Showing papers on "Debye published in 2012"
TL;DR: High electric field impedance spectroscopy on 5-methyl-3-heptanol reveals a field-induced change in the Kirkwood-Fröhlich correlation factor g(K), viewed as evidence for an electric field driven conversion from ring- to chain-type hydrogen-bonded structures.
Abstract: In hydrogen-bonded liquids including monohydroxy alcohols, the prominent Debye process that often dominates the dielectric relaxation behavior is associated with hydrogen bonding, but its microscopic origin has remained unclear to date. High electric field impedance spectroscopy on 5-methyl-3-heptanol reveals a field-induced change in the Kirkwood-Fr\"ohlich correlation factor ${g}_{K}$, viewed as evidence for an electric field driven conversion from ring- to chain-type hydrogen-bonded structures. The concomitant rearrangement of the chain structure is observed to occur on the time scale of the Debye process, suggesting that the Debye peak of monohydroxy alcohols originates from a fluctuation of the net dipole moment via ${g}_{K}$ of the chain structures on a time scale that is largely controlled by viscosity.
106 citations
TL;DR: A bounded plasma where the hot electrons impacting the walls produce more than one secondary on average is studied via particle-in-cell simulation and it is found that no classical Debye sheath or space-charge-limited sheath exists.
Abstract: A bounded plasma where the hot electrons impacting the walls produce more than one secondary on average is studied via particle-in-cell simulation. It is found that no classical Debye sheath or space-charge limited sheath exists. Ions are not drawn to the walls and electrons are not repelled. Hence the unconfined plasma electrons travel unobstructed to the walls, causing extreme particle and energy fluxes. Each wall has a positive charge, forming a small potential barrier or "inverse sheath" that pulls some secondaries back to the wall to maintain the zero current condition.
92 citations
TL;DR: A simple effective model in which dielectric polarization depends both on the electric field and its first time derivative is identified, which enables nearly exact FDTD simulation of light propagation and absorption in silicon in the spectral range of 300-1000 nm.
Abstract: The frequency dependent dielectric permittivity of dispersive materials is commonly modeled as a rational polynomial based on multiple Debye, Drude, or Lorentz terms in the finite-difference time-domain (FDTD) method. We identify a simple effective model in which dielectric polarization depends both on the electric field and its first time derivative. This enables nearly exact FDTD simulation of light propagation and absorption in silicon in the spectral range of 300–1000 nm. Numerical precision of our model is demonstrated for Mie scattering from a silicon sphere and solar absorption in a silicon nanowire photonic crystal.
86 citations
TL;DR: A method is proposed for solving the time-dependent Maxwell's equations via the discontinuous Galerkin finite-element time-domain (DGFETD) method with dispersive media and it is shown to exhibit high-order convergence.
Abstract: A method is proposed for solving the time-dependent Maxwell's equations via the discontinuous Galerkin finite-element time-domain (DGFETD) method with dispersive media. An auxiliary differential equation (ADE) method is used to represent the constitutive relations. The method is applied to Drude materials, as well as to multiple pole Debye and Lorentz materials. An efficient implementation for high-order Runge-Kutta time integration schemes is presented. The method is validated and is shown to exhibit high-order convergence.
79 citations
TL;DR: Comparisons are made among Molecular Dynamics, Classical Density Functional Theory, and Poisson–Boltzmann modeling of the electric double layer for the nonprimitive three component model (3CM) in which the two ion species and solvent molecules are all of finite size.
Abstract: Comparisons are made among Molecular Dynamics (MD), Classical Density Functional Theory (c-DFT), and Poisson-Boltzmann (PB) modeling of the electric double layer (EDL) for the nonprimitive three component model (3CM) in which the two ion species and solvent molecules are all of finite size. Unlike previous comparisons between c-DFT and Monte Carlo (MC), the present 3CM incorporates Lennard-Jones interactions rather than hard-sphere and hard-wall repulsions. c-DFT and MD results are compared over normalized surface charges ranging from 0.2 to 1.75 and bulk ion concentrations from 10 mM to 1 M. Agreement between the two, assessed by electric surface potential and ion density profiles, is found to be quite good. Wall potentials predicted by PB begin to depart significantly from c-DFT and MD for charge densities exceeding 0.3. Successive layers are observed to charge in a sequential manner such that the solvent becomes fully excluded from each layer before the onset of the next layer. Ultimately, this layer filling phenomenon results in fluid structures, Debye lengths, and electric surface potentials vastly different from the classical PB predictions.
