Electronic Structure Configuration Research Articles

Revealing Active Sites in Atomically Dispersed Metal Catalysts on N-Doped Carbon for Electrocatalytic CO2 Reduction By Operando XAS

Single-atom catalysts (SACs) represent a promising class of low-cost materials with high catalytic activity and 100% atom-utilization efficiency, widely applied in energy conversion fields.[1-2] Benefit from tunable coordination environment, the uniformly dispersed SACs on diverse supports exhibit satisfied catalytic ability. More importantly, different from the multiple crystal facets of nanomaterials can serve as active sites. There is only one active site consisting of the single metal atom and coordinated atoms in SACs, thereby improving the selectivity of products some catalytic reactions. Moreover, atomically dispersed metal catalysts exist in cations, which exhibit better stability than traditional nanoparticulated counterparts.[3] However, SACs are susceptible to undergo restructuring during the reactions because of high surface free energy. Therefore, exploring the active sites of SACs through in-situ characterization techniques plays a critical role in studying the reaction mechanism and guiding the design of optimum SACs.As a synchrotron-based element-specific technique, X-ray absorption spectroscopy (XAS) is influential in studying the probed element's structural information. X-ray absorption near edge structure (XANES) spectra reflect the electronic structure, while extended X-ray absorption fine structure (EXAFS) spectra provide local atomic structural information such as coordination number, bond length, and structural disorder. Thus, in-situ XAS is promising and widely used for monitoring electronic structure and atomic configuration changes of SACs during real-time working conditions. We have prepared Ni, Cu single atoms on nitrogen-doped carbon (NiCu@NC-900), which exhibit improved catalytic ability compared with Ni SACs and Cu SACs. Nuclear magnetic resonance results show only ethanol as a liquid product during CO2 reduction reaction (CO2RR). To explore the origin of improved catalytic performance of SACs toward CO2RR, we are going to monitor changes of SACs under open-circuit voltage (OCV) and working conditions by Operando XAS.[1] Li, Z., Ji, S., Liu, Y., Cao, X., Tian, S., Chen, Y., Niu, Z., Li, Y. Chem. Rev. 120, 2 (2020).[2] Ji, S., Chen, Y., Wang, X., Zhang, Z., Wang, D., Li, Y. Chem. Rev. 120, 21 (2020).[3] Speck, F. D., Paul, M. T. Y., Ruiz-Zepeda, F., Gatalo, M., Kim, H., Kwon, H. C., Mayrhofer, K. J. J., Choi, M., Choi, C. H., Hodnik, N., Cherevko, S. J. Am. Chem. Soc. 142, 36 (2020).

Shedding Light on the Electronic Structure and Local Configuration of Fe/N/C Active Sites – an in Situ X-Ray Absorption Spectroscopy Study

