Model Potential Calculations Research Articles

Model dependence of theH2electric dipole moment

Background: Direct measurement of the electric dipole moment (EDM) of the neutron is in the future; measurement of a nuclear EDM may well come first. The deuteron is one nucleus for which exact model calculations are feasible. Purpose: We explore the model dependence of deuteron EDM calculations. Methods: Using a separable potential formulation of the Hamiltonian, we examine the sensitivity of the deuteron EDM to variation in the nucleon-nucleon interaction. We write the EDM as the sum of two terms, the first depending on the target wave function with plane-wave intermediate states, and the second depending on intermediate multiple scattering in the ${}^{3}$$P$${}_{1}$ channel, the latter being sensitive to the off-shell behavior of the ${}^{3}$$P$${}_{1}$ amplitude. Results: We compare the full calculation with the plane-wave approximation result, examine the tensor force contribution to the model results, and explore the effect of short-range repulsion found in realistic, contemporary potential models of the deuteron. Conclusions: Because one-pion exchange dominates the EDM calculation, separable potential model calculations will provide an adequate description of the $^{2}\mathrm{H}$ EDM until such time as a measurement better than $10%$ is obtained.

Structures of AgPd nanoclusters adsorbed on MgO(100): A computational study

The structures of AgPd clusters supported on MgO(100) are investigated via a combination of global optimization searches within an atom–atom potential model and density-functional calculations. Although pure Ag and Pd clusters favor fcc structures in (100) epitaxy with the substrate, it is found that AgPd mixing creates a pocket of stability for polyicosahedral structures in the AgPd/MgO(001) nanoalloy phase diagram. Polyicosahedra are very stable for size around 40 atoms, where they compete with fcc(100) and decahedral structures. For clusters with up to 200 atoms and 50%–50% composition, these last two structural motifs are the most stable ones at the atom–atom potential level. For sizes of 400 atoms, fcc structures in (111) epitaxy with the substrate become close in energy to fcc(100) and Dh clusters, indicating a likely transition to (111) epitaxy for larger sizes.

Triton Electric Form Factor at Low Energies

By using of the Effective Field Theory (EFT) expansion recently developed by the authors, the charge form factor of triton has been calculated up to next-to-next-to-leading order (N2LO). Three-nucleon forces (3NF) are required for renormalization of the three-nucleon system. 3NF effect is predicted for process and it is qualitatively supported by available experimental data. With the 3H binding energy and scattering length as input, we obtain predictions of the pionless EFT for the root-mean-square charge radius of 3H of (1.775 ± 0.023) fm up to N2LO. We show that, by including higher-order corrections, the calculated charge form factor and charge radius of 3H are in satisfactory agreement with the experimental data and modern nucleon-nucleon potential model calculations. Our result converges order by order in low-energy expansion and is also cutoff independent.

STABLE CHROMOMAGNETIC QCD VACUUM AND CONFINEMENT

The stable chromomagnetic vacuum for SU(2) Yang–Mills theory found earlier is shown to give a model for confinement in QCD, using Wilson loop and a linear potential (in the leading order) for quark–antiquark interaction. The coefficient k in this potential is found to be ~ 0.25 GeV2, in satisfactory agreement with nonrelativistic potential model calculations for charmonium. At finite temperature, the real effective energy density found earlier is used to obtain estimates of the deconfining temperature agreeing reasonably with lattice study for SU(2).

The2H Electric Dipole Moment in a Separable Potential Approach

Measurement of the electric dipole moment (EDM) of 2 H or of 3 He may well come prior to the cov- eted measurement of the neutron EDM. Exact model calculations for the deuteron are feasible, and we explore here the model dependence of such deuteron EDM calculations. We investigate in a separable potential approach the relationship of the full model calculation to the plane wave approximation, correct an error in an early poten- tial model result, and examine the tensor force aspects of the model results as well as the e ect of the short range repulsion found in the realistic, contemporary potential model calculations of Liu and Timmermans. We conclude that, because one-pion exchange dominates the EDM calculation, separable potential model calculations should provide an adequate picture of the 2 H EDM until better than 10% measurements are achieved.