77 citations
TL;DR: In this article, a simple model for this correction, based on the Debye and Dugdale-MacDonald approximations, requires as input only readily available parameters of the equation of state, plus the experimental Debye temperature, was found to overestimate the correction by about a factor of two for some solids in diamond and zinc-blende structures.
Abstract: In a standard Kohn-Sham density functional calculation, the total energy of a crystal at zero temperature is evaluated for a perfect static lattice of nuclei and minimized with respect to the lattice constant. Sometimes a zero-point vibrational energy, whose anharmonicity expands the minimizing or equilibrium lattice constant, is included in the calculation or (as here) is used to correct the experimental reference value for the lattice constant to that for a static lattice. A simple model for this correction, based on the Debye and Dugdale-MacDonald approximations, requires as input only readily available parameters of the equation of state, plus the experimental Debye temperature. However, particularly because of the rough Dugdale-MacDonald estimation of Gr\"uneisen parameters for diatomic solids, this simple model is found to overestimate the correction by about a factor of two for some solids in diamond and zinc-blende structures. Using the quasiharmonic phonon frequencies calculated from density functional perturbation theory gives a more accurate zero-point anharmonic expansion (ZPAE) correction. However, the error statistics for the lattice constants of various semilocal density functionals for the exchange--correlation energy are little changed by improving the ZPAE correction. The Perdew-Burke-Ernzerhof generalized gradient approximation (GGA) for solids and the revised Tao-Perdew-Staroverov-Scuseria (revTPSS) meta-GGA, the latter of which is implemented self-consistently here in the band-structure program BAND and applied to a test set of 58 solids, remain the most accurate of the functionals tested, with MAREs below 0.7$%$ for the lattice constants. The most positive and most negative revTPSS relative errors tend to occur for solids for which full nonlocality (missing from revTPSS) may be important.
71 citations
TL;DR: This work demonstrates precise agreement of the computed variances in the estimates of the correlation and the transport coefficient with the extended theory based on the assumption that fluctuations follow a Gaussian process.
Abstract: We present a rigorous Green-Kubo methodology for calculating transport coefficients based on onthe-fly estimates of: (a) statistical stationarity of the relevant process, and (b) error in the resulting coefficient. The methodology uses time samples efficiently across an ensemble of parallel replicas to yield accurate estimates, which is particularly useful for estimating the thermal conductivity of semi-conductors near their Debye temperatures where the characteristic decay times of the heat flux correlation functions are large. Employing and extending the error analysis of Zwanzig and Ailawadi [Phys. Rev. 182, 280 (1969)] and Frenkel [in Proceedings of the International School of Physics “Enrico Fermi”, Course LXXV (North-Holland Publishing Company, Amsterdam, 1980)] to the integral of correlation, we are able to provide tight theoretical bounds for the error in the estimate of the transport coefficient. To demonstrate the performance of the method, four test cases of increasing computational cost and complexity are presented: the viscosity of Ar and water, and the thermal conductivity of Si and GaN. In addition to producing accurate estimates of the transport coefficients for these materials, this work demonstrates precise agreement of the computed variances in the estimates of the correlation and the transport coefficient with the extended theory based on the assumption that fluctuations follow a Gaussian process. The proposed algorithm in conjunction with the extended theory enables the calculation of transport coefficients with the Green-Kubo method accurately and efficiently. © 2012 American Institute of Physics .[ http://dx.doi.org/10.1063/1.3700344]
54 citations
TL;DR: In this paper, He, the first neutral member of the two electron sequence, has been performed under spherical confinement with a view to analyse the effect of pressure on such non linear optical properties.
