Dominant Configurations Research Articles

Icosahedral medium-range order formed in Mg70Zn30 metallic glass: a larger-scale molecular dynamics simulation

A larger-scale Mg70Zn30 alloy system including 100000 atoms has been simulated by using the molecular dynamics method to investigate the icosahedral medium-range order (IMRO) formed in the Mg70Zn30 metallic glass. It is found that the simulated pair distribution function of Mg70Zn30 metallic glass is in good agreement with the experimental results. The glass transition temperature Tg is near 450 K under the cooling rate of 1×1012 K/s. The icosahedral local structures play a critical role in the formation of metallic glass, and they are the dominant local configurations in the Mg70Zn30 metallic glass. The IMRO in the Mg70Zn30 metallic glass is characterized by certain types of extended icosahedral clusters combined by intercross-sharing atoms in the form of chains or dendrites. The size distributions of these IMRO clusters present a magic number sequence of 19, 23, 25, 27, 29, 31, 33, 35, 37, 39, …, and the magic clusters can be classified into three types according to their compactness. The IMRO clusters grow rapidly in a low-dimensional way with cooling, but this growth is limited near Tg.

Simulations of Solid-State Vibrational Circular Dichroism Spectroscopy of (S)-Alternarlactam by Using Fragmentation Quantum Chemical Calculations

Simulations of vibrational circular dichroism (VCD) spectroscopy of optical active aggregates of chiral molecules in the amorphous solid encounter great difficulties in the description of complicated intermolecular interactions by using the conventional quantum mechanical (QM) methods. The fragmentation approach is applied to calculate the VCD spectra of the covalently bonded oligomers and nonbonded molecular aggregates of (S)-alternarlactam, a new fungal cytotoxin with cyclopentenone and isoquinolinone scaffolds. Starting from the statistically averaged configurations that are sampled from the molecular dynamic simulations, the target oligomers or packing systems are divided into several fragments with a proper treatment of boundary effects on the separated segments. Each fragment is embedded in the background point charges centered on the distant atoms to simulate the long-range electrostatic interactions. The total VCD signals are assembled from the rotational strength of all the fragments. Test calculations on the σ-bonded oligomers and molecular aggregates using fragmentation method show good agreement with the conventional QM results. The packing effects on the infrared (IR) absorption and VCD spectroscopies of amorphous (S)-alternarlactam solid are investigated with density of 0.5 and 0.8 g/cm(3), respectively. The fragment-based VCD calculations on (S)-alternarlactam aggregates give a better agreement with experimental spectra than the Boltmann-weighted spectra of various possible monomeric, dimeric, and trimeric configurations. Hydrogen-bonded networks are the dominant packing configurations at the density of 0.5 g/cm(3). The (C═)O···H-N hydrogen-bonding interactions result in the signal splitting of IR and VCD spectra at the C═O stretching vibrational regions. When the density is increased to 0.8 g/cm(3), π-π stacking turns to be the dominating intermolecular interaction pattern. The computational cost of fragmentation calculation scales linearly with the number of the molecular fragments, facilitating the future applications to a wide range of the large-sized chiral systems.

Metal-organic charge transfer can produce biradical states and is mediated by conical intersections

The present paper illustrates key features of charge transfer between calcium atoms and prototype conjugated hydrocarbons (ethylene, benzene, and coronene) as elucidated by electronic structure calculations. One- and two-electron charge transfer is controlled by two sequential conical intersections. The two lowest electronic states that undergo a conical intersection have closed-shell and open-shell dominant configurations correlating with the 4 s 2 and 4 s 1 3 d 1 states of Ca, respectively. Unlike the neutral-ionic state crossing in, for example, hydrogen halides or alkali halides, the path from separated reactants to the conical intersection region is uphill and the charge-transferred state is a biradical. The lowest-energy adiabatic singlet state shows at least two minima along a single approach path of Ca to the π system: ( i ) a van der Waals complex with a doubly occupied highest molecular orbital, denoted , and a small negative charge on Ca and ( ii ) an open-shell singlet (biradical) at intermediate approach (Ca⋯C distance ≈2.5–2.7 Å ) with molecular orbital structure ϕ 1 ϕ 2 , where ϕ 2 is an orbital showing significant charge transfer form Ca to the π-system, leading to a one-electron multicentered bond. A third minimum ( iii ) at shorter distances along the same path corresponding to a closed-shell state with molecular orbital structure has also been found; however, it does not necessarily represent the ground state at a given Ca⋯C distance in all three systems. The topography of the lowest adiabatic singlet potential energy surface is due to the one- and two-electron bonding patterns in Ca-π complexes.

