Simple Potential Model Research Articles

MAGNETIC SUSCEPTIBILITY OF LIQUID METALS

The magnetic susceptibility of a liquid metals is calculated using the first principle Bachelet–Hamann–Schlüter (BHS) model potential and Timbie–White approach. The simple model potential has been used also in this calculations. The results obtained by the using of (BHS) potential are in good agreement with the experiment.

Pronounced in-plane anisotropy of phonon anomalies in YBa(2)Cu(3)O(6.6).

The dispersion of the Cu-O bond-stretching and bond-bending vibrations in YBa(2)Cu(3)O(6.6) has been studied by high resolution inelastic neutron scattering. While the behavior of the bond-bending vibrations can be well accounted for by a simple potential model, the bond-stretching vibrations show a highly anomalous behavior. The displacement pattern of the most anomalous phonons is in principle consistent with dynamic charge stripe formation. However, charge stripes would have to extend along the a axis and not the b axis as inferred from the magnetic fluctuations by Mook et al. [Nature (London) 404, 729 (2000)].

Experimental and theoretical investigation of microsolvation of Na+-ions in the gas phase by high resolution mass spectrometry and global cluster geometry optimization

The hydration of gas phase Na+ ions produced by electrospray ionization was investigated via high resolution time-of-flight mass spectrometry. Water clusters with up to 80 H2O units have been observed with attention to special peculiarities of mass peaks and structures in the envelope of the mass spectra. For solvated Na+-ions no particularly prominent peaks corresponding to “magic numbers” have been observed in the spectra. In addition, the mass spectra of Na+(H2O)n aggregates are compared with those of the H3O+(H2O)n system. As a very first step towards a theoretical understanding of the intensities of the experimental mass spectra we have calculated global minimum structures for the Na+(H2O)n system with n=4–25, using global geometry optimization methods and a simple model potential for this system. Structural and energetic trends and the systematic build-up of solvent shells of the clusters were studied. Within our simple theoretical model we have not observed (symmetric) clathratelike structures with a central Na+-ion inside as global minimum structures. Instead, a structural transition for n between 17 and 18 water units was found, in agreement with the experimental observations. The unimportance of clathratelike structures and the competition between the two structural principles discovered in this work are proposed as an explanation for the notorious lack of “magic” numbers in the mass spectrum of Na+(H2O)n clusters.

Lifetimes of metastable spherical carbon cluster dianions

Semiclassical calculations are made of the lifetimes of metastable spherical carbon cluster dianions using a model potential based on electrostatics. The metastability is attributed to shape resonances resulting from the combination of the long range Coulomb repulsion and the shorter range electron-molecule interaction. Results for fullerene dianions show a strong dependence of the lifetimes on the molecular size. The transition from stable to metastable dianions is estimated to occur when the radius of the spherical molecule is about 5.5 Å. This simple model potential might provide a starting point for experimental and theoretical searches for the existence of stable dianions

Core Ionization Energies, Mean Dipole Moment Derivatives, and Simple Potential Models for B, N, O, F, P, Cl, and Br Atoms in Molecules

Simple potential models relating experimental 1s electron ionization energies for B, N (sp and sp3 hybrids), O, and F atoms; 1s and 2p ionization energies for P atoms; and 2s and 2p ionization energies for Cl atoms as a function of their atomic mean dipole moment derivatives determined from experimental gas phase infrared fundamental band intensities are reported. Potential models using theoretical Koopmans' energies and generalized atomic polar tensor (GAPT) charges are found to form even more precise models than those using experimental data. This is expected because the potential models depend only on the electronic structures of molecules before ionization takes place and do not take into account relaxation effects. If the experimental ionization energies are adjusted by their relaxation energies, models similar to those obtained using Koopmans' energies are determined. The models permit a simple understanding of substituent effects on core ionization energies in terms of atomic charges in molecules. Most of the potential model slopes investigated are shown to be approximately proportional to the inverse atomic radii of the atom being ionized. Core-valence electron repulsion values inferred from the potential models obtained from experimental data are somewhat smaller than those calculated using Slater orbitals of isolated atoms. The potential model intercepts for 1s and 2p electrons are shown to be proportional to the square of the nuclear charge, consistent with their interpretation as core electron ionization energies of neutral atoms. 1s He, Ne, and Ar and 2p Ar, Kr, and Xe core ionization energies obey the linear relationships obtained for the model intercepts. The results suggest that mean dipole moment derivatives obtained from infrared intensities can be interpreted as atomic charges.