Abstract: Pilot calculations on the hyperpolarizability of He, the first neutral member of the two electron sequence, have been performed under spherical confinement with a view to analyse the effect of pressure on such non linear optical properties. Detailed investigations have also been performed for the first time on the hyperpolarizability due to the effect of screened Coulomb potential obtained from a surrounding Debye plasma environment. Variation perturbation theory within coupled Hartree-Fock scheme has been adopted to estimate the non linear optical properties under such external confinement. For a given plasma coupling strength, the hyperpolarizability value is found to reduce systematically with decrease of radius of confinement, while the same is found to increase continuously with increasing plasma coupling strength determined by gradual enhancement of the screening parameter for a given radius of confinement. Under strong confinement the hyperpolarizability value is found to be negative. The estimated free atom hyperpolarizability is consistent with the existing coupled Hartree-Fock result.
47 citations
TL;DR: In this paper, the electrostrictive strain of dipolar polymeric systems is discussed through constitutive equations derived from the Boltzmann statistics and Debye/Langevin formalism.
Abstract: Electrostrictive polymers, as an important category of electroactive polymers, are known to have non-linear response in terms of actuation that strongly affects their dynamic performance and limits their applications. Very few models exist in the literature, and even fewer are capable of making reliable predictions under an electric field. In this paper, electrostrictive strain of dipolar polymeric systems is discussed through constitutive equations derived from the Boltzmann statistics and Debye/Langevin formalism. Macroscopic polarization is expressed as a function of the inherent microscopic parameters of the dielectric material. Electrostrictive strain, polarization and dielectric permittivity are described well by the model in terms of dipole moment and saturation of dipole orientation, allowing the physical definition of the electrostrictive coefficient Q. Maxwell forces generated by dipolar orientation inducing surface charges are also used to explain the electrostrictive strain of polymers. The assessment of this analysis through a comparison with experimental data shows good agreement between reported values and theoretical predictions. These materials are generally used in low-frequency applications, thus the interfacial phenomena that are responsible for low saturation electric field should not be omitted so as not to underestimate or overestimate the low electric field response of the electrostrictive strain.
44 citations
TL;DR: In this article, the effect of Debye plasmas on the dipole, quadrupole, octupole polarizabilities of lithium and sodium atoms using the symplectic algorithm in the framework of the pseudo-state summation technique was investigated.
Abstract: We have carried out calculations to investigate the effect of Debye plasmas on the dipole, quadrupole, octupole polarizabilities of lithium and sodium atoms using the symplectic algorithm in the framework of the pseudo-state summation technique. The polarizabilities of alkali-metal atoms for various Debye lengths are reported for the first time in the literature. The behavior of the transition energies and oscillator strengths for Li and Na in plasma environments is also presented. In free atomic cases, our calculated results are in good agreement with the reported theoretical and experimental results.
42 citations
TL;DR: In this paper, the thermal expansion of rare earth zirconate pyrochlore was studied by the Debye and quasi harmonic approximation combined with the first principles calculations, and the difference between CV and CP was obtained.
Abstract: The thermal expansions of the rare earth zirconate (Ln2Zr2O7, Ln = La, Nd, Sm, and Gd) pyrochlore were studied by the Debye and quasi harmonic approximation combined with the first principles calculations. The difference between CV and CP was obtained. The temperature dependence of thermal expansions is mainly caused by the restoration of thermal energy due to phonon excitations at relatively a low temperature. When the temperature is much higher than Debye temperature, i.e., above 600 K for Ln2Zr2O7 compounds, the volumetric coefficient is increased linearly by increasing the temperature. The calculations are in good agreement with the experiments.
TL;DR: These results suggest that molecules that lack symmetry and possess large dipole moments can perform excellent charge carrier transport contrary to established molecular semiconductor design strategies.
Abstract: A merocyanine dye with an outstandingly large dipole moment of 14 Debye affords thin-film transistors with 0.18 cm(2) V(-1) s(-1) hole mobility and a 10(6) on/off ratio. These results suggest that molecules that lack symmetry and possess large dipole moments can perform excellent charge carrier transport contrary to established molecular semiconductor design strategies.
TL;DR: The solvation experiments show that under constant charge conditions the microscopic polarization fluctuations take place on the time scale of the structural process, and one can demonstrate unambiguously that physical aging is governed by the α-process.