In recent years, the search for platinum group metal-free oxygen reduction reaction (ORR) catalysts as an inexpensive and sustainable replacement for state-of-the-art Pt-based materials has inspired a lot of work in the field of single-atom Fe-based catalysts (often referred to as Fe/N/C). While promising initial ORR-activities were achieved with different synthesis approaches,[1,2] tackling the lack of durability[3] will require a strategic optimization of the employed preparation routes. A comprehensive fundamental understanding of the catalysts active sites’ electronic structure and local configuration under operating conditions lies at the heart of this process and remains a subject of vivid debate. Several studies employing both ex and in situ Mössbauer and X-ray absorption spectroscopies (XAS)[4–7] have brought insights regarding these questions, but a full unambiguous conclusion has not been reached and the community is seeking for novel approaches to unravel such structure-activity and structure-stability relations.With this motivation, we have used in situ XAS to study two catalysts prepared with distinctively different synthesis approaches and widely free of detectable inorganic side phases,[5,8] and that therefore constitute ideal model systems to draw conclusions applicable to this catalyst family. We focused our attention on the spectral 1s → 3d pre-edge feature of the X-ray absorption near edge structure (XANES) spectrum that is sensitive to any changes in the local site symmetry, orbital occupancy, and spin state of the absorbing atom,[9,10] but that to the best of our knowledge has so far not been looked into in the field. In order to achieve adequate energy resolution to discern the relevant features, the spectra were acquired with a Si(311) monochromator at the SuperXAS beamline of the Swiss Light Source employing our group’s spectroelectrochemical flow cell.[11] As illustrated in Fig. 1a, pseudo-Voigt component fits allow tracking the spectral changes as a function of the applied polarization conditions, as qualitatively indicated with green arrows.When using bulk-sensitive techniques like XAS, one must bear in mind that while the catalytic process and expected site changes of interest take place only on the surface sites in contact with the electrolyte, the observed signal also contains the spectral contributions of those sites that remain unaffected by the potential. This is particularly relevant when studying such single-atom Fe-based catalysts, since it is expected that a relatively large (and hard to quantify) fraction of their sites are buried in the catalysts’ bulk. Herein we overcome this intrinsic challenge by performing modulation excitation (ME) experiments using the applied potential as a stimulus (illustrated in Fig. 1b). The technique allows to greatly enhance the signal of the species that changes with said stimulus,[12] and consequently to effectively surmount the lack of surface sensitivity described above. Furthermore, these ME experiments acquired with fluorescence-detected quick-scanning XAS[13] complemented with a multivariate curve resolution (MCR) analysis of the data provide quantitative information about the kinetics of the structural changes undergone by the sites in these Fe/N/C catalysts upon modifying the potential (Fig. 1c).In summary, this work will provide a novel perspective by exploiting the sensitivity of pre-edge XANES features and ME-experiments for the first time for the analysis of Fe/N/C-catalysts allowing insights that will broaden the greatly-needed understanding of the potential-dependent structure of the active sites.[1] E. Proietti et al., Nat. Commun. 2011, 2:416.[2] J. Shui et al., Proc. Natl. Acad. Sci. 2015, 112, 10629–10634.[3] D. Banham et al., J. Power Sources 2015, 285, 334–348.[4] U. I. Kramm et al., Phys. Chem. Chem. Phys. 2012, 14, 11673–88.[5] A. Zitolo et al., Nat. Mater. 20 15, 14, 937–42.[6] Q. Jia et al., Nano Energy 2016, 29, 65–82.[7] S. Wagner et al., Angew. Chemie Int. Ed. 2019, 58, 10486–10492.[8] S. Wagner et al., Hyperfine Interact. 2018, 239:10.[9] T. E. Westre et al., J. Am. Chem. Soc. 1997, 119, 6297–6314.[10] M. Wilke et al., Am. Mineral. 2001, 86, 714–730.[11] T. Binninger et al., J. Electrochem. Soc. 2016, 163, H913–H920.[12] A. Urakawa et al., Chem. Eng. Sci. 2008, 63, 4902–4909.[13] A. H. Clark et al., J. Synchrotron Radiat. 2020, 27, 1–8. Figure 1

Natural Orbital Branching Scheme for Time-Dependent Density Functional Theory Nonadiabatic Simulations.

Real time-time-dependent density functional theory (rt-TDDFT) has now been used to study a wide range of problems, from optical excitation to charge transfer, to ion collision, and to ultrafast phase transition. However, conventional rt-TDDFT Ehrenfest dynamics for nuclear movement lacks a few critical features to describe many problems: the detail balance between state transitions, decoherence for the wave function evolution, and stochastic branching of the nuclear trajectory. There are many-body formalisms to describe such nonadiabatic molecular dynamics, especially the ones based on mixed quantum/classical simulations, like the surface hopping and wave function collapsing schemes. However, there are still challenges to implement such many-body formalisms to the rt-TDDFT simulations, especially for large systems where the excited state electronic structure configuration space is large. Here we introduce two new algorithms for nonadiabatic rt-TDDFT simulations: the first is a Boltzmann factor algorithm which introduces decoherence and detailed balance in the carrier dynamics but uses mean field theory for nuclear trajectory. The second is a natural orbital branching (NOB) formalism, which uses a time-dependent density matrix for electron evolution and a natural orbital set to collapse the wave function upon. It provides the features of decoherence, detailed balance, and trajectory branching. We have tested these methods for a molecule radiolysis decay problem. We found that these methods can be used to study such radiolysis problems in which the molecule is broken into many fragments following complex electronic structure transition paths. The computational time of NOB is similar to that of the original plain rt-TDDFT simulations.

