Energies Of Different Configurations Research Articles

Structural defects induced by Fe-ion implantation in TiO2

X-ray photoelectron spectroscopy and resonant x-ray emission spectroscopy measurements of pellet and thin film forms of TiO2 with implanted Fe ions are presented and discussed. The findings indicate that Fe-implantation in a TiO2 pellet sample induces heterovalent cation substitution (Fe2+ → Ti4+) beneath the surface region. But in thin film samples, the clustering of Fe atoms is primarily detected. In addition to this, significant amounts of secondary phases of Fe3+ are detected on the surface of all doped samples due to oxygen exposure. These experimental findings are compared with density functional theory calculations of formation energies for different configurations of structural defects in the implanted TiO2:Fe system. According to our calculations, the clustering of Fe-atoms in TiO2:Fe thin films can be attributed to the formation of combined substitutional and interstitial defects. Further, the differences due to Fe doping in pellet and thin film samples can ultimately be attributed to different surface to volume ratios.

ANN prediction model for composite plates against low velocity impact loads using finite element analysis

In this paper, the use of Artificial Neural Networks to predict the absorbed energy in the composite plates impacted with low velocity is described. The impact response of a composite laminate depends upon various factors such as thickness, stacking sequence and number of layers. These factors are identified in an earlier study using the sensitivity analysis. These factors have the most prominent effect on the impact resistance of the composite plates. These are studied here with the help of design of experiments so that a suitable data set is obtained. The ability to solve a large number of simulations using FEA gives an advantage in the design optimization with the help of DOE (Design of Experiments). During the study different variations of these factors were tried and the response in terms of the absorbed energy was estimated. The simulation results were then used along with the ANN (Artificial Neural Networks) to fit a function to estimate the amount of absorbed energy. The results from the DOE follow the intuition that the increase of thickness and number of layers increase the performance of the composite plates. The ANN model is trained such that it is able to predict with an acceptable accuracy range the amount of absorbed energy for different configurations of input variables. The paper discusses the codification of input variables so that they can be used to train ANN model. Also, the use of differential evolution algorithm is discussed which is used to select the best possible ANN model based on the maximum error and root mean square error of the ANN models.

Ab initio study of platinum induced reconstructions on Ge(001)–(1×2) surface with dimerization

We present first principle total energy calculation of Pt induced reconstructions on Ge(001)–(1×2) surface with dimerization. Study was undertaken using localized orbitals basis set DFT using SIESTA to compare pure Ge dimerized Ge(001)–(1×2) surface with 0.5 and 1.0 Pt covered dimerized Ge(001)–(1×2) surface with the possibility of homo (Ge–Ge and Pt–Pt) and hetro (Pt–Ge) dimers. From total energy calculation results we calculated dimer bond lengths, buckling angles and formation energy of dimers on Ge(001)–(1×2) surface. By calculating the formation energy of different configurations we find that Ge–Ge buckled dimerized surface has least (−1.23 eV/dimer) and Pt–Pt symmetric dimerized surface has largest (+0.09 eV/dimer) formation energy with respect to unreconstructed surface. We further calculated the electronic DOS and band structure of Ge dimerized as well as Pt dimerized surface to see the change in semiconducting behavior on dimerization. By comparing the DOS and electronic band structure of homo Ge dimerized surface, we found metallicity of Ge(001)–(1×2) surface results from dimer formation. Also by comparing the electronic band structure of homo Ge dimerized surface with unreconstructed surface we find that less number of bands crossing the Fermi level which is perhaps due to the saturation of one dangling bond per Ge surface atom. By introducing Pt at 0.5 and 1.0 coverage in place of Ge, except for homo Pt buckled dimerized surface having 1.0 coverage of Pt, we find in all other cases increase in number of bands are crossing the Fermi level, indicating strong metallic behavior of Ge(001)–(1×2) surface.

Effects of hydrostatic pressure on the Coulomb-bound states in GaAs–Ga 1− xAl xAs semiconductor superlattices

The effects of hydrostatic pressure on the Coulomb-bound states in GaAs–Ga 1− x Al x As and GaAs–AlAs semiconductor superlattices are theoretically studied. Calculations of the impurity binding energies for different configurations of the system and for various values of the hydrostatic pressure are performed in the framework of the parabolic-band and effective-mass schemes, and within the variational procedure. The hydrostatic-pressure dependence on the exciton energy is also obtained, and theoretical results are compared and found in good agreement with available experimental measurements.