Abstract: Binary solutions of 2-ethyl-1-hexanol (2E1H) with 2-ethyl-1-hexyl bromide (2E1Br) are investigated by means of dielectric, shear mechanical, near-infrared, and solvation spectroscopy as well as dielectrically monitored physical aging. For moderately diluted 2E1H the slow Debye-like process, which dominates the dielectric spectra of the neat monohydroxy alcohol, separates significantly from the α-relaxation. For example, the separation in equimolar mixtures amounts to four decades in frequency. This situation of highly resolved processes allows one to demonstrate unambiguously that physical aging is governed by the α-process, but even under these ideal conditions the Debye process remains undetectable in shear mechanical experiments. Furthermore, the solvation experiments show that under constant charge conditions the microscopic polarization fluctuations take place on the time scale of the structural process. The hydrogen-bond populations monitored via near-infrared spectroscopy indicate the presence of a critical alcohol concentration, xc ≈ 0.5‐0.6, thereby confirming the dielectric data. In the pure bromide a slow dielectric process of reduced intensity is present in addition to the main relaxation. This is taken as a sign of intermolecular cooperativity probably mediated via halogen bonds. © 2012 American Institute of Physics.
TL;DR: In this paper, the electronic band structure and elastic constants of SnS 2 and SnSe 2 have been calculated by using density functional theory (DFT) and the calculated band structures show that SnS2 andsnSe2 are both indirect band gap semiconductors.
Abstract: The electronic band structure and elastic constants of SnS 2 and SnSe 2 have been calculated by using density-functional theory (DFT). The calculated band structures show that SnS 2 and SnSe 2 are both indirect band gap semiconductors. The upper valence bands originate mainly from S p and Sn d electrons, while the lowest conduction bands are mainly from (S, Se) p and Sn s states. The calculated elastic constants indicate that the bonding strength along the [100] and [010] direction is stronger than that along the [001] direction and the shear elastic properties of the (010) plane are anisotropic for SnS 2 and SnSe 2 . Both compounds exhibit brittle behavior due to their low B/G ratio. Relationships among volumes, the heat capacity, thermal expansion coefficients, entropy, vibrational energy, internal energy, Gibbs energy and temperature at various pressures are also calculated by using the Debye mode in this work.
TL;DR: In this article, the dependences of structure, elastic, electronic properties and Debye temperature of M 2 AlC (M V, Nb and Ta) ternary compounds on pressure were investigated based on density functional theory.
TL;DR: In this article, the transport coefficients for Coulomb collision processes in correlated plasmas are calculated within the binary collision approximation, and the results show excellent agreement with molecular dynamics simulations of temperature relaxation.
Abstract: Transport coefficients for Coulomb collision processes in correlated plasmas are calculated within the binary collision approximation. Considering plasmas with flowing Maxwellian distributions and Debye screened particle potentials as an example, it is shown that the friction and energy exchange densities are similar to those in weakly coupled plasmas, but a generalized Coulomb logarithm, Ξ(Λ), is required. This asymptotes to lnΛ in the weakly coupled limit, but is significantly modified when Λ≲102 due to large angle collisions. The results show excellent agreement with molecular dynamics simulations of temperature relaxation.
TL;DR: In this paper, the authors used the classical trajectory Monte Carlo (CTMC) method in the collision energy ranging from 1'keV/amu to 500'kV/AMU to determine the charge exchange and ionization cross sections.
Abstract: Charge exchange and ionization processes in O8+ +H collision system in a Debye plasma are studied using the classical trajectory Monte Carlo (CTMC) method in the collision energy ranging from 1 keV/amu to 500 keV/amu Total charge exchange and ionization cross sections have been determined in both screening and unscreening environments In the unscreened case, partial cross sections for transfer into individual n shells of the projectile have also been determined An interesting and remarkable feature of sudden increase in the ionization cross sections at lower velocities is discussed in terms of the CTMC framework Results are analyzed in light of available theoretical and experimental results The cross sections dependencies on Debye screening lengths have been investigated, and plasma screening effect on charge exchange and ionization cross sections has been found throughout the collision energies range, but is particularly pronounced at low projectile collision energies The sudden rise in the ionization cross sections towards lower energies is explained qualitatively in terms of the multiple encounter model
TL;DR: In this paper, the authors developed an expression for Sherwood number for combined pressure-driven and electroosmotic flow in a porous rectangular microchannel, which quantifies the mass transfer of an electrically neutral solute in the microchannel and is useful for designing microfluidic devices and porous media flows.
