An efficient method for accurate evaluation of the site–site direct correlation functions of molecular fluids

The direct correlation function (DCF) plays an important role in liquid integral-equation theories and non-mean-field applications of the classical density functional theory (DFT). While for a simple fluid the DCF can easily be calculated from the radial distribution function via the Fourier transform and/or, for special cases, can be derived from analytical solutions of the Ornstein–Zernike equation, computation of the site–site DCFs of a molecular fluid is more challenging because of numerical issues associated with solving the matrix integral equations. This paper describes a new theoretical method for accurate evaluation of the site–site DCFs of molecular fluids by combination of molecular simulation and analytical asymptotic analysis. The computational procedure entails four steps: (1) molecular simulation is used to calculate the site–site total correlation functions (TCFs) in real space; (2) the reference-interaction-site model (RISM) is used to calculate the site–site DCFs in Fourier space at large wavenumbers; (3) asymptotic expressions are derived for the TCFs and DCFs in the limit of small wavenumbers; and (4) site–site DCFs over the entire range are obtained by interpolation of the asymptotic results. The numerical procedure has been illustrated by application to bulk SPC/E water. Accurate evaluation of the site–site DCFs for water lays a foundation for future applications of the DFT to aqueous systems with atomic details.     

Substitutional adsorption of K on Fe(100)

The adsorption of potassium on Fe(100) was studied by time-of-flight forward scattering and recoiling spectroscopy (TOF-SARS), low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). After heating to 650 K of the potassium saturated surface the formation of a p(3 × 3) potassium superstructure was observed by LEED. TOF-SARS experiments ruled out the adsorption of potassium in the on-top, bridge and four-fold hollow site. The only site which is in agreement with all experimental results is the substitutional site where K replaces an Fe atom of the topmost layer of the crystal. This is the first time a substitutional adsorption site has been found on a bcc surface. On an fcc surface such an adsorption site has been found recently for adsorption of sodium and potassium on Al(111).     

Splitting of vibrational mode of CO isotopic species in the unstable trapping site in argon matrix

FTIR spectra of the antisymmetric stretch and symmetric bending vibrational modes of isotopic species of CO₂ trapped in argon matrix are recorded at 5 K with a resolution of . A splitting of the mode is observed for the different species in the trapping site termed unstable. Considering this splitting and the width of the line observed for this unstable site, one expects this site to be larger than the stable one. A theoretical model is elaborated to interpret this splitting. Calculations performed at harmonic level of approximation for vibrational modes in a single substitutional site (S1) and a double substitutional one (S2) in a fcc distorted argon lattice to minimize the free energy of the molecule-matrix system, show the splitting to occur in the larger site. The latter result leads us to invert former attribution of the sites: the stable site is a single substitutional one, whereas the unstable site is a double substitutional one. Received: 27 January 1998 / Revised: 30 July 1998 / Accepted: 17 August 1998     

铝和银在铱（111）宽范围吸附的稳定性、原子构型及电子特性研究

利用第一性原理密度泛函理论研究了铝和银在铱的111面的宽范围吸附特性。基于密度泛函理论计算了覆盖度在0.11ML到2.00ML的结构稳定性、原子构型及平均结合能。对于铝原子在铱111面的吸附,最稳定的结构是铝原子覆盖度为0.5ML位于密堆六方空位（hcp-hollow）,相应的结合能为-4.68eV;对于亚层铝原子的吸附,最稳定结构是铝原子覆盖度为1.00ML时位于octahedral位置,相应的结合能为-5.28eV。对于覆盖度为2.00ML的满覆盖度混合结构的表层及亚层吸附,最稳定结构是Al位于六方密堆及八方密堆位置,相应的结合能为-4.70eV。这意味着当铝原子以满覆盖度吸附在铱的111面上时,趋向于在铱的111面的亚层形成化学键,而非吸附于表层。相比于铝吸附在铱111面,银的吸附特性呈现出很大的不同,面心位置更为稳定,在覆盖度为0.25ML时其结合能为3.89eV,略微高出密堆六方位置处3.88eV的结合能。     

Density of states of the binary alloy in the coherent potential approximation

The density of states of an electron in a binary alloy in the tight binding model is calculated in the single site coherent potential approximation (CPA) as a function of the concentration and the site energy difference. The fluctuations in the site energies due to the random environment is taken into account approximately by giving width to the site energy probability distribution function, which is normally a sum of two delta functions with proper weight factor.     