Isomer spectroscopy ofCd127

The spin and configurational structure of excited states of Cd-127, the two-proton and three-neutron hole neighbor of Sn-132, has been studied. An isomeric state with a half-life of 17.5(3) mu s was populated in the fragmentation of a Xe-136 beam on a Be-9 target at a beam energy of 750 MeV/u. Time distributions of the delayed gamma transitions and gamma gamma coincidence relations were exploited to construct a decay scheme. The observed yrast (19/2)(+) isomer is proposed to have dominant configurations of nu(h(11/2)(-3))pi(g(9/2)(-1), p(1/2)(-1)), nu(h(11/2)(-2)d(3/2)(-1))pi(g(9/2)(-2)), and nu(h(11/2)(-2), s(1/2)(-1))pi(g(9/2)(-2)) and to decay by two competing stretched M2 and E3 transitions. Experimental results are compared with the isotone Sn-129. The new information provides input for the proton-neutron interaction and the evolution of neutron hole energies in nuclei around the doubly magic Sn-132 core.

The Full Story of the Electron Configurations of the Transition Elements

The dominant electronic valence configurations of atoms in chemical substances of a transition element of group G in period n is (n − 1)dGns0. Transition-metal chemistry is d orbital chemistry. In contrast, the ground states of free, unbound atoms derive, in most cases, from configurations (n − 1)dG−1ns1 or (n − 1)dG−2ns2. Five features must be considered to resolve this paradox. Point (i), the d−s orbital−energy distance in relation to the averaged differences of dd, ds, and ss two-electron repulsion energies, has often been discussed in the literature. However, four equally important features are (ii) the different two-electron repulsion energies within the d shell; (iii) the spin−orbit coupling energies in the d shell;and, in particular, (iv) the “d-orbital collapse” below the s level for increasing nuclear charge around group 3 and (v) the different perturbations of the valence (n − 1)d and ns orbitals when a free atom enters an alloy or compound. There is a conceptual difference between spectroscopic ground states and chemically dominant ground configurations. The ground states of unbound atoms mentioned in most chemistry textbooks have little meaning in chemistry.

Theoretical Basis and Correct Explanation of the Periodic System: Review and Update

Long-standing questions on the theoretical basis of the periodic system have been answered in recent years. A specific type of periodicity is imposed on all elements by the main groups just before and after the noble gasses. The upper np shells of these elements are unique because of their stabilized energies and the large gaps to the next higher nd and (n+1)s shells. In contrast to closed np6 shells, closed ns2 or nd10 shells are not particularly stable because they can easily hybridize with nearby np or (n+1)s shells, respectively. It is misleading to consider the electron configurations of the ground states of free neutral atoms as the dominant configurations of the chemical elements, that is, of bound atoms in chemical substances. It blurs the fact that the dominant valence configuration of, for example, chemically bound carbon is 2s12p3 or of a transition element of group G is (n+1)dGns0. The textbook wisdom that (n+1)s is below nd and is occupied before nd strictly applies only to a few of all cases. The common periodicity of elements most probably ends in period 7.