Abstract: An expression for Sherwood number is developed from first principles for combined pressure-driven and electroosmotic flow in a porous rectangular microchannel. This quantifies the mass transfer of an electrically neutral solute in the microchannel and is useful for designing microfluidic devices and porous media flows. The convective-diffusive species balance equation, coupled with the velocity field, is solved within the mass transfer boundary layer utilizing similarity method. From the simulations, it is observed that the Sherwood number increases as the electric double layer near the channel wall becomes more compact (as manifested through a decrease in the Debye length), and it reaches a constant value around the scaled Debye length of 40. The Sherwood number becomes constant at higher Debye lengths as electrokinetic effects become negligible. A detailed analysis of dependence of Reynolds number, dimensionless permeation velocity, ratio of driving force and scaled Debye length on Sherwood number is presented. © 2011 American Institute of Chemical Engineers AIChE J, 58: 1693–1703, 2012
TL;DR: In this paper, an analytical theory and numerical simulations of the dipolar response of hydrated proteins in solution were presented and the effective dielectric constant representing the average dipole moment induced at the protein by a uniform external field was calculated.
Abstract: The paper presents an analytical theory and numerical simulations of the dipolar response of hydrated proteins in solution. We calculate the effective dielectric constant representing the average dipole moment induced at the protein by a uniform external field. The dielectric constant shows a remarkable variation among the proteins, changing from 0.5 for ubiquitin to 640 for cytochrome c. The former value implies a negative dipolar susceptibility, that is a dia-electric dipolar response and negative dielectrophoresis. It means that ubiquitin, carrying an average dipole of ≃240 D, is expected to repel from the region of a stronger electric field. This outcome is the result of a negative cross-correlation between the protein and water dipoles, compensating for the positive variance of the intrinsic protein dipole in the overall dipolar susceptibility. In contrast to the neutral ubiquitin, charged proteins studied here show para-electric dipolar response and positive dielectrophoresis. The study suggests that the dipolar response of proteins in solution is strongly affected by the coupling of the protein surface charge to the hydration water. The protein-water dipolar cross-correlations are long-ranged, extending ~2 nm from the protein surface into the bulk. A similar correlation length of about 1 nm is seen for the electrostatic potential produced by the hydration water inside the protein. The analysis of numerical simulations suggests that the polarization of the protein-water interface is highly heterogeneous and does not follow the standard dielectric results for cavities carved in dielectrics. The polarization of the water shell gains in importance, relative to the intrinsic protein dipole, at high frequencies, above the protein Debye peak. The induced interfacial dipole can be either parallel or antiparallel to the protein dipole, depending on the distribution of the protein surface charge. As a result, the high-frequency absorption of the protein solution can be either higher or lower than the absorption of water. Both scenarios have been experimentally observed in the THz window of radiation.
TL;DR: In this paper, the authors derived the Cole-Cole expression for the complex dielectric permittivity and showed that the exponent α involved in that expression is equal to the fractal dimension of the spatial-temporal self-similar ensemble characterizing the structure of the medium and the relaxation process occurring in it.
Abstract: Based on a self-similar spatial-temporal structure of the relaxation process, we construct a microscopic model for a non-Debye (nonexponential) dielectric relaxation in complex systems. In this model, we derive the Cole-Cole expression for the complex dielectric permittivity and show that the exponent α involved in that expression is equal to the fractal dimension of the spatial-temporal self-similar ensemble characterizing the structure of the medium and the relaxation process occurring in it. We find a relation between the macroscopic relaxation time and the micro- and mesoparameters of the system. We obtain a generalized Cole-Cole expression for the complex dielectric permittivity involving log-periodic corrections that occur because of a discrete scaling invariance of the fractal structure generating the relaxation process on the mesoscopic scale. The found expression for the dielectric permittivity can be used to interpret dielectric spectra in disordered dielectrics.
TL;DR: In this paper, an effective method is proposed to greatly improve the thermal transport across the interface between two solids with dissimilar phonon spectra by inserting a 3-unit-cell-thick interlayer whose Debye temperature is approximately the square root of the product of the Debye temperatures of the two soliders.