Comparative quantum chemistry study of the F-center in lanthanum trifluoride

In this study we report the results of first principle quantum chemical modeling of electronic structure and spatial configuration of LaF3 crystal containing the F-center. Theoretical predictions of atomic and electronic properties of LaF3 are given by means of density functional theory. From our modeling we predict that the F-center placed at fluorine ion position between the lanthanum planes at tysonite LaF3 is the most energetically stable. In order to interpret the structure of the optical absorption spectrum of X-irradiated LaF3 we performed calculations of excited and transition states of the F-center within the embedded cluster formalism using time dependent density functional theory. Predicted transition energies and oscillator strengths of LaF3 containing the F-center are in line with those recently measured.

Controllable Synthesis of Co@CoOx/Helical Nitrogen-Doped Carbon Nanotubes toward Oxygen Reduction Reaction as Binder-free Cathodes for Al-Air Batteries.

Efficient and stable electrocatalysts for oxygen reduction reaction and freestanding electrode structure were developed to reduce the use of polymer binders in the cathode of metal-air batteries. Considering the unique geometrical configurations of helical carbon nanotubes (CNTs) and improved properties compared with straight CNTs, we prepared high-purity Co@CoOx/helical nitrogen-doped carbon nanotubes (Co@CoOx/HNCNTs) on a carbon fiber paper by hydrothermal and single-step in situ chemical vapor deposition strategies. Under an optimized growth time (1 h), the synthesized Co@CoOx/HNCNTs provide richer edge defects and active sites and show prominent electrocatalytic performance toward oxygen reduction reaction (ORR) under alkaline media compared with Co@CoOx/HNCNTs-0.5 h and Co@CoOx/HNCNTs-2 h. The soft X-ray absorption spectroscopy technique is used to investigate the influences of different growth times on the electronic structure and local chemical configuration of Co@CoOx/HNCNTs. Furthermore, the Al-air coin cell employing Co@CoOx/HNCNTs-1 h as the binder-free cathode exhibits an open-circuit voltage of 1.48 V, a specific capacity of 367.31 mA h g-1 at the discharge current density of 1.0 mA cm-2, and a maximum power density (Pmax) of 3.86 mW cm-2, which are superior to those of Co@CoOx/HNCNTs-0.5 h and Co@CoOx/HNCNTs-2 h electrodes. This work provides valuable insights into the development of scalable binder-free cathodes, exploiting HNCNT composite materials with an outstanding electrocatalytic performance for ORR in Al-air systems.

Interface Engineering of Hierarchical Branched Mo‐Doped Ni3S2/NixPy Hollow Heterostructure Nanorods for Efficient Overall Water Splitting