Lifetimes and branching ratios of excited states inLa−,Os−,Lu−,Lr−, andPr−

Relativistic configuration-interaction calculations have been performed for all possible $E1$, $M1$, and $E2$ transitions between bound anion states of ${\mathrm{La}}^{\ensuremath{-}}$ and ${\mathrm{Os}}^{\ensuremath{-}}$. We pay particular attention to $E1$ transitions in each case that may be of use in laser cooling of these anions. Although the ${\mathrm{La}}^{\ensuremath{-}}$ transition energy is approximately one-third of the ${\mathrm{Os}}^{\ensuremath{-}}$ transition, our results indicate that the Einstein $A$ coefficient is nearly two orders of magnitude larger, which would lead to more efficient laser cooling. We have also explored long-lived opposite-parity excited states in ${\mathrm{Lu}}^{\ensuremath{-}}$ and ${\mathrm{Lr}}^{\ensuremath{-}}$ which are restricted to $M2$ decay by selection rules. Finally, in ${\mathrm{Pr}}^{\ensuremath{-}}$, we find sufficient mixing between a weakly bound excited $4{f}^{2}5{d}^{2}6{s}^{2}$ state with a nearby $4{f}^{3}6{s}^{2}6p$ resonance to result in a lifetime similar to that of the other excited anion states, despite the fact that the dominant configurations of these $M1$ and $E2$ transitions differ by two electrons.

Mechanistic study of arsenate adsorption on lithium/aluminum layered double hydroxide

Lithium/aluminum layered double hydroxide intercalated by chloride (Li/Al LDH-Cl) was considered as a superior adsorbent for anionic compounds. This study aimed to establish the adsorption behavior of arsenate on Li/Al LDH-Cl by employing X-ray absorption spectroscopy (XAS) and adsorption kinetics. The XAS analysis demonstrated that inner-sphere complexes were the dominant arsenate adsorption configurations on the planar surfaces and edges of Li/Al LDH-Cl. Based on a kinetic study, arsenate adsorption on Li/Al LDH-Cl could be separated into fast and slow reactions. This biphasic arsenate adsorption behavior was partially attributable to: (i) two different adsorption sites associated with Li, exposing on planar surfaces, and Al, existing on the edges of double hydroxyl layers, and (ii) micropore adsorption sites within the Li/Al LDH-Cl surfaces. Activation energies derived by the Arrhenius equation indicated that the diffusion process was the rate-limiting step of arsenate adsorption on Li/Al LDH-Cl.

Molecular dynamics investigation of mixed-alkali borate glasses: Short-range order structure and alkali-ion environments

Structural properties of mixed-alkali borate glasses, $0.3[(1\ensuremath{-}x){\text{Li}}_{2}\text{O-}x{\text{Cs}}_{2}\text{O}]\ensuremath{-}0.7{\text{B}}_{2}{\text{O}}_{3}$ and $0.3[(1\ensuremath{-}x){\text{Li}}_{2}\text{O-}x{\text{Na}}_{2}\text{O}]\ensuremath{-}0.7{\text{B}}_{2}{\text{O}}_{3}$, have been studied by molecular dynamics simulations at $T=300\text{ }\text{K}$ and for several values of the alkali mixing parameter, $x$, to explore structural foundations of the mixed-alkali effect (MAE). The short-range order (SRO) structure was found to consist of borate tetrahedra, $\text{B}{\ensuremath{\emptyset}}_{4}^{\ensuremath{-}}$, and of neutral, $\text{B}{\ensuremath{\emptyset}}_{3}$, and charged, $\text{B}{\ensuremath{\emptyset}}_{2}{\text{O}}^{\ensuremath{-}}$, triangular units [$\ensuremath{\emptyset}=\text{bridging}$ oxygen atom]. The abundance of $\text{B}{\ensuremath{\emptyset}}_{4}^{\ensuremath{-}}$ units was found to decrease from Li to Cs and to exhibit negative deviation from linearity in Li-Cs glasses. However, no appreciable change in SRO structure was detected in mixed Li-Na glasses. Even though alkali metal $(M)$ ions occupy in mixed glasses sites, i.e., coordination environments with O atoms, similar to those formed in single alkali borates, mixing was found to affect the $M\text{-O}$ bonding properties of dissimilar alkalis in an opposite manner. Thus, for both systems investigated here the Li ion-coordination environment was found to become better defined and the Li-O interactions to strengthen upon alkali mixing, whereas the Cs-O and Na-O interactions become progressively weakened. The origin of these trends was traced to cationic environments formed around nonbridging oxygen (NBO) atoms in glass; it was found that the dominant cation configurations around NBOs consist of dissimilar cations in mixed-alkali glasses. The formation of dissimilar ion pairs affects by polarization effects the bonding and vibrational properties of metal ions in their oxide sites. This was demonstrated for Li-Cs glasses by both experimental and calculated far infrared spectra, where the metal ion-oxide site vibrations are strongly active. It was discussed that the preference of unlike-alkali ion pairing around NBOs and the consequent drastic reduction in the number of NBOs that sense like-cations could provide a structural explanation for the MAE.