A model study of dickite intercalated with formamide and N-methylformamide

Local geometry and orientation of intercalated molecules of formamide (FA) and N-methylformamide (NMFA) in the clay mineral dickite (D) was studied by means of Density Functional Theory (DFT) calculations. Ten configurations with different orientation of the intercalated molecule were investigated for both D_FA and D_NMFA intercalates. Four groups of relaxed structures sorted by the calculated total electronic energy were found in both cases. The experimental geometry of the D_FA intercalate was denoted as the most stable structure from the investigated models. The differences in the total electronic energy of all D_FA configurations are within the interval of ∼92 kJ/mol. On one hand FA forms intercalates specifically and a close relation between the orientation of the FA molecules in the interlayer space and the stability of a particular configuration has been observed. On the other hand, N-methylformamide forms intercalated structures non-specifically. Small differences in the total energy, not larger than 18 kJ/mol, are observed for different orientations of the NMFA molecules The reorientation of the intercalated molecules has only a small effect on the stabilization of the D_NMFA intercalate what is in contrast with the D_FA intercalate. It was also observed that the experimental D_NMFA configuration is not the most stable. A small variation of the total electronic energy of different configurations correlates with small changes of the orientation of the dipole moment of the intercalated NMFA molecule.

Self-consistent field study of the alignment by an electric field of a cylindrical phase of block copolymer

Self-consistent field theory is applied to a film of cylindrical-forming block copolymer subject to a surface field which tends to align the cylinders parallel to electrical plates, and to an external electric field tending to align them perpendicular to the plates. The Maxwell equations and self-consistent field equations are solved exactly, numerically, in real space. By comparing the free energies of different configurations, we show that for weak surface fields, the phase of cylinders parallel to the plates makes a direct transition to a phase in which the cylinders are aligned with the field throughout the sample. For stronger surface fields, there is an intermediate phase in which cylinders in the interior of the film, aligned with the field, terminate near the plates. For surface fields which favor the minority block, there is a boundary layer of hexagonal symmetry at the plates in which the monomers favored by the surface field occupy a larger area than they would if the cylinders extended to the surface.

Electronic configurations and the periodic table for superheavy elements

The periodicity of the chemical properties of elements is associated with the repetition of the configurations of valence electrons with respect to the quantum number l , which dictates the angular distribution of wave functions in the nonrelativistic approximation and, thus, determines the chemical properties. However, for superheavy elements ( Z > 111), which have been successfully synthesized in the last years [1‐4], relativistic effects are crucial and lead to a noticeable violation of simple regularities [5, 6]. In early works on the electronic configurations of superheavy elements [5‐7], configuration interaction was not considered and some calculations were performed by the Dirac‐Fock‐ Slater method with approximate inclusion of exchange, which is less accurate than the Dirac‐Fock method. In this work, we performed multiconfiguration Dirac‐Fock calculations with inclusion of the total angular momentum coupling scheme and the Breit correction. The lowest lying term was taken to be the energy of a given configuration. When these energies for different configurations were very close to one another, configuration interaction was included in the calculations [8]. The calculations covered the elements with 119 ≤ Z ≤ 164. Configuration interaction calculations were necessary since the total energies of different configurations differ little from one another in many cases [5]. Therefore, inclusion of configuration interaction can lead to a change of the ground-state configuration. In particular, it is worth noting that about one half of the ground-state configurations obtained in this work differ from those reported in [7], where no configuration interaction was included and the exchange energy was considered by an approximate statistical method. Dirac‐Fock calculations were performed for many heavy elements [9], but configuration interaction was not included. These data are compared with our data in Tables 1 and 2. This comparison shows that configuration interaction is crucial in more than 30% of cases since its consideration leads to another ground-state configuration. The calculated electronic configurations of some elements (up to Z = 120) were reported in [10‐ 14].

Hysteresis in small arrays of interacting magnetic nanoparticles

The out-of-plane hysteresis loops of small arrays of magnetic nanoparticles, under the influence of an external field applied perpendicular to the array and the dipolar interaction are investigated. The particles are assumed to have a perpendicular anisotropy energy that tends to align the magnetic moments to be perpendicular to the array. The magnetization is found to exhibit a plateaux-and-jumps structure as the external field is swept up and down. These jumps are associated with jumps in the energy of the system, and correspond to transition from one configuration of the moment orientation to another. The energy of different configurations of the magnetic moments for a 3×3 array in the limit of weak dipolar interaction is analyzed, as a means to understand the hysteresis loop. These jumps are more pronounced in arrays of smaller sizes and when the dipolar interaction is weak. The configuration of magnetic moments at zero external field as the field is swept up and down is found to be highly sensitive to the dipolar interaction.