AbstractRational design and construction of bifunctional electrocatalysts with excellent activity and durability is imperative for water splitting. Herein, a novel top‐down strategy to realize a hierarchical branched Mo‐doped sulfide/phosphide heterostructure (Mo‐Ni3S2/NixPy hollow nanorods), by partially phosphating Mo‐Ni3S2/NF flower clusters, is proposed. Benefitting from the optimized electronic structure configuration, hierarchical branched hollow nanorod structure, and abundant heterogeneous interfaces, the as‐obtained multisite Mo‐Ni3S2/NixPy/NF electrode has remarkable stability and bifunctional electrocatalytic activity in the hydrogen evolution reaction (HER)/oxygen evolution reaction (OER) in 1 m KOH solutions. It possesses an extremely low overpotential of 238 mV at the current density of 50 mA cm−2 for OER. Importantly, when assembled as anode and cathode simultaneously, it merely requires an ultralow cell voltage of 1.46 V to achieve the current density of 10 mA cm−2, with excellent durability for over 72 h, outperforming most of the reported Ni‐based bifunctional materials. Density functional theory results further confirm that the doped heterostructure can synergistically optimize Gibbs free energies of H and O‐containing intermediates (OH*, O*, and OOH*) during HER and OER processes, thus accelerating the catalytic kinetics of electrochemical water splitting. This work demonstrates the importance of the rational combination of metal doping and interface engineering for advanced catalytic materials.

Configurationally Stable Chiral Dithia-Bridged Hetero[4]helicene Radical Cation: Electronic Structure and Absolute Configuration.

A stable chiral hetero[4]helicene radical cation was synthesized and characterized by UV/Vis absorption and EPR spectroscopy, as well as X-ray crystallography. For the first time, a combination of chiroptical methods involving ECD, ORD, and VCD, supported by quantum mechanical predictions, enabled the elucidation of the absolute configuration of such open-shell helical species.

Electronic Structure and Spin Configuration Trends of Single Transition Metal Impurity in Phase Change Material

Fe doped GST has shown experimentally the ability to alter its magnetic properties by phase change. In this work, we use screened exchange hybrid functional to study the single neutral substitutional 3d transition metal (TM) in crystalline GeTe and GeSb2Te4. By curing the problem of local density functional (LDA) such as over delocalization of the 3d states, we find that Fe on Ge/Sb site has its majority d states fully occupied while its minority d states are empty, which is different than previous predicted electronic configuration by LDA. From early transition metal Cr to heavier Ni, the majority 3d states are gradually populated until fully occupied and then the minority 3d states begin to be filled. In order to study the magnetic contrast, we use lower symmetry crystalline GeTe and GeSb2Te4 as the amorphous phases, respectively, which has been proposed to model the medium range disordering. We find that only Co substitution in r-GeSb2Te4 and s-GeSb2Te4 shows magnetic contrast. The experimental magnetic contrast for Fe doped GST may be due to additional TM-TM interaction, which is not included in our model. It can also be possible that these lower symmetry crystalline models are not sufficient to characterize the magnetic properties of real 3d TM doped amorphous GST.

Highly active nickel-cobalt/nanocarbon thin films as efficient water splitting electrodes.

Developing low cost, highly active and stable electrocatalysts for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) using the same electrolyte has remained a major challenge. Herein, we report a novel and robust material comprised of nickel-cobalt nanoparticles coated on a porous nitrogen-doped carbon (NC) thin film synthesized via a two-step pulsed laser deposition technique. The optimized sample (Ni0.5Co0.5/NC) achieved the lowest overpotentials of 176 mV and 300 mV at a current density of 10 mA cm-2 for HER and OER, respectively. The optimized OER activity might be attributed to the available metal oxide nanoparticles with an effective electronic structure configuration and enhanced mass/charge transport capability. At the same time, the porous nitrogen doped carbon incorporated with cobalt and nickel species can serve as an excellent HER catalyst. As a result, the newly developed electrocatalysts manifest high current densities and strong electrochemical stability in overall water splitting, outperforming most of the previously reported non-precious metal-based catalysts.

Modern ab initio valence bond theory calculations reveal charge shift bonding in protic ionic liquids.

