier-induced spin-spiral ground state in the one-dimensional model system, the spin stiffness of bcc-Fe, and the spin stiffness in a zigzag graphene nanoribbon. We confirmed that our implementation gives good agreement with experiments. Based on these results, we believe that our implementation of the generalized Bloch theorem using an LCPAO is useful for predicting the properties of complex magnetic materials. We also predicted a large reduction (enhancement) of spin stiffness for the electron (hole) doping of zigzag-edge graphene nanoribbon ferromagnetic states.

Reliability and applicability of magnetic-force linear response theory: Numerical parameters, predictability, and orbital resolution

We investigated the reliability and applicability of so-called magnetic force linear response method to calculate spin-spin interaction strengths from first-principles. We examined the dependence on the numerical parameters including the number of basis orbitals and their cutoff radii within non-orthogonal LCPAO (linear combination of pseudo-atomic orbitals) formalism. It is shown that the parameter dependence and the ambiguity caused by these choices are small enough in comparison to the other computation approach and experiments. Further, we tried to pursue the possible extension of this technique to a wider range of applications. We showed that magnetic force theorem can provide the reasonable estimation especially for the case of strongly localized moments even when the ground state configuration is unknown or the total energy value is not accessible. The formalism is extended to carry the orbital resolution from which the matrix form of the magnetic coupling constant is calculated. From the applications to Fe-based superconductors including LaFeAsO, NaFeAs, BaFe$_2$As$_2$ and FeTe, the distinctive characteristics of orbital-resolved interactions are clearly noticed in between single-stripe pnictides and double-stripe chalcogenides.

Efficient Calculations with Multisite Local Orbitals in a Large-Scale DFT Code CONQUEST.

Multisite local orbitals, which are formed from linear combinations of pseudoatomic orbitals from a target atom and its neighbor atoms, have been introduced in the large-scale density functional theory calculation code CONQUEST. Multisite local orbitals correspond to local molecular orbitals so that the number of required local orbitals can be minimal. The multisite support functions are determined by using the localized filter diagonalization (LFD) method [ Phys. Rev. B 2009 , 80 , 205104 ]. Two new methods, the double cutoff method and the smoothing method, are introduced to the LFD method to improve efficiency and stability. The Hamiltonian and overlap matrices with multisite local orbitals are constructed by efficient sparse-matrix multiplications in CONQUEST. The investigation of the calculated energetic and geometrical properties and band structures of bulk Si, Al, and DNA systems demonstrate the accuracy and the computational efficiency of the present method. The representability of both occupied and unoccupied band structures with the present method has been also confirmed.

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.

The d-Band Structure of Pt Nanoclusters Correlated with the Catalytic Activity for an Oxygen Reduction Reaction

This paper presents the correlation between the d-band center of Pt nanoclusters and the catalytic activity for an oxygen reduction reaction (ORR). The size-dependent d-band center was determined for Pt nanoclusters deposited on glassy carbon substrates by photoelectron spectroscopy using synchrotron radiation light. It is found that the d-band center moves toward the Fermi level (EF) with decreasing cluster size, as predicted by the first-principles calculations using linear combinations of pseudoatomic orbitals and norm-conserved pseudopotentials. The ORR activity of Pt nanoclusters measured by linear sweep voltammetry with the rotating disk electrode method reveals that the activity is diminished with decreasing cluster size and with moving the d-band center toward the EF. This is consistent with the d-band model claiming that the closer the d-band center is to EF the larger the oxygen adsorption energy is.

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.

Structures of 13-atom clusters of fcc transition metals byab initioand semiempirical calculations

We report the results of ab initio calculations of the structures and magnetic moments of ${\mathrm{Ni}}_{13}$, ${\mathrm{Pd}}_{13}$, ${\mathrm{Pt}}_{13}$, ${\mathrm{Cu}}_{13}$, ${\mathrm{Ag}}_{13}$, and ${\mathrm{Au}}_{13}$ that were performed using a density-functional method that employs linear combinations of pseudoatomic orbitals as basis sets (SIESTA). Our structural results for ${\mathrm{Pt}}_{13}$, ${\mathrm{Cu}}_{13}$, ${\mathrm{Ag}}_{13}$, and ${\mathrm{Au}}_{13}$ show that a buckled biplanar structure (BBP) is more stable than the icosahedral configuration, in keeping with results obtained recently by Chang and Chou [Phys. Rev. Lett. 93, 133401 (2004)] using the Vienna ab initio simulation package with a plane-wave basis. However, for ${\mathrm{Ni}}_{13}$ and ${\mathrm{Pd}}_{13}$ we found that the icosahedral structure is more stable than BBP. For all these clusters, two semiempirical methods based on spherically symmetric potentials both found the icosahedral structure to be the more stable, while the modified embedded atom model method, which uses a direction-dependent potential, found BBP to be the more stable structure. When low-energy structures found in recent ab initio studies of ${\mathrm{Pt}}_{13}$, ${\mathrm{Cu}}_{13}$, and ${\mathrm{Au}}_{13}$ other than Chang and Chou were optimized with SIESTA, those reported for ${\mathrm{Pt}}_{13}$ and ${\mathrm{Cu}}_{13}$ were found to be less stable than BBP, but the two-dimensional planar configuration reported for ${\mathrm{Au}}_{13}$ proved to be more stable than BBP.

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.

RELAXATION EFFECTS OF THE (111) SURFACE OF Si AND GaAs

The method of linear combination of pseudoatomic orbitals is applied to calculated the surface electronic structure of semiconductors- Besides the form factors of pseudo-potential, the method does not introduce any adjustable parameters. The electronic states and wave functions of the ideal and relaxed (111) surfaces of Si and GaAs arc calculated. The results of Si is compared with Appelbaum and Hamann's results of self-consistent calculation, it is found that both results are consistent well about the position of surface energy levels and distribution of surface electronic charges. It is shown that the surface energy levels shift upward when the Si or Ga atoms at the surface move inward, and shift downwarol when the As atoms at the surface move outward. At the same time the properties of the wave function of surface states often show more obvious changes.