Resonant enhancements of high-order harmonic generation

Solving the one-dimensional time-dependent Schr\"odinger equation for simple model potentials, we investigate resonance-enhanced high-order harmonic generation, with emphasis on the physical mechanism of the enhancement. By truncating a long-range potential, we investigate the significance of the long-range tail, the Rydberg series, and the existence of highly excited states for the enhancements in question. We conclude that the channel closings typical of a short-range or zero-range potential are capable of generating essentially the same effects.

Molecular Dynamics Simulation of Metal Oxides Including Ag

AgPb 6 CO 9 crystal has been simulated by means of molecular dynamics calculational method. A new simple interatomics potential model for this crystal was developed so that the crystal structure remains quite stable for all the simulation time steps. By analysis of the mean squared displacement, we studied the atomic motions in this crystal structure in detail. Molecular Dynamics superconductivity

Chemical Order of Manganese-Antimony Liquid Alloys Obtained Experimentally by the Neutron “Null Matrix” Method

The Bhatia-Thornton concentration-concentration partial structure factor S CC ( q ) is a strong indicator of the order in liquid alloys. Manganese is one of the four metals, which has a negative neutronic scattering length, while the antimony one is positive. Thus for a defined composition (Mn 60 Sb 40 ) the linear combination of the two neutronic scattering lengths weighted by the atomic compositions is zero (zero alloy). We present here the results of neutron diffraction on the Mn 60 Sb 40 "null matrix" alloy at 950C, which is proportional to the Bhatia-Thornton S CC ( q ) ("null matrix" method). The total structure factor of Mn 40 Sb 60 at 800C has also been measured. The main peak of the experimental S CC ( q ) is a proof of a strong chemical order in this alloy. This order is confirmed in the real space by the Fourier transform of the structure factor. To interpret our experimental results, one generally uses effective potentials determined with the pseudopotential formalism. But transition metal pseudopotentials are not easy to handle especially in alloys. We used different simple effective potential models: hard spheres with constant diameters, hard spheres with composition dependent diameters, shouldered hard spheres models. We first show that the hard sphere model cannot reproduce the experimental results if we postulate that the hard sphere diameters do not change on alloying. Then we fit the hard sphere diameters on the experimental structure factor of the alloy. This schema can correspond physically to a charge transfer between the two components. We show that it can no more explain the experiment. The addition of attractive and (or) repulsive contributions to the different interatomic potentials give satisfactory results. The best results are obtained with attractive contributions between unlike atoms and repulsive ones between identical atoms. We have also shown that the parameters obtained for Mn 60 Sb 40 give also good results for the structure factor at a second composition: Mn 40 Sb 60 .

Surface Spectroscopy Utilizing Field Electron Emission from Thermal-field Treated Tips in STM

We have demonstrated the potentials of a field emission scanning tunneling microscope (FE-STM), in which the STM tip can be operated as a stable field electron emission gun. The tips were treated by a thermal-field (T-F) method for remolding and cleaning. A well-controlled T-F treated tip can determine the electric field between tip and sample in the STM as a geometrical boundary condition. Using the treated tip the electron standing waves excited in a vacuum gap between tip and sample were analyzed, and the electric field near sample surfaces was evaluated by assuming a simple potential model. Furthermore, by irradiating sample surfaces with a high-energy electron beam emitted from the tip, we obtained electron energy loss spectra (EELS) and Auger electron spectra from semiconductors, HOPG and metals.

