On exchange coupling and bonding in the Gd2@C80 and Gd2@C79N endohedral dimetallo-fullerenes

A series of computational experiments performed with various methods belonging to wave-function and density functional theories approaches the issue of bonding regime and exchange coupling in the title compounds. Gd₂@C₈₀ is computed with a very weak exchange coupling, the sign depending on the method, while Gd₂@C₇₉N has resulted with a strong coupling and ferromagnetic ground state, irrespective of the computational approach. The multi-configuration calculation and broken symmetry estimation are yielding closely coincident coupling constants, of about J ～ 400 cm^?1. No experimental estimation exists, but the ferromagnetic ground state of Gd₂@C₇₉N is confirmed from paramagnetic resonance data. The different behaviour is due to particularities of electron accommodation in the orbital scheme. The exchange effects localised on atom lead to preference for parallel alignment of the electrons placed in the 4f and 5d lanthanide shells, determining also a ferromagnetic inter-centre coupling. The structural insight is completed with a ligand field analysis of the density functional theory results in the context of frozen density embedding. The energy decomposition analysis of bonding effects is also discussed. Finally, with the help of home-made codes (named Xatom+Xsphere), a model for the atom encapsulated in a cage is designed, the exemplified numeric experiments showing relevance for the considered endohedral metallo-fullerene issues.     

Surface chemistry: from vibrational spectroscopy to photoemission spectromicroscopy

Vibrational spectroscopy and core-level photoelectron spectroscopy are two of the most powerful tools for surface chemical analysis. We have used both techniques in conjunction with others to study hydrocarbon radical interactions with metal and semiconductor surfaces as well as thin film deposition processes. These include the investigations of CH₂ and CH₃ reactions on Cu by high resolution electron energy loss spectroscopy (HREELS), and GaN formation from Ga(CH₃)₃ and NH₃ on SiC by SR-PES. The vibrational analysis has revealed rich surface chemistry of the radicals while the photoemission study has led us to develop new approach for probing ultrafine structures. The effort involves the construction of a scanning photoemission spectromicroscope (SPEM) coupled to a synchrotron radiation light source. Recent results are presented in this brief report.     

A first-principle investigation of NO2 adsorption behavior on Co,Rh, and Ir-embedded graphitic carbon nitride: Looking for highly sensitive gas sensor

First-principle calculations were performed to investigate the adsorption behavior of NO₂ gas on the pristine graphitic carbon nitride (gCN) and transition metals (TM)-embedded gCN systems (TM = Co, Rh, and Ir elements) in order to explore the sensing capabilities of gCN systems as toxic gas sensor. The results of adsorption energy revealed that NO₂ gas was physisorbed on the pristine gCN, whereas this gas was strongly chemisorbed on the TM-embedded gCN. Additionally, it was found that the interaction of NO₂ gas with Ir-embedded gCN (−4.47 eV) is much higher than those of the Co and Rh-embedded systems, alluding to its suitability as a highly sensitive gas sensor. The obtained results displayed that the electronic and magnetic properties of the gCN systems remarkably modulated by chemisorption of NO₂ gas. The strong interactions between the TM-embedded gCN and NO₂ gas induced dramatic changes on the conductivity of the systems and led a large reduction in the band gap energy. The results of spin-polarized band structure and density of states indicated that with adsorption of NO₂ gas over the Rh- and Ir-embedded gCN, the magnetic moment of these systems remarkably reduced from 0.10 to 0.07 and 0.01 μB, respectively. Additionally, the results of partial density of states indicated that with adsorption of NO₂ gas over the pristine and TM-embedded gCN systems, the sharp peaks close to the Fermi energy levels of TM-embedded gCN were significantly increased in comparison with the pristine gCN, thanks to the large charge transfer from d-orbitals of the TM atoms to p-orbitals of NO₂ gas. Furthermore, the results of optimized structure showed that with embedding Co-, Rh-, and Ir-elements and also adsorption of NO₂ gas on the gCN, the initial planar structure of the pristine gCN automatically became wrinkle. Finally, based on the obtained results, it can be concluded that the high adsorption energy and considerable charge transfer between NO₂ gas and Ir-embedded gCN make this system as an excellent candidate for NO₂ gas sensor applications.     