Topological phase transition in a RNA model in the de Gennes regime

We study a simplified model of the RNA molecule proposed by Vernizzi in the regime of strong concentration of positive ions in solution. The model considers a flexible chain of equal bases that can pairwise interact with any other one along the chain while preserving the property of saturation of the interactions. In the regime considered, we observe the emergence of a critical temperature Tc separating two phases that can be characterized by the topology of the predominant configurations: in the large temperature regime, the dominant configurations of the molecule have very large genera (on the order of the size of the molecule), corresponding to a complex topology, whereas in the opposite regime of low temperatures the dominant configurations are simple and have the topology of a sphere. We determine that this topological phase transition is of first order and provide an analytical expression for Tc. The regime studied for this model exhibits analogies with the dense polymer systems studied by de Gennes.

Theoretical Study on the f−f Transition Intensities of Lanthanide Trihalide Systems

The photoabsorption intensities of intra-4f(N) transitions (f-f transitions) in lanthanide systems have been extensively studied with the semiempirical Judd-Ofelt theory. The oscillator strengths of most f-f transitions are insensitive to a change of surrounding environment because 4f electrons are shielded by closed-shell 5s and 5p electrons from outside. However, there are some exceptional transitions, so-called hypersensitive transitions, whose intensities are very sensitive to a change of surrounding environment, and the reason for this hypersensitivity has not been clarified. In this study, we calculated the oscillator strengths of lanthanide trihalides (LnX(3); Ln = Pr, Tm; X = Br, I) with the multireference spin-orbit configuration interaction method and obtained reasonably accurate values. To clarify the cause of hypersensitivity, we examined various possible effects on the oscillator strengths, such as molecular vibration, f-d mixing, ligand to metal charge transfer (LMCT), and intraligand excitation, and concluded that the effect of molecular vibration is very small and that the oscillator strengths of most f-f transitions including hypersensitive transitions arise from both the LMCT and dynamic-coupled intraligand excitations through their configuration mixings with the dominant configurations of 4f(N).

Two-quasiparticle isomers and bands ofNd154,156andSm156,158,160

The decay of a new $3.2\text{\ensuremath{-}}\ensuremath{\mu}\mathrm{s}$, $({4}^{\ensuremath{-}})$ isomeric state at 1298.0 keV has been observed using $\ensuremath{\gamma}$-ray spectroscopy at the Lohengrin mass spectrometer of the Institut Laue-Langevin. By comparison with theoretical calculations this state was assigned a $\ensuremath{\nu}5/2[642]\ensuremath{\bigotimes}\ensuremath{\nu}3/2[521]$ dominant configuration. Prompt $\ensuremath{\gamma}$-ray data from a spontaneous fission experiment have also been analyzed allowing the observation of a collective band on top of this isomeric state, the identification of a new band on top of the previously reported (${5}^{\ensuremath{-}}$) isomer of $^{156}\mathrm{Nd}$ and the extension of collective bands on top of (${5}^{\ensuremath{-}}$) isomers of the neighboring $^{156,158}\mathrm{Sm}$ nuclei. Quasiparticle rotor model calculations reported in this work correctly predict the energies and decay patterns of these bands. A new (${5}^{\ensuremath{-}}$) isomer in $^{160}\mathrm{Sm}$ has also been observed. The calculations predict $\ensuremath{\nu}5/2[642]\ensuremath{\bigotimes}\ensuremath{\nu}5/2[523]$ dominant configurations for all these (${5}^{\ensuremath{-}}$) isomers.