TL;DR: The degenerate paths and repeated paths considerably simplify the interpretation of scattering within the coated sphere, thus making it possible to catalog the contributions of the various paths.
Abstract: Although scattering of light by a coated sphere is much more complicated than scattering by a homogeneous sphere, each of the partial wave amplitudes for scattering of a plane wave by a coated sphere can be expanded in a Debye series. The Debye series can then be rearranged in terms of the various reflections that each partial wave undergoes inside the coated sphere. For a given number of internal reflections, it is found that many different Debye terms produce the same scattered intensity as a function of scattering angle. This is called path degeneracy. In addition, some of the ray trajectories are repeats of those occurring for a smaller number of internal reflections in the sense that they produce identical time delays as a function of scattering angle. These repeated paths, however, have a different intensity as a function of scattering angle than their predecessors. The degenerate paths and repeated paths considerably simplify the interpretation of scattering within the coated sphere, thus making it possible to catalog the contributions of the various paths.
TL;DR: A mesoscale simulation method based on multiparticle collision dynamics (MPCD) for the electrohydrodynamics of polyelectrolytes with finite Debye lengths using the Debye-Hückel approximation to reproduce the rapid rise in the electrophoretic mobility with respect to the degree of polymerization for the shortest polymer lengths.
Abstract: We introduce a mesoscale simulation method based on multiparticle collision dynamics (MPCD) for the electrohydrodynamics of polyelectrolytes with finite Debye lengths. By applying the Debye-Huckel approximation to assign an effective charge to MPCD particles near charged monomers, our simulations are able to reproduce the rapid rise in the electrophoretic mobility with respect to the degree of polymerization for the shortest polymer lengths followed by a small decrease for longer polymers due to charge condensation. Moreover, these simulations demonstrate the importance of a finite Debye length in accurately determining the mobility of uniformly charged polyelectrolytes and net neutral polyampholytes.
TL;DR: In this paper, the effect of Debye plasmas screening on the doubly excited non-autoionizing 1,3Pe and 1-3Do states of Li II, Be III, B IV, and C V using highly correlated exponential wave functions within the framework of Ritz variational principle was investigated.
Abstract: We have carried out non-relativistic calculations to investigate the effect of Debye plasmas screening on the doubly excited nonautoionizing 1,3Pe and 1,3Do states of Li II, Be III, B IV, and C V using highly correlated exponential wave functions within the framework of Ritz variational principle. The 2p2 3Pe, 2p3p 1Pe, and 2p3d 1,3Do states energies, and wavelengths for the 2p3p 1Pe → 2p3d 1Do, 2p2 3Pe → 2p3d 3Do, 2p3p 3Pe → 2p3d 3Do, and 2p3p 3Pe → 2p3d 3Do transitions for different Debye lengths are reported. Comparisons are made with the existing results. Results for B IV and C V for different screening parameters are reported for the first time in the literature. Transition wavelengths show interesting behavior with increasing screening parameters and nuclear charge.
TL;DR: In this paper, the authors studied the thermodynamics of a crystalline solid by applying q-deformed algebras and found that q deformation acts as a factor of disorder or impurity, modifying the characteristics of a semiconductors.
Abstract: We study the thermodynamics of a crystalline solid by applying q -deformed algebras. We based part of our study on both Einstein and Debye models, exploring primarily q -deformed thermal and electric conductivities as a function of Debye specific heat. The results revealed that q -deformation acts as a factor of disorder or impurity, modifying the characteristics of a crystalline structure, as for example, in the case of semiconductors.
TL;DR: In this article, the authors analyzed particle electrophoresis in the thin-double-layer limit for asymptotically large applied electric fields, where fields scaling as, being the dimensionless Debye thickness, were considered.