Model Potential Approach to Core Polarisation in SCF Calculations

The paper discusses the applicability of simple model potentials used to describe the valence-core electron correlation (core polarisation in a semiclassical picture) within fully relativistic self-consistent-field Dirac-Fock calculations. The influence of core polarisation on oscillator strengths is demonstrated for medium and heavy systems with one and two valence electrons and is compared with other correlation effects (e.g. intravalence correlation), where applicable. Different forms of core polarisation potential and induced transition dipole moment are presented and discussed, together with limitations of the core polarisation picture. Though the emphasis is on the use of core polarisation potentials within SCF calculations, these potentials are also widely employed within a valence electron effective model potential approach. Some recent results obtained with different effective model potentials for Ag and Au isoelectronic sequences are presented and compared with SCF calculations. Finally, our earlier results obtained with the core polarisation picture are compared with the most recent unpublished results where the valence-core electron correlation (core polarisation) is explicitly represented in the configuration interaction (CI) scheme and spin-orbitals are generated by employing an ab initio model potential technique with noninteger outermost core shell occupation number, intended to improve the efficiency of the CI representation of the valence-core correlation.

Quantum, intramolecular flexibility, and polarizability effects on the reproduction of the density anomaly of liquid water by simple potential functions

The ability of simple potential functions to describe the properties of liquid water at a range of thermodynamic state points has been explored. These simple potential function models represent a water molecule by a set of sites, either rigid or flexible relative to each other, that interact with a simple, generally classical, Hamiltonian, which has parameters that are empirically determined. Calculations on several models that include intramolecular flexibility, electronic polarization or quantum mechanical effects have been performed. The consequences of altering these parameters have been systematically examined to determine factors of importance in reproducing properties of pure liquid water. It is found that simple four-site models that incorporate classical intramolecular flexibility or electronic polarization do not improve the description of the density anomaly of liquid water. Quantum statistical mechanical path integral calculations on the classical rigid nonpolarizable TIP5P model [J. Chem. Phys. 112, 8910 (2000)] and the classical flexible nonpolarizable TIP4F model indicate that although quantum mechanical effects destructure the rigid model, they improve the radial distribution and energy distribution properties of the flexible model. In addition, although quantum effects make the density behavior of the rigid model worse, they improve the density behavior of the flexible model. Path integral calculations have also been performed on quantum D2O TIP5P water; this leads to a temperature of maximum density that is higher and to a more structured liquid than results from calculations on quantum H2O TIP5P water. A similar effect is seen with calculations on a five-site rigid model, TIP5P(PIMC), which was parameterized using path integral rather than classical Monte Carlo calculations.

Influence of intermediate states on photoelectronspectra

We investigate the information contained in the photoelectronspectra (PES) of electrons emitted from an atom, molecule, orcluster under the influence of a laser pulse. Basic patterns areevaluated in a simple potential well model. The results arecorroborated by realistic applications to the cluster Na9+ calculated within a time-dependent density functionalformalism. The final PES are composed from an interplay betweenfeatures of the laser pulse and spectral properties of theemitting system. We discuss the mutual influences and thepossibilities for disentangling the various effects.

Quantum rotational states of PF3 molecule pair on Ru([formula omitted]) surface

The azimuthal angular distribution of PF3 molecules on Ru(001) could be semi-quantitatively explained down to a temperature of 20 K by considering that the molecules form a classical gear network on a triangular lattice under the influence of a simple model potential. The limitations of such a treatment are clarified by analysing the quantum rotational states of a PF3 molecule pair on Ru(001). The quantum rotational states are calculated numerically on an adiabatic potential energy surface obtained through ab initio molecular orbital (MO) calculations. The temperature at which the quantum effects become appreciable is estimated. The quality of the simple model potential is also examined in comparison with the potential obtained through MO calculations.