Frustrated Ising Model on a Diamond Hierarchical Lattice

A frustrated Ising model on a diamond hierarchical lattice is studied. We obtain the exact partition function of this model and calculate the transition temperature, specific heat, entropy, magnetization, and ferromagnetic correlation function. Depending on the magnitude of a parameter giving the frustration, there exist three types of ground states: ferromagnetic, classical spin-liquid with highly developed short-range order, and paramagnetic. The dependence of the zero-temperature entropy on the frustration parameter has an infinite number of steps. The temperature dependence of the specific heat exhibits many peaks with decreasing temperature and entropy loss. The dominant spin configurations at low temperatures are also specified.

Electron propagator studies of molecular anions

Electron propagator (EP) or Green's function (GF) methods have been successfully employed to compute electron affinities for a large number of molecules having either a closed-shell or single-valence-hole dominant electronic configuration. The accuracy of such calculations, if carried out through third order, has often been comparable to that of reasonable configuration interaction (CI) calculations. However, as a computational tool, EP methods have not yet been adequately developed in relation to general open-shell molecules and atoms, and only recently have they begun to be generalized to describe states having two or more dominant configurations. Thus although GFs look promising as methods for ab initio calculation, much formal and programming work remains to be done before this approach can be said to compete with CI methods. On the other hand, GFs provide us with a mechanism for focusing on the one-electron (EP) or two-electron [polarization propagator (PP)] features of any problem. It is this fact that makes GFs an attractive route for developing chemical models which may or may not make use of experimental data. Although such a viewpoint has been widely used in solid-state physics, not enough work has been done on making models based upon the EP or PP. In this article, both the computational and formal history of GFs, as they apply particularly to molecular anion studies, are overviewed. The author's opinions concerning the current status and future development of the area constitute a large part of the presentation.

Magnetic structure and interactions in the quasi-one-dimensional antiferromagnetCaV2O4

${\text{CaV}}_{2}{\text{O}}_{4}$ is a spin-1 antiferromagnet, where the magnetic vanadium ions have an orbital degree of freedom and are arranged on quasi-one-dimensional zigzag chains. The first- and second-neighbor vanadium separations are approximately equal suggesting frustrated antiferromagnetic exchange interactions. High-temperature susceptibility and single-crystal neutron-diffraction measurements are used to deduce the dominant exchange paths and orbital configurations. The results suggest that at high temperatures ${\text{CaV}}_{2}{\text{O}}_{4}$ behaves as a Haldane chain, but at low temperatures, it is a spin-1 ladder. These two magnetic structures are explained by different orbital configurations and show how orbital ordering can drive a system from one exotic spin Hamiltonian to another.

Theoretical study of the electronic states of CuCl2

The electronic states of the CuCl(2) molecule are studied by several theoretical methods. We report geometries, excitation energies, vibrational frequencies, rotational constants, and transition dipole moments. With the purpose to describe the correlation energy accurately enough, a set of diffuse secondary 3d(') orbitals is introduced, thus resulting in a large active space of 21 electrons in 17 orbitals. By restricting the active space and selecting dominant configurations, the results of the general multireference second-order perturbation theory with this large active space agree very well with the experimental ones. It is found that the so-called (2)Pi(u) state is asymmetric linear and the (2)Sigma(u)(+) state is bent at the minima on their adiabatic potential energy surfaces, whereas the other five gerade states are centrosymmetric linear. After including the spin-orbit coupling, the (I)(2)Pi(g3/2)-(I)(2)Pi(g1/2) splitting is computed to be 415 cm(-1), in excellent agreement with the experimental value of about 480 cm(-1).