Electron spin resonance studies of 199HgH, 201HgH, 199HgD and 201HgD isolated in neon and argon matrices at 4K: an electronic structure investigation

Various isotopomers of the mercury hydride radical (HgH) have been generated in a microwave discharge and trapped in neon and argon matrices at 4 K for electron spin resonance (ESR) investigations. Both the dipolar (A_dip) and isotropic (A_iso) components of the nuclear hyperfine interactions have been directly measured for ¹⁹⁹Hg, ²⁰¹Hg, H and D. Electronic structure information for HgH in its X²Σ ground state obtained from the hyperfine data is compared with theoretical results from several different computational methods. The hyperfine interactions in HgH are unusually large with A_iso(¹⁹⁹Hg) = 6859(3), A_dip(¹⁹⁹Hg) = 446(3), A_iso(H) = 730(2) and A_dip(H) = 0(2) MHz. A standard analysis of the hyperfine interactions demonstrates the need for a more in-depth theoretical treatment of HgH that should include relativistic effects. An interesting shift in spin density is observed when deuterium replaces hydrogen in HgH. The decreased spin density on deuterium, which was demonstrated in earlier studies, can now be more fully investigated since these new measurements confirm an associated increase in spin density on mercury     

Topological analysis of charge density in heptasulfur imide (S7NH) from isolated molecule to solid

Intra and intermolecular interactions of heptasulfur imide (S₇NH) are investigated in terms of topological properties analyses, such analyses are applied to both experimental (multipole model) and theoretically calculated (DFT and PDFT calculations) charge densities of the isolated molecule and of the crystal. The same analyses are also applied to a multipole model density obtained from theoretically (PDFT) derived structural amplitudes. The covalent bond character of S-N, N-H and S-S bonds are well described in terms of density, ρ_b, and total energy density, H_b, at the bond critical point r_c, though it is clear that the S-S bonds are weaker shared interactions than those of N-H and S-N bonds. Lone pair electron regions of sulfur and nitrogen atoms are revealed as the local charge concentration site from the Laplacian of charge density. The even weaker intermolecular interactions are well characterized; these include the N-H?S hydrogen bonding, N?S binding interactions and S?S binding interactions. All these intermolecular binding interactions are closed-shell interactions. The Laplacian of charge density demonstrates a directional intermolecular binding interaction. The corresponding intermolecular binding energies are derived by MP2/6-311+G(d,p) calculations. Atomic graph of each atom of the molecule is described in detail by the vertices, edges and faces of the polyhedron around the nucleus to illustrate such directional interactions.     

Energy barriers for trimethylaluminum reaction with varying surface hydroxyl density

Energy barriers for trimethylaluminum (TMA) reaction with varying surface hydroxyl (-OH) density were investigated using density functional theory. When the surface -OH density increased from 0 to 6.8/nm² on Si (0 0 1) surfaces, the energy barrier for TMA reaction with the surfaces decreased due to attractive interactions between TMA and -OH. When the surface -OH density further increased to 9.2/nm² on a-SiO₂ (0 0 1) surfaces, however, the trend was reversed. This was because the attractive interactions between TMA and -OH were decreased due to the attractive interactions of -OH's themselves via hydrogen bond at this high surface -OH density.     

O2、 CO2和H2O在UN(001)表面化学吸附的密度泛函理论研究

采用广义梯度近似GGA,修正Perdew-Burke-Ernzerhof交换-关联泛函,以及周期性切片模型对O₂、CO₂和H₂O在UN(001)表面的化学吸附行为进行非自旋极化水平的密度泛函理论计算. 在四个对称性化学位置条件下,对化学吸附能与分子和UN(001)表面之间距离的关系曲线进行优化. 结果表明O₂、CO₂和H₂O分子的最稳定吸附位置分别为桥式平行、空心平行和桥式H     

Density of states,charge densities and autocorrelation function of the electronic ground state in orthorhombic sulfur

The molecular orbitals of the S₈ molecule have been calculated selfconsistently without any fitting parameters using the LCAO-MO-method in the STO-4G basis. The ground state charge density, energy spectrum and autocorrelation function of the one-electron density matrix (Fourier transformed Compton profile) are presented and compared with experimental data obtained from the orthorhombic crystal. While the energy spectrum agrees with photoemission data the autocorrelation function shows a strong anisotropy between directions parallel and perpendicular to the plane of the ring. This anisotropy is interpreted in terms of intra- and intermolecular interactions.     