The nature of bonding interactions between the cation and the anion of an ionic liquid is at the heart of understanding ionic liquid properties. A particularly interesting case is a special class of ionic liquids known as protic ionic liquids. The extent of proton transfer in protic ionic liquids has been observed to vary according to the interacting species. Back proton transfer renders protic ionic liquids volatile and to be considered as inferior ionic liquids. We try to address this issue by employing modern ab initio valence bond theory calculations. The results indicate that the bonding in the cation and the anion of a prototypical ionic liquid, ethylammonium nitrate, is fundamentally different. It is neither characteristic of covalent/polar covalent bonding nor ionic bonding but rather charge shift bonding as a resonance hybrid of two competing ionic molecular electronic structure configurations. An investigation of other analogous protic ionic liquids reveals that this charge shift bonding seems to be a typical characteristic of protic ionic liquids while the ionic solid analogue compound ammonium nitrate has less charge shift bonding character as compared to protic ionic liquids. Further the extent of charge shift bonding character has been found to be congruent with the trends in many physicochemical properties such as melting point, conductivity, viscosity, and ionicity of the studied ionic liquids indicating that percentage charge shift character may serve as a key descriptor for large scale computational screening of ionic liquids with desired properties.

Nanoparticle Superlattices as Efficient Bifunctional Electrocatalysts for Water Splitting.

The solar-driven water splitting process is highly attractive for alternative energy utilization, while developing efficient, earth-abundant, bifunctional catalysts for both oxygen evolution reaction and hydrogen evolution reaction has remained as a major challenge. Herein, we develop an ordered CoMnO@CN superlattice structure as an efficient bifunctional water-splitting electrocatalyst, in which uniform Co-Mn oxide (CoMnO) nanoparticles are coated with a thin, continuous nitrogen-doped carbon (CN) framework. The CoMnO nanoparticles enable optimized OER activity with effective electronic structure configuration, and the CN framework serves as an excellent HER catalyst. Importantly, the ordered superlattice structure is beneficial for enhanced reactive sites, efficient charge transfer, and structural stability. This bifunctional superlattice catalyst manifests optimized current densities and electrochemical stability in overall water splitting, outperforming most of the previously reported single- or bifunctional electrocatalysts. Combining with a silicon photovoltaic cell, this CoMnO@CN superlattice bifunctional catalyst enables unassisted solar water splitting continuously for ∼5 days with a solar-to-hydrogen conversion efficiency of ∼8.0%. Our discovery suggests that these transition metal oxide-based superlattices may serve as a unique structure modality for efficient bifunctional water splitting electrocatalysts with scale-up potentials.

Atomistic study of LaNbO4; surface properties and hydrogen adsorption

We have calculated fundamental properties of pure and hydrogen-covered (010), (101), (100) and (001) surfaces of the low temperature monoclinic phase of LaNbO4 (LN). The (010) surface was the most stable one, exhibiting electronic structure and local geometric configurations similar to bulk. As the first stage of proton migration into the electrolyte, the ability of LN surfaces to split H2 molecules was probed indirectly by calculating the adsorption energy of H atoms on two of the LN surfaces. H adsorption on the (010) surface was found to be strongly endothermic, and thus cannot contribute much in splitting H2. The adsorption energy on the relatively unstable (101) surface was on the other hand approximately −0.6 eV, in the right range for surface H2 to be catalyzed beneficially. H adsorption on this surface was induced by surface states in the band gap of the clean surface. Since the unstable (101) surface is not abundant, the rate of dissociative adsorption of H2 on the LN surface can be anticipated to be very low. Application of the energies to simple adsorption isotherm calculations for typical proton conducting fuel cells (PCFCs) operating temperatures correspondingly showed very low H coverage, and it is not expected that LaNbO4 surfaces can contribute much to the H2 activation reaction of a PCFC anode.