Theoretical study on the dual fluorescence of 2-(4-cyanophenyl)-N,N-dimethylaminoethane and its deactivation pathway

Low-lying singlet states and photophysical deactivation pathways of the electronically decoupled 2-(4-cyanophenyl)-N,N-dimethylaminoethane (PCN2NM) have been investigated by the density functional theory and CASSCF/CASPT2 approaches. PCN2NM in the ground state has two dominant configurations of the alkyl-twisted and alky-stretched structures with an interconversion barrier of 2.5 kcal/mol. The predicted vibrationally resolved weak absorption and fluorescence emission of (1)(S(1)-L(b)) exhibits clearly well-resolved vibronic features, whereas (1)(S(1)-CT) state corresponding to the redshifted fluorescence has no fine structure. Due to the presence of low-energy (1)(S(1)/S(0))(CI), radiationless decay of the excited PCN2NM to the ground state is facile, once the excess energy beyond the barrier is available. Present results show reasonable agreement with experimental observations available and provide a basis for understanding of the dual fluorescence of the electronically decoupled species.

Top-quark pair production near threshold at LHC

The next-to-leading order analysis for the cross section for hadroproduction of top-quark pairs close to threshold is presented. Within the framework of non-relativistic QCD a significant enhancement compared to fixed-order perturbation theory is observed which originates from the characteristic remnant of the 1S peak below production threshold of top-quark pairs. The analysis includes all color-singlet and color-octet configurations of top-quark pairs in S-wave states and, for the dominant configurations, it employs all-order soft-gluon resummation for the hard parton cross section. Numerical results for the Large Hadron Collider at $\sqrt{s}=14~$ TeV and $\sqrt{s}=10~$ TeV and also for the Tevatron are presented. The possibility of a top-quark mass measurement from the invariant-mass distribution of top-quark pairs is discussed.

Folding Peptides Into Lipid Bilayer Membranes

The folding and integration of peptides into lipid bilayers remains one of the most intriguing processes in biophysics, as it cannot be directly observed at sufficient temporal and spatial resolutions. From a physical chemistry perspective transfer of solvated peptides into a hydrocarbon phase should follow a two-stage pathway, where helical segments fold at the phase boundary before inserting, due to the large energetic penalty associated with de-solvating exposed peptide bonds.The adsorption, folding and membrane insertion of a model peptide (WALP) was studied via microsecond-timescale molecular simulations at atomic resolution. Both an implicit model and an explicit lipid bilayer setup were used. The implicit simulations generally follow the theoretically predicted two-stage pathway. The vastly increased sampling yields fully converged thermodynamic properties such as the free-energy of folding and membrane insertion. In contrast, the explicit bilayer simulations show that after spontaneous adsorption the peptide immediately crosses the polar interface to locate at the hydrophobic membrane core. Remarkably, there is no interfacial state and the dominant configurations are deeply inserted unfolded and beta-hairpin conformers. The native trans-membrane helix formed for several hundred nanoseconds is not stable. At present the reasons for this unexpected behavior remain unclear.View Large Image | View Hi-Res Image | Download PowerPoint Slide

Folding Peptides into Lipid Bilayer Membranes.

The adsorption, insertion, and folding of a synthetic 16-residue WALP peptide was studied at physiological time scales (>μs) by atomic detail molecular dynamics simulation using a fully explicit DPPC/DMPC lipid bilayer setup. The temperature was elevated to 80 °C/44 °C respectively to increase sampling. After spontaneous adsorption the peptide crosses the polar interfaces to locate at the hydrophobic bilayer core. Remarkably, insertion occurs before folding, and the dominant configurations are inserted beta-hairpins. For the DPPC simulation a trans-membrane helix formed but was not stable. Unfolded membrane insertion of WALP was first observed by Nymeyer and co-workers using a replica exchange method. However, both results are in stark contrast to current theory and simulations with implicit membrane models, which rule out unfolded insertion into the hydrophobic core. At present the exact reasons for this unexpected behavior cannot be unambiguously determined, due to the lack suitable experimental and simulation data to compare to. Nevertheless, the results demonstrate that simulation studies can now in principle provide atomic detail insights into complex biophysical phenomena at physiologically relevant time scales. Future effort must now concentrate on suitable ways to verify current force fields and methodologies for such simulations.