An alternative approach on the calculation of cohesive energy density and isothermal compressibility of alkali metal halides

In the present paper, new and useful theoretical methods for the estimation of cohesive energy density (Ced) and isothermal compressibility (k_T) of alkali metal halides are described. The mentioned theoretical methods include the use of Kaya molecular hardness equation published by us in recent years. Cohesive energy density and isothermal compressibility of alkali metal halides were calculated in the framework of mentioned theoretical methods and the results obtained were compared with both experimental data and the results of previous theoretical methods proposed to calculate the aforementioned quantities, namely cohesive energy density and isothermal compressibility. It is important to note that the results obtained in the study are in good agreement with the available experimental data and with the results of previous theoretical methods. Additionally, we also investigated the correlation with lattice energy of cohesive energy density and isothermal compressibility for alkali halides.     

TiO基态 (X 3 Δr) 的势能函数与光谱常数

应用群论及原子分子反应静力学方法推导了TiO分子基态(X³Δ_r)的离解极限.采用不同的计算方法,包括密度泛函B3LYP,B3P86,BP86,B3PW91和MP2,MP4方法,结合不同基组计算了TiO分子基态的平衡核间距、能量和振动频率.研究表明,使用B3LYP方法,对O原子使用6-311+G基组,Ti原子使用6-311+ +G^**基组时计算得到的平衡几何结构、分子离解能和谐振频率与实验值符合得最好.使用优选出的方法和基组对T 关键词： TiO 势能函数 光谱常数 密度泛函理论     

On the theory of interactions and thermodynamic properties of carbon atoms in hcp and fcc iron

General expressions are derived for deformational interactions of interstitial atoms in an hcp interstitial alloy MeX _c taking into account Me-X interactions of any range. These expressions are used for obtaining quantitative estimates of both deformational and chemical interactions of carbon atoms in hcp iron-carbon alloys. The results show that the interactions of interstitial atoms in hcp alloys noticeably differ from analogous interactions in cubic alloys, being more anisotropic, more extended, and more sharply oscillating with increasing distance. Analytical expressions are obtained for the thermodynamic contribution of interstitial atoms in disordered MeX _c alloys using both the simple mean field approximation and more exact cluster methods. Comparison of the results of such calculations for austenite with the results of simulation based on the Monte Carlo technique and with experimental data shows that the accuracy of the mean field approximation for MeX _c alloys is very low due to strong repulsion of neighboring interstitial atoms. On the other hand, the accuracy of cluster methods remains high for all temperatures and concentrations under consideration.     

On the possibility of investigation of the mass composition and energy spectra of primary cosmic ray (PCR) in the energy range from 1015 to 1017 eV using EAS data

Summary A method allowing one to determine the mass composition of primary cosmic radiation by means of simultaneous analysis of model and experimental data is presented in this paper. The most important part of this work is the quantitative comparison of multivariate distributions and the use of methods of nonparametric statistics for probability density estimation in a multivariate space of features. To check the method offered, events withE ₀>500TeV were generated by the Monte Carlo method. The showers generated were preliminarily processed by algorithms used in experimental data handling. The apparatus-induced distortions of the measured EAS characteristics have been taken into account. The method allows one to select an experimental event initiated by primary protons and iron nuclei with an efficiency of (70÷80)%. Also a new multivariate method of incident particle energy estimation based on the nonparametric regression is described. The method proposed, together with the above-mentioned multivariate EAS classification, allows one to determine the energy spectrum of incident protons and nuclei. Detection and investigation of the products of interaction of these particles with the atmosphere will allow us to study proton-nuclei and nuclei-nuclei interactions at energies from 10¹⁵ to 10¹⁷ eV.     