铝和银在铱（111）宽范围吸附的稳定性、原子构型及电子特性研究

利用第一性原理密度泛函理论研究了铝和银在铱的111面的宽范围吸附特性。基于密度泛函理论计算了覆盖度在0.11ML到2.00ML的结构稳定性、原子构型及平均结合能。对于铝原子在铱111面的吸附,最稳定的结构是铝原子覆盖度为0.5ML位于密堆六方空位（hcp-hollow）,相应的结合能为-4.68eV;对于亚层铝原子的吸附,最稳定结构是铝原子覆盖度为1.00ML时位于octahedral位置,相应的结合能为-5.28eV。对于覆盖度为2.00ML的满覆盖度混合结构的表层及亚层吸附,最稳定结构是Al位于六方密堆及八方密堆位置,相应的结合能为-4.70eV。这意味着当铝原子以满覆盖度吸附在铱的111面上时,趋向于在铱的111面的亚层形成化学键,而非吸附于表层。相比于铝吸附在铱111面,银的吸附特性呈现出很大的不同,面心位置更为稳定,在覆盖度为0.25ML时其结合能为3.89eV,略微高出密堆六方位置处3.88eV的结合能。     

Chemisorption of Au on Si（001） surface

The chemisorption of one monolayer of Au atoms on an ideal Si(001) surface is studied by using the self-consistent tight binding linear muffin-tin orbital method. Energies of the adsorption system of a Au atom on different sites are calculated. It is found that the most stable position is A site (top site) for the adsorbed Au atoms above the Si(001) surface. It is possible for the adsorbed Au atoms to sit below the Si(001) surface at the B_1 site(bridge site), resulting in a Au－Si mixed layer. This is in agreement with the experiment results. The layer projected density of states is calculated and compared with that of the clean surface. The charge transfer is also investigated.     

Adsorption of carbon monoxide on Pd(3 1 1) and (2 1 1) surfaces

Adsorption of carbon monoxide on Pd(3 1 1) and (2 1 1) stepped surfaces has been investigated by the extended London-Eyring-Polyani-Sato (LEPS) method constructed using a 5-parameter Morse potential. The calculated results show that there exist common characteristics of CO adsorption on the two surfaces. At low coverage, CO occupies threefold hollow site of the (1 1 1) terrace and is tilted with respect to the surface normal. Among the threefold hollow sites on the (1 1 1) terrace, the nearer the site is to the step, the greater is the influence of the step. The twofold bridge site on the (1 0 0) step is also a stable adsorption site at high coverage. Because of the different lengths of the (1 1 1) terraces, the (3 1 1) and (2 1 1) stepped surfaces have different characteristics. A number of new sites are exposed on the boundary regions, including the fourfold hollow site (H₄) of the (3 1 1) surface and the fivefold hollow site (H₅) of the (2 1 1) surface. At high coverage, CO resides in the H₅ site of the (2 1 1) surface, but the H₄ site of the (3 1 1) surface is not a stable adsorption site. This study further shows that the on-top site on the (1 0 0) step of Pd(3 1 1) is a stable adsorption site, but the same type of site on Pd(2 1 1) is not.     

Strain and interfacial engineering to accelerate hydrogen evolution reaction of two-dimensional phosphorus carbide

Hydrogen,regarded as a promising energy carrier to alleviate the current energy crisis,can be generated from hydrogen evolution reaction(HER),whereas its efficiency is impeded by the activity of catalysts.Herein,effective strategies,such as strain and interfacial engineering,are imposed to tune the catalysis performance of novel two-dimensional(2D)phosphorus carbide(PC)layers using first-principle calculations.The findings show that P site in pristine monolayer PC(ML-PC)exhibits higher HER performance than C site.Intriguingly,constructing bilayer PC sheet(BL-PC)can change the coordinate configuration of P atom to form 3-coordination-P atom(3-co-P)and 4-coordination-P atom(4-co-P),and the original activity of 3-co-P site is higher than the 4-co-P site.When an external compressive strain is applied,the activity of the 4-co-P site is enhanced whereas the external strain can barely affect that of 3-co-P site.Interestingly,the graphene substrate enhances the overall activity of the BL-PC because the graphene substrate optimizes the?GH*value of 4-co-P site,although it can barely affect the HER activity of 3-co-P site and ML-PC.The desirable properties render 2 D PC-based material promising candidates for HER catalysts and shed light on the wide utilization in electrocatalysis.     