Electronic Structure and Bonding of an Ionic Molecular Adsorbate: c-C5H5 on Cu{111}

Self-assembled monolayers containing conjugated π systems find application in organic electronics to functionalize and modify the electronic properties of metals and metal oxides. Isolated cyclopentadienyl is an aromatic molecular anion similar in size to benzene that, unlike benzene, adsorbs quite strongly even on coinage metal surfaces. In this study, the electronic structure, bonding, and minimum energy configuration of cyclopentadienyl (c-C5H5 or Cp) adsorbed on Cu{111} are calculated via first-principles density functional theory (DFT). The Cu{111} surface has been modeled within a (2√3 × 2√3)R30° cell, and the adsorbed Cp has been found to reside preferentially on the hollow sites, with a binding energy of 1.73 eV. Electronic population analysis reveals a net charge transfer of ∼1.1 electrons from the metal to the Cp, indicating that the adsorption is dominated by ionic bonding. The surface diffusion barrier between two adjacent hollow sites was calculated to be 55 meV, in good agreement with previously reported measurements by helium spin echo (HeSE) spectroscopy. It was found that lateral interactions do not significantly influence the binding energy and mobility of the adsorbate. The physical–chemical properties of this strongly bound but weakly mutually interacting molecular adsorbate suggest that Cp could become a model system for ionically adsorbed molecular adsorbates.

Magnetic field-induced martensite–austenite transformation in Fe-substituted NiMnGa ribbons

We report As-75 nuclear quadrupole resonance (NQR) studies on oxypnictide superconductors LaFeAsO1-xFx (x=0.08, 0.15) and LaNiAsO1-xFx (x=0, 0.06, 0.10, 0.12). In LaFeAsO0.92F0.08 (T-c=23 K), nuclear spin-lattice relaxation rate 1/T-1 shows no coherence peak just below T-c and decreases with decreasing temperature accompanied by a hump structure at T similar to 0.4T(c), which is a characteristic of the multigap superconductivity. In the normal state, the quantity 1/T1T increases with decreasing temperature to T-c, indicating that the existence of antiferromagnetic correlation originating from its multiple electronic band structure. On the other hand, LaNiAsO1-xFx shows a clear Hebel-Slichter (coherence) peak just below T-c, evidencing that the LaNiAsO1-xFx is a BCS superconductor. In the normal state, 1/T-1 T is constant in the temperature range 4 <= T <= 10 K for all LaNiAsO1-xFx, which indicates electron correlations are weak. We suggest that the contrasting behavior of both superconductivity and electron correlations in LaFeAsO0.92F0.08 and LaNiAsO1-xFx between them relate to the difference of electronic band structure configuration. We also provide a possible interpretation for the pseudogap-like behavior in the normal state observed in both compounds. (C) 2010 Elsevier Ltd. All rights reserved.

Electronic Structure and Bonding Configuration of Histidine on Ge(100)

The electronic structures and bonding configuration of histidine on Ge(100) have been investigated with various sample treatments using core-level photoemission spectroscopy (CLPES). Interpretation of the Ge 3d, C 1s, N 1s, and O 1s core level spectra being included in these systems revealed that both the imino nitrogen in the imidazole ring and the carboxyl group in the glycine moiety concurrently participate in the adsorption of histidine on a Ge(100) surface at 380 K. Moreover, we could clearly confirm that the imino nitrogen with a free lone pair in the imidazole group adsorbs on Ge(100) more strongly than the carboxyl group in the glycine moiety by examining systems annealed at various temperatures.

As-NQR study of superconductivity in LaRAsO1−xFx (R=Fe and Ni)

We report 75As nuclear quadrupole resonance (NQR) studies on oxypnictide superconductors LaFeAsO1−xFx (x=0.08, 0.15) and LaNiAsO1−xFx (x=0, 0.06, 0.10, 0.12). In LaFeAsO0.92F0.08 (Tc=23K), nuclear spin-lattice relaxation rate 1/T1 shows no coherence peak just below Tc and decreases with decreasing temperature accompanied by a hump structure at T∼0.4Tc, which is a characteristic of the multigap superconductivity. In the normal state, the quantity 1/T1T increases with decreasing temperature to Tc, indicating that the existence of antiferromagnetic correlation originating from its multiple electronic band structure. On the other hand, LaNiAsO1−xFx shows a clear Hebel–Slichter (coherence) peak just below Tc, evidencing that the LaNiAsO1−xFx is a BCS superconductor. In the normal state, 1/T1T is constant in the temperature range 4≤T≤10K for all LaNiAsO1−xFx, which indicates electron correlations are weak. We suggest that the contrasting behavior of both superconductivity and electron correlations in LaFeAsO0.92F0.08 and LaNiAsO1−xFx between them relate to the difference of electronic band structure configuration. We also provide a possible interpretation for the pseudogap-like behavior in the normal state observed in both compounds.