Kinetic analysis of the ionization ratio effects on a three‐species plasma

In partially ionized plasmas, the energy transferred to electrically charged species by the electromagnetic field can be partly channelized to the population of neutrals, due to interspecies collisional processes. Depending on the relative density of neutrals, these effects may govern the collective plasma dynamics by drastically modifying particle dynamics and energy‐transport processes with respect to the fully ionized plasma‐approximation models. In this work, the influence of the ionization ratio r_i on a partially ionized plasma is analysed by means of a three‐species one‐dimensional kinetic model to compute transient and steady state velocity‐dependent distribution functions. The conservative collision operators accounting for charge–charge and charge–neutral interactions allow studying several plasma scenarios with the same entire number of particles per unit of volume but for an increasing r_i parameter, in the presence of a modulated signal‐like electric field. For a sequence of plasma scenarios of fixed r_i, ranging from typical weakly ionized to highly ionized plasma values r_i ～ 10^?7–10^?4, the mass species flows are examined. These flows behave linearly with respect to r_i up to a value r_i ? 10^?5 from which the quasi‐linear dependence is critically altered. The convection–diffusion equations are solved with the semianalytical Propagator Integral Method, which behaves well to deal with conservative operators, density, and field discontinuities, allowing for the use of collision terms of disparate time and spatial characteristic scales. The results can be relevant to a wide class of plasma systems and to analyse the ionization ratio effects on transport coefficients.     

The nature of hydrogen bonding in R22(8) crystal motifs – a computational exploration

R²₂(8), a commonly occurring motif in organic crystals, has been examined here through ab initio and density functional theory (DFT) methods. The 11 R²₂(8) motifs reported by Allen et al. have been classified into five types; their structural features, hydrogen-bonding patterns and the kind of interactions stabilising these motifs have been studied. Results reveal that the electronegativity of donor atoms plays a major role in directing the hydrogen bonds, whereas their positions in the motif have been found to be less important. Quantum theory of atoms in molecules (QTAIM) and reduced density gradient-based non-covalent-interaction analyses have been used to understand the weak interactions between monomers. Homonuclear interactions within the motifs have been found to be stronger with higher covalent character at the bond critical points than heteronuclear interactions. In addition, a localised molecular orbital energy decomposition analysis (LMOEDA) has been accomplished to provide useful insight into various long- and short-range interactions that contribute to the total stabilisation energies. The stabilising n → σ* interactions have been explained using natural bond orbital (NBO) analysis. Overall, this study provides the essential criteria for an organic crystal to be in an R²₂(8) motif and further discusses the different combinations of hydrogen-bonding features within the R²₂(8) motifs.     

A density functional theory study on the adsorption of CO and O2 on Cu-terminated Cu2O （111） surface

The adsorptions of CO and 02 molecules individually on the stoichiometric Cu-terminatcd Cu20 （111） surface are investigated by first-principles calculations on the basis of the density functional theory. The calculated results indicate that the CO molecule preferably coordinates to the Cu2 site through its C atom with an adsorption energy of-1.69 eV, whereas the 02 molecule is most stably adsorbed in a tilt type with one O atom coordinating to the Cu2 site and the other O atom coordinating to the Cul site, and has an adsorption energy of -1.97 eV. From the analysis of density of states, it is observed that Cu 3d transfers electrons to 2π orbital of the CO molecule and the highest occupied 5σ orbital of the CO molecule transfers electrons to the substrate. The sharp band of Cu 4s is delocalized when compared to that before the CO molecule adsorption, and overlaps substantially with bands of the adsorbed CO molecule. There is a broadening of the 2π orbital of the 02 molecule because of its overlapping with the Cu 3d orbital, indicating that strong 3d-2π interactions are involved in the chemisorption of the 02 molecule on the surface.     