Structural and electronic properties of boron-doped double-walled silicon carbide nanotubes

The effects of boron doping on the structural and electronic properties of (6,0)@(14,0) double-walled silicon carbide nanotube (DWSiCNT) are investigated by using spin-polarized density functional theory. It is found that boron atom could be more easily doped in the inner tube. Our calculations indicate that a Si site is favorable for B under C-rich condition and a C site is favorable under Si-rich condition. Additionally, B-substitution at either single carbon or silicon atom site in DWSiCNT could induce spontaneous magnetization.     

硅铝酸银分子筛吸附氖原子的第一性原理计算

采用基于第一性原理的密度泛函理论(DFT)和局域密度近似(LDA)方法,优化计算硅铝酸银分子筛吸附Ne原子体系的几何结构,能量,电子能带和电荷密度分布。结果表明,硅铝酸银为层状的周期结构,具有直径为a=5.390 Å的孔道。在分子晶体孔道的轴线上,桥O原子附近(I)和表面Ag+离子附近(II)的能量均有利于对Ne原子的吸附。尽管Ne(I)的能量最低,但是SiO4四面体排斥产生的能垒在动力学上不利于Ne原子的吸附。电子能带和电荷分布显示,Ne(II)原子主要受库仑极化的影响,其电子能带的能量较高,Ne(I)原子与桥O原子之间的共价作用能够降低对应的电子能带能量。     

硅铝酸银分子筛吸附氖原子的第一性原理计算

采用基于第一性原理的密度泛函理论(DFT)和局域密度近似(LDA)方法,优化计算硅铝酸银分子筛吸附Ne原子体系的几何结构,能量,电子能带和电荷密度分布。结果表明,硅铝酸银为层状的周期结构,具有直径为a=5.390 Å的孔道。在分子晶体孔道的轴线上,桥O原子附近(I)和表面Ag+离子附近(II)的能量均有利于对Ne原子的吸附。尽管Ne(I)的能量最低,但是SiO4四面体排斥产生的能垒在动力学上不利于Ne原子的吸附。电子能带和电荷分布显示,Ne(II)原子主要受库仑极化的影响,其电子能带的能量较高,Ne(I)原子与桥O原子之间的共价作用能够降低对应的电子能带能量。     

Random turn walk on a half line with creation of particles at the origin

We consider a version of random motion of hard core particles on the semi-lattice 1,2,3,… , where in each time instant one of three possible events occurs, viz., (a) a randomly chosen particle hops to a free neighboring site, (b) a particle is created at the origin (namely, at site 1) provided that site 1 is free and (c) a particle is eliminated at the origin (provided that the site 1 is occupied). Relations to the BKP equation are explained. Namely, the tau functions of two different BKP hierarchies provide generating functions respectively (I) for transition weights between different particle configurations and (II) for an important object: a normalization function which plays the role of the statistical sum for our non-equilibrium system. For time t→∞ we obtain the asymptotic configuration of particles obtained from the initial empty state (the state without particles).     

Vibrational enhancement of the effective donor-acceptor coupling

This letter deals with a simple three-site model for charge transfer phenomena in a one-dimensional donor (D)-bridge (B)-acceptor (A) system coupled with vibrational dynamics of the B site. It is found that, in a certain range of parameters, the vibrational coupling leads to an enhancement of the effective donor-acceptor electronic coupling as a result of the formation of the polaron on the B site. This enhancement of the charge transfer efficiency is maximum at the resonance, where the effective energy of the fluctuating B site coincides with the donor (acceptor) energy.     

Muon Spin Dynamics and Sites in GdNi5

In GdNi₅, the muon localizes at the 3f interstitial site and below 80K a second muon site becomes populated, the so-called 6_ring site. This ring site is metastable and the muon hops to the 3f site. We determine the mean time of stay of the muon in the ring site and show that it is governed by a multi-phonon quantum diffusion process. The coincidence energy is measured to be E _a =272(10)K and the tunneling matrix element J=0.11(2)meV.     