Computer simulation of highly compressed molecular hydrogen

The first entry in the Periodic Table, hydrogen has the simplest electronic structure, but remains one of the most intriguing elements. At low temperatures and low pressures hydrogen crystallizes as an insulating quantum molecular solid [1]. At extreme pressure electrons would no longer remain stable localized in covalent molecular bonds and hydrogen would form a dense plasma [2]. Metallization of hydrogen fluid was recently discovered in dynamic hightemperature shock-compression experiments using two-stage light-gas gun [3] at pressures above 140 GPa and temperatures near 3000 K. Absence of closed atomic electronic shells makes hydrogen extremely compressible and stable in condensed phase. Softness of intermolecular repulsion in hydrogen is very important in statistical mechanical theory. It is just what makes hydrogen different from many other substances, and therefore the well-known and useful molecular models like hard spheres or dumbbells cannot be applied to hydrogen without essential modification. Diatomic structure of H2 molecule is stable even at extreme conditions. At lower temperatures hydrogen remains diatomic up to the highest pressure reached in experiments. High-energy barriers dividing molecular and metallic phases prevent the metallization transition at least up to 250 GPa. Hydrogen solid must be compressed more then ten times [1] before chemical bonds broke and electrons delocalize. Within that range of density intermolecular and intramolecular forces, as well as intermolecular distances and equilibrium bond lengths become comparable. Notable change of vibronic frequency [1] and squeezing of chemical bonds appear at high density. These make the conventional rigid-molecule models non-applicable to highly compressed hydrogen. Without doubt, ab initio path-integral Monte Carlo simulations [4] are able, in principle, to describe the behavior of dissociating hydrogen fluid at any density from pure molecular state up to fully ionized plasma. This approach, requiring all the performance of modern supercomputers, is promising but still semi-quantitative. Our approach is based on simple atom-atom potential model for non-rigid molecules (AAP-approximation) [7], shortly outlined in the next section. It provides a satisfactory description of principal Hugoniot and double-shock experimental data up to four-time compression of liquid hydrogen and deuterium (pressure range up to 40 GPa). At higher compressions predicted pressures become significantly higher and energies essentially lower

Are lithium hydride clusters purely ionic? Study using model potentials and density-functional theory

A study of the LinHn and Lin+1Hn+ clusters with n up to 7 and Li14H13+ using both density-functional theory (DFT) and a model potential is presented. The combined use of these two methods has shown great efficiency. It has enabled us to perform exhaustive explorations of the potential energy surfaces and to study a large number of isomers. Our results show that the structures of the Li–H clusters are very close to typical ionic structures and that the Li–H bond is almost totally ionic. Moreover, the stability of the small clusters is not directly related to the number of ionic bonds formed, and families of structures exist throughout the sizes studied. Our study has also shown that a very simple ionic model potential yields good starting structures and not unreasonable energies. This simple model, however, exhibits two problems: The number of minima in the potential is too large compared to DFT, and its accuracy is by far insufficient to enable one to interpret experimental results. Finally, a good agreement is observed between B3LYP results and the available experimental data.

Mean Square Fluctuations of Hydrogen Atoms and Water-Biopolymer Interactions in Hydrated Saccharides

We have used the elastic neutron scattering technique to investigate the dynamics of the two main saccharidic components of starch: amylose and amylopectin. The measurements were carried out in the temperature range of 20 to 320 K and at different hydration levels from the dry state up to 0.47 g saccharide/g D 2O. In the dry samples, the atomic dynamics is harmonic up to approximately 300 K. In the hydrated samples a “glass-like” transition leading to an anharmonic dynamics is observed. The onset of the anharmonicity occurs at temperatures that increase from ∼180 K to 260 K upon decreasing hydration from 0.5 to 0.1 g saccharide/g D 2O. This behavior is qualitatively similar to that observed in hydrated globular proteins, but quantitative differences are present. Assuming a simple asymmetric double-well potential model, the temperature and hydration dependence of the transition have been described in terms of few physical parameters.