Hierarchical fractional-step approximations and parallel kinetic Monte Carlo algorithms

We present a mathematical framework for constructing and analyzing parallel algorithms for lattice kinetic Monte Carlo (KMC) simulations. The resulting algorithms have the capacity to simulate a wide range of spatio-temporal scales in spatially distributed, non-equilibrium physiochemical processes with complex chemistry and transport micro-mechanisms. Rather than focusing on constructing exactly the stochastic trajectories, our approach relies on approximating the evolution of observables, such as density, coverage, correlations and so on. More specifically, we develop a spatial domain decomposition of the Markov operator (generator) that describes the evolution of all observables according to the kinetic Monte Carlo algorithm. This domain decomposition corresponds to a decomposition of the Markov generator into a hierarchy of operators and can be tailored to specific hierarchical parallel architectures such as multi-core processors or clusters of Graphical Processing Units (GPUs). Based on this operator decomposition, we formulate parallel Fractional step kinetic Monte Carlo algorithms by employing the Trotter Theorem and its randomized variants; these schemes, (a) are partially asynchronous on each fractional step time-window, and (b) are characterized by their communication schedule between processors.The proposed mathematical framework allows us to rigorously justify the numerical and statistical consistency of the proposed algorithms, showing the convergence of our approximating schemes to the original serial KMC. The approach also provides a systematic evaluation of different processor communicating schedules. We carry out a detailed benchmarking of the parallel KMC schemes using available exact solutions, for example, in Ising-type systems and we demonstrate the capabilities of the method to simulate complex spatially distributed reactions at very large scales on GPUs. Finally, we discuss work load balancing between processors and propose a re-balancing scheme based on probabilistic mass transport methods.     

Super-resolved parallel MRI by spatiotemporal encoding

Recent studies described an “ultrafast” scanning method based on spatiotemporal (SPEN) principles. SPEN demonstrates numerous potential advantages over EPI-based alternatives, at no additional expense in experimental complexity. An important aspect that SPEN still needs to achieve for providing a competitive ultrafast MRI acquisition alternative, entails exploiting parallel imaging algorithms without compromising its proven capabilities. The present work introduces a combination of multi-band frequency-swept pulses simultaneously encoding multiple, partial fields-of-view, together with a new algorithm merging a Super-Resolved SPEN image reconstruction and SENSE multiple-receiving methods. This approach enables one to reduce both the excitation and acquisition times of sub-second SPEN acquisitions by the customary acceleration factor R, without compromises in either the method’s spatial resolution, SAR deposition, or capability to operate in multi-slice mode. The performance of these new single-shot imaging sequences and their ancillary algorithms were explored and corroborated on phantoms and human volunteers at 3 T. The gains of the parallelized approach were particularly evident when dealing with heterogeneous systems subject to major T₂/T₂* effects, as is the case upon single-scan imaging near tissue/air interfaces.     

银团簇上吸附氧原子与氢气作用的研究

The interactions between Ag_nO^-(n=1-8) and H₂ (or D₂) were explored by combination of the mass spectroscopy experiments and density function theory (DFT) calculations. The experiments found that all oxygen atoms in Ag_nO^-(n=1-8) are inert in the interactions with H₂ or D₂ at the low temperature of 150 K, which is in contrast to their high reactivity with CO under the same condition. These observations are parallel with the preferential oxidation (PROX) of CO in excess hydrogen catalyzed by dispersed silver species in the condensed phase. Possible reaction paths between Ag_nO^-(n=1-8) and H₂ were explored using DFT calculations. The results indicated that adsorption of H₂ on any site of Ag_nO^-(n=1-8) is extremely weak, and oxidation of H₂ by any kind of oxygen in Ag_nO^-(n=1-8) has an apparent barrier strongly dependent on the adsorption style of the "O". These experiments and theoretical results about cluster reactions provided molecule-level insights into the activity of atomic oxygen on real silver catalysts.     

Multiple-scattering calculations for 1s photoelectron angular distributions from single oriented molecules in the energy region above 50 eV

1s photoelectron angular distributions from fixed-in-space CO₂, NO₂, BF₃ and CH₃F molecules have been calculated by X-ray photoelectron diffraction (XPD) theory with muffin-tin-type molecular potential. For all the molecules, the calculated results show good agreements with those by density functional theory in the energy region ?100 eV. Furthermore, for all the molecules experimental data on the angular distributions in such energy region are well reproduced by the XPD theory. These intensive studies lead to a rather general rule that the XPD theory is an adequate tool to describe high-energy photoelectron angular distributions for any single oriented molecules.