Simulating temperature programmed desorption directly from density functional calculations: How adsorbate configurations relate to desorption features

DFT calculations are used to explain states in TPD spectra by relating the differential adsorption energies for different configurations to the desorption features. A plot of the number of molecules versus these temperatures strongly resembles the TPD spectrum. The calculations were also used to obtain pairwise additive potentials. These potentials were used in a dynamic Monte Carlo model to simulate the TPD spectrum. The spectra obtained from both approaches are in good agreement, validating the approximation that lateral interactions can be added on a pairwise basis.

Stability and magnetism of Ni–Fe alloyed overlayers on Fe(001)

The surface ordered alloy formation of Ni/Fe(001) overlayer system is explored, using the tight-binding linear muffin-tin orbital (TB-LMTO) method. The alloy formation for one- and two- surface layer of Ni–Fe on Fe(001) are considered by calculating the total and surface formation energies for different configurations of Ni overlayers on Fe(001). The preferred ground states for different configurations are determined and Ni (alloyed with Fe) is found to remain at the Fe surface. The magnetic profile for these configurations is found comparable to the experimental predictions.

Calibration of embedded-cluster method for defect studies in amorphous silica

We performed validation and calibration of the embedded-cluster method in order to study its applicability to modeling defects in amorphous silica, where defect models are not well established. As test systems we used well-studied ${E}_{4}^{\ensuremath{'}}$ and ${E}_{1}^{\ensuremath{'}}$ centers in $\ensuremath{\alpha}$-quartz and compared our results with the experimental data and the results of periodic density-functional-theory calculations. Our method can reproduce the essential physics of both centers in good agreement with experiment. The relative energies of different configurations obtained in our calculations are consistent for the two centers and are in qualitative agreement with experiment. The calculated hyperfine parameters agree with the experimental values within 20%. The defect-induced relaxation propagates for more than 10 \AA{} at the puckered side of the ${E}_{1}^{\ensuremath{'}}$ center. We discuss factors which determine the accuracy of calculations of relative defect energies in crystalline and amorphous silica.

QM/MM Car-Parrinello molecular dynamics study of the solvent effects on the ground state and on the first excited singlet state of acetone in water.

We present a hybrid Car-Parrinello quantum mechanical/molecular mechanical (QM/MM) approach that is capable of treating the dynamics of molecular systems in electronically excited states in complex environments. The potential energy surface in the excited state is described either within the restricted open-shell Kohn-Sham (ROKS) formalism or within time-dependent density functional theory (TDDFT). As a test case, we apply this technique to the study of the solvent effects on the ground state and on the first excited singlet state of acetone in water. Our results demonstrate that for this system a purely classical description of the solvent is sufficient, since inclusion of the first solvent shell of 12 water molecules into the quantum system does not show a significant effect on this transition. The excited-state energies calculated with ROKS are red shifted by a constant value compared to the TDDFT results, while the relative variations of the excitation energy for different configurations are in very good agreement. The experimentally observed blue shift of the excitation energy in going from gas phase to condensed phase is well reproduced. Excited-state dynamics carried out with ROKS yield the relaxation of the solute and the rearrangement of the solvent structure on a picosecond timescale. The calculated Stokes shift is in reasonable agreement with experimental data.

A triple point in an alloy with modulated compositional order

We consider a triangular lattice whose sites can be occupied by atoms A or B, which interact with each other up to second neighbours. The concentration of A atoms, x, is fixed. Using the mean-field approximation the free energy is written in terms of the occupation probabilities of the sites, which are evaluated by a classical statistical formalism. In this way, the free energies of different configurations can be compared with each other. It is found that in a certain range of values of the interaction constants a triple-point exists in the ( x, T) phase diagram, where the disordered phase (D) coexists with a commensurate (C) and an incommensurate (I) modulated phase. This triple point is not of the Lifshitz type. The C–D and the C–I transitions are discontinuous, while the I–D transition is continuous.

