Electron-ion collisional dissociative recombination at high ambient ammonia densities

A plot of the measured electron—ion recombination coefficient against the number density of ambient ammonia shows an unexpectedly early fall which, it is suggested, may be due to the collisional coupling between the dissociative states and the other states being weakened by the growth in the clustering.     

Efficient combination of Wang-Landau and transition matrix Monte Carlo methods for protein simulations

An efficient combination of the Wang-Landau and transition matrix Monte Carlo methods for protein and peptide simulations is described. At the initial stage of simulation the algorithm behaves like the Wang-Landau algorithm, allowing to sample the entire interval of energies, and at the later stages, it behaves like transition matrix Monte Carlo method and has significantly lower statistical errors. This combination allows to achieve fast convergence to the correct values of density of states. We propose that the violation of TTT identities may serve as a qualitative criterion to check the convergence of density of states. The simulation process can be parallelized by cutting the entire interval of simulation into subintervals. The violation of ergodicity in this case is discussed. We test the algorithm on a set of peptides of different lengths and observe good statistical convergent properties for the density of states. We believe that the method is of general nature and can be used for simulations of other systems with either discrete or continuous energy spectrum.     

Olefin adsorption on silica-supported silver salts--a DFT study

Recent experiments have shown that silver salts supported on mesoporous silicas display excellent adsorption selectivities of ethylene over ethane and propylene over propane. Employing the techniques of density functional theory, we have investigated the fundamental bases of this separation process by examining silver salts dispersed on model silica surfaces. Our model system includes Ag+ cations, their counteranions, silica supports, and surface silanols. Both adsorption geometries and energetics of ethylene and propylene were explored. Our results indicate that the nature of the Ag-olefin interaction is predominantly hybridization between Ag d and olefin pi states, which is supported by analyses of electron density difference plots and density of states. The counteranions, such as NO3- were found to interact strongly with surface silanols through multiple hydrogen bonds but have limited effect on the adsorption energy of olefins on the Ag+ cations. The current work supports recent experiments, which indicate that Ag-salt/silica may be a very promising adsorbent for olefin/paraffin separation.     

Intermediate case radiationless decay: the excited state dynamics of pyrazine

This paper reports new measurements of the non-exponential fluorescence decay of pyrazine, covering the time range 0.1-200 ns. The restits of our measurements remove the inconsistencies between experiment and the Frad-Lahmani-Tramer-Tric model of the radiationless decay in this molecule. In particular, our data show that the triplet component of the mixed singlet-triplet levels does increase with increasing triplet density of states. The effective density of triplet levels determined from our experimental data exceeds the theoretical density of vibrational levels. We propose that at the excitation levels achieved in the triplet manifold there is extensive scrambling of rotational states, but that conservation of nuclear spin states permits a level with total angular momentum quantum number f to couple to only (2J + 1/4) of the 2J+1 rotational levels built on one vibration. The appropriate theoretical density of states to be compared with experiment is then obtained by multiplying the vibrational density of states by J2. Good agreement is found between experimentally determined and calculated densities of states.     

Charge recombination in CuPc/PTCDA thin films

The recombination kinetics of photogenerated charge carriers in perylene-3, 4, 9, 10-tetracarboxylic dianhydride (PTCDA) and copper phthalocyanine (CuPc) thin films grown by organic molecular beam deposition have been studied using transient absorption spectroscopy. Optical excitation is observed to generate long-lived polaron states, which exhibit power law recombination dynamics on time scales from microseconds to milliseconds. Studies as a function of excitation density and temperature, and comparison between heterostructures and PTCDA single layers, all indicate that this power law behavior results from trapping of PTCDA- polarons in localized states, with an estimated trap state density of approximately 6 x 10(17) polarons cm(-3). This recombination behavior is found to be remarkably similar to that previously observed for polymer/fullerene blends, suggesting that it may be generic to a range of semiconducting materials.     

Calculating energy levels of isomerizing tetra-atomic molecules. II. The vibrational states of acetylene and vinylidene

A general, full-dimensional computational method for the accurate calculation of rotationally and vibrationally excited states of tetra-atomic molecules is further developed. The resulting computer program may be run in serial and parallel modes and is particularly appropriate for molecules executing wide-amplitude motions and isomerizations. An application to the isomerizing acetylene/vinylidene system is presented. Large-scale calculations using a coordinate system based on orthogonal satellite vectors have been performed in six dimensions and vibrational term values and wave functions for acetylene and vinylidene states up to approximately 23 000 cm(-1) above the potential minimum have been determined. This has permitted the characterization of acetylene and vinylidene states at and above the isomerization barrier. These calculations employ more extensive vibrational basis sets and hence consider a much higher density of states than in any variational calculations reported hitherto for this system. Comparison of the calculated density of states with that determined empirically suggests that our calculations are the most realistic achieved for this system to date. Indeed more states have been converged than in any previous study of this system. Calculations on lower lying excited states of acetylene based on HC-CH diatom-diatom coordinates give nearly identical results to those based on orthogonal satellite vectors. Comparisons are also made with calculations based on HH-CC diatom-diatom coordinates.     

Search for stratospheric bromine reservoir species: theoretical study of the photostability of mono-, tri-, and pentacoordinated bromine compounds

Previous work has shown that pentacoordinated bromine compounds have their lowest excited electronic states shifted to the blue relative to monocoordinated bromine molecules, and that this shift may be large enough to render them photostable in the lower stratosphere. Our earlier work has also shown that certain pentacoordinated bromine compounds are thermodynamically stable relative to their mono- or tricoordinated isomers, suggesting that if a bromine stratospheric reservoir species exists, it may be a pentacoordinated compound. In this study we have examined the singlet and triplet excited electronic states of several bromine compounds, using time dependent density functional theory, to assess their photostability under stratospheric conditions and in order to elucidate the nature of lowest excited states in mono-, tri-, and pentacoordinated bromine molecules. The triplet states have been included due to the strong spin-orbit mixing in bromine. We have found several pentacoordinated bromine/oxygen compounds that could be photostable in the lower stratosphere, but we have also found that monovalent bromine compounds where the bromine atom is bonded to an atom with no lone-pair p-electrons is far and away the most photostable. Attachment/detachment electron density plots have been useful in ascertaining the nature of the excited electronic states and their likely path to photodissociation.     