A study into the effect of subtle structural details and disorder on the terahertz spectrum of crystalline benzoic acid

The phonon modes of crystalline benzoic acid have been investigated using terahertz time-domain spectroscopy, rigid molecule atom-atom model potential and plane-wave density functional theory lattice dynamics calculations. The simulation results show good agreement with the measured terahertz spectra and an assignment of the terahertz absorption features of benzoic acid is made with the help of both computational methods. Focussing on the strongest interactions in the crystal, we describe each vibration in terms of distortions of the benzoic acid hydrogen bonded dimers that are present in the crystal structure. The terahertz spectrum is also shown to be highly sensitive to the location of the carboxylic acid hydrogen atoms in the cyclic hydrogen-bonded dimers and we have systematically explored the influence of the observed disorder in the hydrogen atom positions on the lattice dynamics.

Classical three-center model potential calculations for ion-H2O collisions

We present a study of the ionization and capture processes in collisions of H+, He2+ and C6+ with water molecules. Single and total cross sections are obtained in the energy range 20 keV/amu ≤ E ≤ 10 MeV/amu, in the framework of the CTMC method, using a three-center model potential to describe the target molecule. We study the asymptotic behaviour of the total ionization cross section and we propose scaling laws for capture and ionization cross sections.

Model for the inverse isotope effect of FeAs-based superconductors in theπ-phase-shifted pairing state

The isotope effects for Fe based superconductors are considered by including the phonon and magnetic fluctuations within the two band Eliashberg theory. We show that the recently observed inverse isotope effects of Fe, $\alpha_{Fe} \approx -0.18 \pm 0.03 $,\cite{Shirage0903.3515} as well as the large positive isotope exponent ($\alpha\approx 0.35$) can naturally arise for the magnetically induced sign revered s-wave pairing state within reasonable parameter range. Either experimental report can not be discarded from the present analysis based on the parameter values they require. The inverse and positive isotope effects mean, respectively, the interband and intraband dominant eletron-phonon interaction. We first make our points based on the analytic result from the square well potential model and present explicit numerical calculations of the two band Eliashberg theory.

Lattice dynamics of orthorhombic perovskite yttrium manganite,YMnO3

The lattice dynamics of yttrium manganite (YMnO3) has been investigated by means of a shell model with pair-wise interionic interactionpotential. The experimental data of crystal structure and Raman and infrared frequenciescompare well with the lattice dynamical calculations. The phonon dispersioncurves found along three high symmetry directions and the density of states ofYMnO3 have also been calculated from this model. The computed phonon density of states is used toderive the macroscopic thermodynamic quantities like the Debye temperature and specificheat. The crystal structure data computed from this model are in good agreement with theavailable experimental data measured by neutron powder diffraction. We have made acomparative study of the structures derived from the potential model calculations for bothLaMnO3 andYMnO3. Symmetry vectors obtained through group theoretical analysis at the zone centre pointwere employed to classify the phonon frequencies obtained into their irreduciblerepresentations. The computed Raman and infrared frequencies have shown goodagreement with the measured data.

Model Potential Calculations of Oscillator Strength Spectra of Rydberg Li Atoms in External Fields

By combining the B-spline basis set with model potential (B-spline + MP), we present oscillator strength spectra of Rydberg Li atoms in external fields. The photoabsorption spectra are analyzed. Over the narrow energy ranges considered in this paper, the structure of the spectra can be independent of the initial state chosen for a given atom. Our results are in good agreement with previous high-precision experimental data and theoretical calculations, where the R-matrix approach together with multichannel quantum defect theory (R-matrix+MQDT) was used. It is suggested that the present methods can be applied to deal with the oscillator strength spectra of Rydberg atoms in crossed electric and magnetic fields.