Open-shell van der Waals complexes of the coinage metals: Cu…H 2O, Ag…H 2O, Cu…H 2S, and Ag…H 2S

The interaction potential curves for complexes between the coinage metal atoms (Me ≡ Cu, Ag) and H 2X (X ≡ O, S) molecules are calculated for three selected configurations. Two of them represent H-bonded structures while the third corresponds to direct interactions between Me and the lone pair carrier of XH 2. Most calculations have been carried out at the level of the restricted open-shell Hartree-Fock (ROHF) formalism followed by a perturbation treatment of the electron correlation contribution (CASPT2). For some selected points the ROHF-based spin-adapted coupled cluster method (ROHF SA CCSD) has been used. The relativistic effects are accounted for through the mass-velocity and Darwin (MVD) corrections. We found remarkable differences in the size and the sign of MVD corrections to interaction energies for different configurations. For all structures bonded through the hydrogen of H 2X (H-bonded structures) the ROHF interaction potentials turn out to be essentially repulsive, the attractive force due to induction effects being negligible in comparison with the valence repulsion for all but very large intersystem distances. However, the direct Me…O interaction produces a significant minimum even at the level of the ROHF approximation. At the correlated levels of approximations all structures of all systems are attractive with binding energies of the order of 1 kcal mol −1. In the case of the water molecule complexes, lone pair interaction is favoured so that the Me…OH 2 bonding resembles the usual H-bonds with the Me atom playing the role of the electron acceptor. It is suggested that the Me-bonding in systems of the form Me…OH 2 may represent the coinage metal counterparts of the H-bonding. On the contrary, H-bonding is found to give the most stable structures for the Me…H 2S complexes, the interaction energies being slightly higher than those for the corresponding complexes with the water molecule.

Calculation of the Free Energy of Different Configurations of {001}Σ=13 Grain Boundary in Silicon by the Quasiharmonic Method

Calculation of the Free Energy of Different Configurations of {001}Σ=13 Grain Boundary in Silicon by the Quasiharmonic Method

Resonant spin-flip Raman scattering on photoexcited carriers inp-typeCd0.95Mn0.05Te crystals

The exchange interaction with magnetic impurities has been studied in p-type ${\mathrm{Cd}}_{0.95}$${\mathrm{Mn}}_{0.05}$Te crystals by means of spin-flip Raman scattering (SFRS). An additional line, active in crossed polarizations, was observed in the Raman spectra under excitation above the energy gap. The spectral shift of the line depends strongly on magnetic field and changes from \ensuremath{\Delta}\ensuremath{\nu}=28 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ to \ensuremath{\Delta}\ensuremath{\nu}=135 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ as the field increases from H=1 T to H=10 T. This SFRS line displays resonant behavior with excitation energies close to the exciton transitions. Analysis of the selection rules and resonance energies for different configurations of scattering enables us to attribute this line to SFRS by photoexcited electrons created in the crystal by band-to-band optical excitation. This conclusion is confirmed by the superlinear behavior of the SFRS intensity as a function of the excitation power. These results provide prospects for the study of the s-d exchange interaction in p-type semimagnetic semiconductors using the SFRS method.

An electrostatic investigation of the motion of alkali metal cations through polyacetylene

The electrostatic energy of the alkali-metal-doped polyacetylene lattice is calculated for two possible structures of the dopant sublattice. It is shown that the lower energy one is that in which the dopant ions in different channels are related by simple lattice translations. The migration of a single alkali metal ion through the channel is modelled by considering a unit cell consisting of several hundred atoms and eight channels, in one of which the dopant is a single ion. By calculating the electrostatic contribution to the lattice energy for different configurations of this ion, the energy barrier for ion migration is estimated to be 0.1 eV.

Small Vacancy Clusters in Nickel

AbstractThe method of molecular dynamics is used to compute binding energies of different configurations of vacancy clusters in nickel. The interatomic pair potential is obtained according to the pseudopotential theory. It is found that in the range from divacancy to hexavacancy various atomic configurations are formed which can be considered as nuclei of dislocation loops, voids, and new crystal lattices.

The shapes of islands of chemisorbed atoms as a probe of long-range interadatom interactions

When ordered superlattices of adatoms are produced by attractive electronic indirect interactions, islands form at low coverages. At low temperatures, the shapes of these domains should bear the mark of these interactions. (For example, at zero temperature a c(2 × 2) island on a square lattice should be square if the second neighbor attraction is at least roughly three times the third neighbor attraction.) First, asymptotic and explicit evaluations of the energies of different configurations are presented. Then some thermal disordering effects are included in a quasicontinuous way. Polygonal-like shapes should be observable at low temperatures, below (possibly much below) 100K. Complications due to three-adatom interactions can arise in model calculations. Island shapes should be observable using LEED; the adlayer-induced spots should exhibit star-like patterns. Problems such as diffusion and heterogeneity limit the size of islands, making this a problem of metastability. The (111) faces of fcc crystals most likely are the best substrate on which to seek polygonal (i.e., hexagonal) shapes. In any case these ideas provide another aspect by which to compare Monte Carlo simulations with experimental observations.