Sites and tours in orthoalgebras and orthomodular lattices

A block of an orthoalgebra (or of an orthomodular lattice) is a maximal Boolean subalgebra. A site is the intersection of two distinct blocks. L is block (site)-finite if there are only finitely many blocks (sites). We introduce a certain type of subalgebra of an orthoalgebra which is a subortholattice if the orthoalgebra is an ortholattice (and therefore an orthomodular lattice) and which is block finite if the orthoalgebra is site finite. The construction yields a cover of a site-finite orthoalgebra or orthomodular lattice L by block-finite substructures of the same type and having the same center as L. Every site-finite orthomodular lattice is commutator finite.In memory of Charles H. Randall.     

Adsorption of hydrogen and of oxygen on an open metal surface-HREELS investigation at Ni(311)

The adsorption behavior of hydrogen and oxygen on the stepped Ni(311) surface has been investigated by HREELS. A series of metastable phases was found for hydrogen adsorption at low temperatures with a succession of different adsorption sites indicated by the following loss peaks: 55 and 149 meV for the threefold site, shifting with higher coverage to 65 and 155 meV, respectively; 40 and 90 meV for the fourfold site, shifting to 35 and 85 meV with coverage; and 110 and 124 meV for an additional site between close packed rows. Room temperature adsorption of hydrogen leads to the reconstruction of the surface with occupation of three- and fourfold sites, represented by loss peaks at 60 and 145 meV for the threefold site and 74 meV for the fourfold site. This phase is the thermodynamically stable one. Oxygen is most likely initially adsorbed on a bridge site (loss peak at 66 meV). The stepped surface is already oxidized at very low exposures to oxygen, as seen by the characteristic vibration for oxide islands at 55 meV and later by the Fuchs-Kliewer mode of NiO at 68 meV.     

Atomic Layer Deposition of Al2O3 on H-Passivated GeSi： Initial Surface Reaction Pathways with H/GeSi（100）-2 × 1

The reaction mechanisms of Al（CH3 ）3 （TMA） adsorption on H-passivated GeSi（100）-2 × 1 surface are investigated with density functional theory. The Si-Ge and Ge-Ge one-dimer cluster models are employed to represent the GeSi（100）-2 × 1 surface with different Ge compositions. For a Si-Ge dimer of a H-passivated SiGe surface, TMA adsorption on both Si-H^＊ and Ge-H^＊ sites is considered. The activation barrier of TMA with the Si-H^＊ site （1.2eV） is higher than that of TMA with the Ge-H^＊ site （0.91 eV）, which indicates that the reaction proceeds more slowly on the Si-H^＊ site than on the Ge-H^＊ site. In addition, adsorption of TMA is more energetically favorable on the Ge-Ge dimer than on the Si-Ge dimer of H-passivated SiGe.     

A method for systematic site-to-bond conversion of directed graphs

A graph-theoretic method for the complete derivability of bond graphs from their site counterparts is described. Bond perimeter, valence, and cyclomatic number distributions as well as spatial extent measures can be systematically generated when coupled with a site valence partition in the original graph. Relevant consequences for bond configurations include the facts that (i) percolation perimeter and cyclomatic number distributions are equivalent, (ii) geometrical susceptibilities are not independent, and (iii) a critical site/bond ratio exists.     

A covalent model for the bonding of adsorbed hydrocarbon fragments on the (111) face of platinum

A systematic molecular orbital study of the location of CH_n and C-CH_n (n = 1, 2, 3) species on a Pt(111) single-crystal surface has been performed by using both a cluster model and a band structure calculation within the framework of the Extended Hückel Theory (EHT). These species may be involved in the hydrogenation/dehydrogenation processes of hydrocarbons. The observed dependence of the adsorption site on the number of hydrogens in the CH_n fragments suggests that any C-H bond breaking in CH_n species must involve a change of adsorbate bonding site. The carbon is found to be located on the surface in such a way as to complete its tetravalancy. Thus, CH occupies a three-fold coordinated hollow site, CH₂ a two-fold coordinated bridge site and CH₃ a one-fold coordinated top site. C-CH₃ is found to be perpendicular to the surface in a three-fold hollow site in agreement with experimental observations. It is also found that a displacement of C-CH₂-R to a top site makes a β C-R cleavage easier.