The structure of liquid tetrachlorides CCl4, SiCl4, GeCl4, TiCl4, VCl4, and SnCl4

Neutron diffraction measurements have been carried out for determining the total structure factor of liquid CCl4, SiCl4, GeCl4, TiCl4, VCl4, and SnCl4. The data were interpreted using the reverse Monte Carlo method, where the procedure started from results of molecular dynamics calculations. It is demonstrated that simple repulsive interatomic potential models are suitable for describing the most important structural features qualitatively. Based on detailed analyses of particle configurations, it is shown that “corner-to-face” type near-neighbor arrangements, that have been promoted for the interpretation of these structures over the last 20 years, are actually very rare, their occurrence being around 5% only. Instead, the dominance of “corner-to-corner” type orientational correlations is found.

Electronic energy spectrum of nitride single heterostructure field effect transistors

Abstract A Thomas–Fermi-based model for the electronic energy spectrum in nitride single heterostructure field effect transistors is introduced. The high values of the sheet density charges induced at an AlGaN/GaN interface guarantee the validity of the approximation. As a result, a simple analytical potential model becomes available for device designing. Single electron states contain information of the many-body Coulomb effects in the gas through the Hartree and exchange potentials. Qualitative features of the potential profile and energy spectrum are well reproduced. Comparison with reported self-consistent Schrodinger–Poisson calculations gives a very good quantitative agreement as well.

On the migdal-watson approach to FSI effects in meson production in NN collisions

The influence of the nucleon-nucleon final-state interaction (FSI) on properties of the meson-production amplitude near threshold is discussed. For nucleon-nucleon interaction, a simple Yamaguchi potential and realistic potential models are considered. It is shown that FSI effects cannot be factorized from the production amplitude. The absolute magnitude of FSI effects depends on the momentum transfer (or on the mass of the produced meson) and hence is not universal. Only in the case of the production of rather heavy mesons like η′ or φ FSI do effects become universal. The Jost function approach to FSI effects is critically examined.

Coulomb breakup of7Lifor nuclear astrophysics

A new Coulomb breakup experiment was performed for ${}^{7}\mathrm{Li}$ with an improved experimental technique and theoretical treatment. Energy spectra of $\ensuremath{\alpha}$ particles and tritons were examined to find the signature of post-Coulomb acceleration in the breakup of ${}^{7}\mathrm{Li}$ at ${E}_{\ensuremath{\alpha}t}=0.$ The data revealed the delayed nature of nonresonant breakup of astrophysical relevance that stems from quantum tunneling. Semiclassical discussions are presented of the lifetime of continuum states in ${}^{7}\mathrm{Li}$ and distortion of relative kinetic energies between $\ensuremath{\alpha}$ and t by post-Coulomb acceleration. Dynamical calculations of Coulomb breakup were performed by solving a time-dependent Schr\"odinger equation. A simple potential model of ${}^{7}\mathrm{Li}$ was employed. The dynamical calculations reasonably reproduced experimental cross sections for both resonant and nonresonant breakup with two key ingredients: higher order effects and mixture of $E1$ and $E2$ multipoles. Considering the dominant role of the first-order $E1$ nature in adiabatic Coulomb breakup, cross sections in the ${v}_{\ensuremath{\alpha}}>~{v}_{t}$ branch at $7\ifmmode^\circ\else\textdegree\fi{}$--$15\ifmmode^\circ\else\textdegree\fi{}$ for ${}^{64}\mathrm{Zn}$ and ${}^{90}\mathrm{Zr}$ were used to deduce astrophysical S factors $S(E)$ for $t(\ensuremath{\alpha},\ensuremath{\gamma}{)}^{7}\mathrm{Li}.$ They exhibit a moderate energy dependence at small energies. The strongly energy-dependent $S(E)$ resulted from the previous Coulomb breakup experiment based on cross sections with ${v}_{\ensuremath{\alpha}}<~{v}_{t};$ they are most likely Coulomb distorted and are revised in the present work.