Magnesium and calcium cation-ligand interactions within the pseudopotential approach. I. Cation-water interaction

Ab-initio model potential calculations were performed in water-Ca2+ and water-Mg2+ systems. It was found that the results obtained are reasonable, but that the interaction energy is slightly overestimated. An analysis of these results indicates that the use of minimal basis set—within the pseudopotential approach—is a promising technique. The interaction energy was improved by correcting the electrostatic contribution and performing counterpoise calculations.

Molecular calculations with the MODPOT, VRDDO, and MODPOT/VRDDO procedures. V. Ab initio and MODPOT LCAO-MO-SCF calculations on the chlorofluoromethanes

Ab initio LCAO-MO-SCF calculations were carried out on the chlorofluoromethanes CF4, CF3Cl, CF2Cl2, CFCl3, and CCl4 both with a minimum STO 4–3G basis set and with a larger, extended s. p basis set. Ab initio effective core model potential (31G/MODPOT) calculations were then carried out on this same series of chlorofluoromethanes. Valence orbital energies, core orbital energies, total energies, gross atomic populations, total overlap populations, and dipole moments are presented for comparison among the various computational results as well as to measured experimental properties. The results of the 31G/MODPOT calculations compared favorably with those from the ab initio larger extended s. p basis set, which themselves were in better agreement with experiment than those from the ab initio minimum 4–3G set.

IR Spectroscopy and Theory of Cu+(H2O)Ar2 and Cu+(D2O)Ar2 in the O−H (O−D) Stretching Region: Fundamentals and Combination Bands

Copper-water ion-molecule complexes with attached argon atoms, Cu(+)(H(2)O)Ar(2), are produced in a supersonic molecular beam by pulsed laser vaporization. These systems are mass-selected in a reflectron time-of-flight spectrometer and studied with infrared photodissociation spectroscopy. The vibrational spectra for these complexes are characteristic of many cation-water systems, exhibiting symmetric and asymmetric O-H stretch fundamentals, and weaker features at higher frequency that have been tentatively assigned to combination bands. Using isotopically substituted spectra and model potential calculations, we are able to assign the combination bands to a water torsional vibration (frustrated rotation) in combination with the asymmetric stretch fundamental. This combination band assignment is likely to apply to IR spectra of many cation-water complexes.

Separation of acetic acid from xylose by nanofiltration

Lignocellulose has drawn great attention in the bioethanol industry as an alternative feedstock for ethanol production due to its renewability, abundance and non-food crop characteristics. Acid hydrolyzation of lignocellulose releases sugars (mainly d-xylose) and several derivatives. The sugars in the hydrolyzate are then converted into ethanol by fermentation. Since acetic acid is believed to be one of the inhibitors which limit the yield of ethanol, it is beneficial to remove acetic acid from the hydrolyzates before fermentation. In this study, a Desal-5 DK nanofiltration (NF) membrane was used to separate acetic acid from xylose, using a synthetic acetic acid–xylose solution as the model. The Desal-5 DK membrane had an isoelectric point of 3.7 and a pore size of 0.83 nm based on streaming potential measurement and model calculation. It was found that both the solution pH and the applied pressure affected the separation performance. The observed retention of xylose and acetic acid varied from 28% to 81% and −6.8% to 90%, respectively, depending on the solution pH and the applied pressure. The maximum separation factor was 5.4 when the system was operated at pH 2.9 and 24.5 bar. In addition, negative retention of acetic acid was observed only in the presence of xylose. The results suggested that intermolecular interactions play an important role in the separation of xylose and acetic acid.

Charm and Charm spectroscopy

Recent developements in D mixing physics and charm spectroscopy will be discussed. Focus will be on the BaBar experimental results for the D mixing: first evidence of the D 0 -mixing (hadronic D 0 decays), lifetime difference and time-dependent Dalitz plot analysis of D 0 → K + π − π 0 . Then, recent results on charm spectroscopy will be presented with particular focus on the new D s states that have been discovered in the last few years. Some of these states were not expected theoretically: their masses, widths, quantum numbers, and decay modes do not fit the existing spectroscopic classication, which is based mostly on potential model calculations.