Abstract: We analyse particle electrophoresis in the thin-double-layer limit for asymptotically large applied electric fields. Specifically, we consider fields scaling as , being the dimensionless Debye thickness. The dominant advection associated with the intense flow mandates a uniform salt concentration in the electro-neutral bulk. The large tangential fields in the diffuse part of the double layer give rise to a novel ‘surface conduction’ mechanism at moderate zeta potentials, where the Dukhin number is vanishingly small. The ensuing electric current emerging from the double layer modifies the bulk electric field; the comparable transverse salt flux, on the other hand, is incompatible with the nil diffusive fluxes at the homogeneous bulk. This contradiction is resolved by identifying the emergence of a diffusive boundary layer of thickness, resembling thermal boundary layers at large-Reynolds-number flows. The modified electric field within the bulk gives rise to an irrotational flow, resembling those in moderate-field electrophoresis. At leading order, the particle electrophoretic velocity is provided by Smoluchowski’s formula, describing linear variation with applied field.
TL;DR: In this paper, an upper-level undergraduate project is described in which students perform high-level ab initio computational scans of the potential energy curves for Ne2 and Ar2 and obtain the respective Lennard-Jones (LJ) potential parameters σ and e for the dimers.
Abstract: An upper-level undergraduate project is described in which students perform high-level ab initio computational scans of the potential energy curves for Ne2 and Ar2 and obtain the respective Lennard-Jones (LJ) potential parameters σ and e for the dimers. Using this information, along with the summation of pairwise interactions in the face-centered cubic structures of the crystalline solids, the students determine the cohesive energies of solid Ne and Ar. By applying the Debye theory of solids, they then adjust these values to take zero-point vibrational effects into account after obtaining the respective Debye temperatures from published low-temperature heat capacity data. The students also calculate the lattice parameters and solid densities. An assessment of their work is made by comparing their results with experimental data.
TL;DR: A fast approximation algorithm is presented that efficiently computes the summation to any prescribed accuracy ϵ in linear time, based on a hierarchical spatial decomposition of the molecule coupled with local harmonic expansions and translation of these expansions.
Abstract: Debye summation, which involves the summation of sinc functions of distances between all pair of atoms in three dimensional space, arises in computations performed in crystallography, small/wide angle X-ray scattering (SAXS/WAXS) and small angle neutron scattering (SANS). Direct evaluation of Debye summation has quadratic complexity, which results in computational bottleneck when determining crystal properties, or running structure refinement protocols that involve SAXS or SANS, even for moderately sized molecules. We present a fast approximation algorithm that efficiently computes the summation to any prescribed accuracy in linear time. The algorithm is similar to the fast multipole method (FMM), and is based on a hierarchical spatial decomposition of the molecule coupled with local harmonic expansions and translation of these expansions. An even more efficient implementation is possible when the scattering profile is all that is required, as in small angle scattering reconstruction (SAS) of macromolecules. We examine the relationship of the proposed algorithm to existing approximate methods for profile computations, and show that these methods may result in inaccurate profile computations, unless an error bound derived in this paper is used. Our theoretical and computational results show orders of magnitude improvement in computation complexity over existing methods, while maintaining prescribed accuracy.
TL;DR: In this paper, an ab initio equation of motion method is introduced to calculate the temperature-dependent mean square vibrational amplitudes which appear in the Debye-Waller factors in x-ray absorption, xray scattering, and related spectra.
Abstract: Department of Physics, University of Washington, Seattle, WA 98195(Dated: September 1, 2011)An ab initio equation of motion method is introduced to calculate the temperature-dependentmean square vibrational amplitudes which appear in the Debye-Waller factors in x-ray absorption,x-ray scattering, and related spectra. The approach avoids explicit calculations of phonon-modes,and is based instead on calculations of the displacement-displacement time correlation functionfrom ab initio density functional theory molecular dynamics simulations. The method also yieldsthe vibrational density of states and thermal quantities such as the lattice free energy. Illustrations ofthe method are presented for a numberof systems and compared with other methods and experiment.I. INTRODUCTION
TL;DR: In this article, the effect of the screened Coulomb potential on positronium formation process was investigated by using the Debye-Huckel potential, and the present results are compared with available theoretical calculations.
Abstract: Positronium (Ps) formation cross sections (n = 1, 2) in positron-hydrogen collisions in Debye plasma environment are calculated using the screening approximation model for various Debye screening lengths from the Ps formation thresholds to 50 eV. The effect of the screened Coulomb potential on Ps formation process is investigated by using the Debye-Huckel potential. The present results are compared with available theoretical calculations.