Response of Mn overlayers on Fe to external magnetic fields: Electronic structure calculations

The behavior of Mn overlayers on Fe(0 0 1) under the influence of external magnetic fields is investigated. The electronic charge distribution, local magnetic moments as well as their couplings are determined as a function of the external field by solving self-consistently a tight binding Hamiltonian, parameterized to ab initio TBLMTO calculations. Our method allows to trace back the field-dependent average magnetization of the system to its electronic structure and magnetic configuration. We show how in the non-collinear framework the response of the system is markedly different to what is found in the collinear framework. If metastable magnetic configurations exist, the external field can be used for tuning the system between some of them because the system stays in some of those metastable states even after switching off the external field.

Gradient-based direct normal-mode analysis

A formulation of a direct, iterative method for obtaining the lowest eigenvalues and eigenvectors of a Hessian matrix is presented. Similar to the iterative schemes in electronic structure configuration interaction calculations (methods due to Lanczos, Davidson, and others), the mass-weighted Hessian matrix K is not constructed explicitly; instead, its operation on a basis vector (a direction in the 3N Cartesian configuration space of the atoms) is computed based on the principles of dynamical equations of motion. By noting that the time derivative of the gradient vector in the harmonic force field is related to the particles' momenta via dg/dt = Kp, a Hessian-vector product can be computed on the fly by finite differentiation of the gradient along the direction specified by the p vector. Thus, only two evaluations of the gradient are required per Davidson-like iteration per root, which leads to a linear scaling behavior of the computational effort with the number of atoms (provided that the evaluation of the gradient scales linearly). Preliminary results are presented for a 27,000-atom 4He nanodroplet.

A unified scheme for ab initio molecular orbital theory and path integral molecular dynamics

We present a general approach for accurate calculation of chemical substances which treats both nuclei and electrons quantum mechanically, adopting ab initio molecular orbital theory for the electronic structure and path integral molecular dynamics for the nuclei. The present approach enables the evaluation of physical quantities dependent on the nuclear configuration as well as the electronic structure, within the framework of Born–Oppenheimer adiabatic approximation. As an application, we give the path integral formulation of electric response properties—dipole moment and polarizability, which characterize the changes both in electronic structure and nuclear configuration at a given temperature when uniform electrostatic field is present. We also demonstrate the calculation of a water molecule using the present approach and the result of temperature and isotope effects is discussed.

Coulomb correlation and magnetic ordering in double-layered manganites: LaSr 2Mn 2O 7

A detailed study of the electronic structure and magnetic configurations of the 50% hole-doped double-layered manganite LaSr 2Mn 2O 7 is presented. We demonstrate that the on-site Coulomb correlation ( U) of Mn d electrons (i) significantly modifies the electronic structure, magnetic ordering (from FM to AFM), and interlayer exchange interactions, and (ii) promotes strong anisotropy in electrical transport, reducing the effective hopping parameter along the c-axis for electrically active e g electrons. This finding is consistent with observations of anisotropic transport—a property which sets this manganite apart from conventional 3D systems. A half-metallic band structure is predicted with both the LSDA and LSDA+U methods. The experimentally observed A-type AFM ordering in LaSr 2Mn 2O 7 is found to be energetically more favorable with U⩾7 eV. A simple interpretation of interlayer exchange coupling is given within double and super-exchange mechanisms based on the dependencies on U of the effective exchange parameters and e g state sub-band widths.