Energies ofB9(1/2+)andC10(02+)

A potential-model calculation has given reasonably precise values for the energies of the 1/2{sup +} first excited state of {sup 9}B and the excited 0{sup +} state of {sup 10}C. We here extend this calculation to allow for nonzero widths of the 1/2{sup +} and 5/2{sup +} levels of {sup 9}Be and {sup 9}B that are involved in the calculation. The {sup 9}B 1/2{sup +} peak energy (but not the resonance energy) appears to be well determined. The {sup 10}C 0{sup +} energy is determined sufficiently well to make it unlikely that a state recently observed in {sup 10}C at 4.20 MeV could be the 0{sup +} state, as has been suggested.

Description of N = 7 ∼ 9 Isotones with a Modified Single-Particle Potential

We study the level structures of N = 7 ∼ 9 isotones and their mirror nuclei in the framework of the single-particle potential model. Considering the limitation of the conventional potential-model calculation, the isospin-dependent l2 coupling is newly introduced in the average potential. The modified model gives a unified description for the structures of all studied nuclei. Calculations self-consistently produce the s-d level inversion in N = 9 isotones and their mirror nuclei. Meanwhile, the s-p level inversion in the mirror nuclei 11Be and 11N is reproduced. The study confirms the neutron halo structures in 11Be(2s1/2), 11Be(1p1/2), 12B(2s1/2), 14B(2s1/2), 13C(2s1/2), 15C(2s1/2) and the proton halo structure in 17F(2s1/2). The agreement between theory and experiment indicates that the inclusion of the l2 coupling is a feasible way to explain the abnormal structures of exotic light nuclei.

Electron collisions with sulfur compounds SO, SO2 and SO2AB (A, B = Cl, F): various total cross sections

This paper reports our theoretical investigations into electron scattering with SO, SO2, SO2Cl2, SO2ClF and SO2F2 in the energy range from the ionization threshold to 2000 eV. The present studies are prompted by the occurrence and applications of these molecular species in atmospheric and astrophysical systems, together with a paucity of data for SO and SO2AB (A, B = Cl, F) species. Complex model potential calculations are carried out to find total elastic as well as inelastic cross sections, and ionization cross sections are retrieved from inelastic cross sections in a semi-empirical approximation. Comparisons with other data are made and discussed.

First observation of the splittings of theE1p-wave amplitudes in low energy deuteron photodisintegration and its implications for the Gerasimov-Drell-Hearn Sum Rule integrand

Angular distributions of the cross section and linear analyzing powers have been measured for the $d(\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\gamma}},n)p$ reaction at the High Intensity $\ensuremath{\gamma}$-ray Source with linearly polarized beams of 14 and 16 MeV. The outgoing neutrons were detected using the Blowfish detector array, consisting of 88 liquid scintillator detectors with large solid angle coverage. The amplitudes of the reduced transition matrix elements were extracted by means of fits to the data and good agreement was found with a recent potential model calculation of the splittings of the triplet $p$-wave amplitudes. The extracted amplitudes are used to reconstruct the Gerasimov-Drell-Hearn sum rule integrand for the deuteron and are compared to theory.

Molecular Polarization Effects on the Relative Energies of the Real and Putative Crystal Structures of Valine.

The computer-generation of the crystal structures of the α-amino acid valine is used as a challenging test of lattice energy modeling methods for crystal structure prediction of flexible polar organic molecules and, specifically, to examine the importance of molecular polarization on calculated relative energies. Total calculated crystal energies, which combine atom-atom model potential calculations of intermolecular interactions with density functional theory intramolecular energies, do not effectively distinguish the real (known) crystal structures from the rest of the low energy computer-generated alternatives when the molecular electrostatic models are derived from isolated molecule calculations. However, we find that introducing a simple model for the bulk crystalline environment when calculating the molecular energy and electron density distribution leads to important changes in relative total crystal energies and correctly distinguishes the observed crystal structures from the set of computer-generated possibilities. This study highlights the importance of polarization of the molecular charge distribution in crystal structure prediction calculations, especially for polar flexible molecules, and suggests a computationally inexpensive approach to include its effect in lattice energy calculations.