Anharmonic effect on unimolecular reactions with application to the photodissociation of ethylene

The importance of anharmonic effect on dissociation of molecular systems, especially clusters, has been noted. In this paper, we shall present a theoretical approach that can carry out the first principle calculations of anharmonic canonical and microcanonical rate constants of unimolecular reactions within the framework of transition state theory. In the canonical case, it is essential to calculate the partition function of anharmonic oscillators; for convenience, the Morse oscillator potential will be used for demonstration in this paper. In the microcanical case, which involves the calculation of the total number of states for the activated complex and the density of states for the reactant, we make use of the fact that both the total number of states and the density of states can be expressed in the inverse Laplace transformation of the partition functions and that the inverse Laplace transformation can in turn be carried out by using the saddle-point method. We shall also show that using the theoretical approach presented in this paper the total number of states and density of states can be determined from thermodynamic properties and the difference between the method used in this paper and the thermodynamic model used by Krems and Nordholm will be given. To demonstrate the application of our theoretical approach, we chose the photodissociation of ethylene at 157 and 193 nm as an example.     

Density of electron states in crystalline systems calculated in the presence and absence of the magnetic field

It is demonstrated in terms of explicit calculations that the average density of states in a crystalline system remains unaffected by the presence of the external magnetic field. This is so on condition the magnetic field is weak enough to provide us with a large number of the Landau levels per energy unit. For a special case of a two‐dimensional crystalline system, the equivalence of the density of states obtained in the presence and the absence of the magnetic field can be shown in an analytic way. For three dimensions, the tightly bound s‐electron states in crystals having cubic symmetry are taken into consideration. Here the density of states calculated in the presence of the magnetic field is compared numerically with that of the field‐absent Bloch states acquired with the aid of Jelitto's method. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Limitations of density functional theory in application to degenerate states

It is shown that the claims that density functional theory (DFT) can handle orbitally degenerate states are ungrounded. The constraint search formulation of DFT allows one to determine a set of densities and eigenvalues for the degenerate term that, however, are neither observables, nor can they be used to solve the system of coupled equations for the nuclear motions to obtain observables, as in the wave function presentation. A striking example of the failure of the existing versions of DFT to describe degenerate states is provided by the Berry phase problem: the strong dependence of the results on the phase properties of the electronic wave function that are smeared out in the density formulation. The solution of the Jahn-Teller E-e problem illustrates these statements. For nondegenerate states with the full wave function taken in the adiabatic approximation as a product of the electronic and nuclear parts, the formulation of DFT is rigorous if and only if the dependence of the electronic wave function on nuclear coordinates is ignored. This lowers the accuracy of the results, in general, and may lead to erroneous presentation as in the case of molecular systems in strong magnetic fields. © 1997 by John Wiley & Sons, Inc.     

Structural,electronic, and optical properties of Cd-halide-based inorganic LiCdX3 (X = F,Cl) perovskites for photovoltaic applications: A density functional theory study

Halide base perovskite LiCdX₃ (X = F, Cl) is tested by CASTEP (Cambridge Serial Total Energy Package) based on density function theory (DFT). The presented discussion is to explore the structural, electronic, and optical properties of LiCdX₃ (X = F, Cl). The calculated values of the lattice parameter are found to be 3.8 Å and 5.27 Å of LiCdF₃ and LiCdCl₃ respectively. The ideal structure of LiCdX₃ (X = F, Cl) is cubic and dynamically stable. Electronic properties show that materials are semiconductors. The results from band structure are further evaluated by the total and partial density of states. The partial and total density of states confirms the degree of localization of electrons. In optical properties, the highest absorption coefficient is observed in LiCdCl₃. The material is half metallic and has a narrow indirect band gap which may be used in photovoltaic applications.     

Is the 3MLCT the only photoreactive state of polypyridyl complexes?

By means of Delta-SCF and time-dependent density functional theory (DFT) calculations on [Ru(LL)3]2+ (LL = bpy = 2,2'-bipyridyl or bpz = 2,2' -bipyrazyl) complexes, we have found that emission of these two complexes could originate from two metal-to-ligand charge-transfer triplet states (3MLCT) that are quasi-degenerate and whose symmetries are D3 and C2. These two states are true minima. Calculated absorption and emission energies are in good agreement with experiment; the largest error is 0.14 eV, which is about the expected accuracy of the DFT calculations. For the first time, an optimized geometry for the metal-centered (MC) state is proposed for both of these complexes, and their energies are found to be almost degenerate with their corresponding 3MLCT states. These [RuII(LL)(eta1-LL)2]2+ MC states have two vacant coordination sites on the metal, so they may react readily with their environment. If these MC states are able to de-excite by luminescence, the associated transition (ca. 1 eV) is found to be quite different from those of the 3MLCT states (ca. 2 eV).     

Method for efficient computation of the density of states in water-explicit biopolymer simulations on a lattice

We present a method for fast computation of the density of states of binary systems. The contributions of each of the components to the density of states can be separated based on the conditional independence of the individual components' degrees of freedom. The conditions establishing independence are the degrees of freedom of the interfacial region between the two components. The separate contributions of the components to the density of states can then be calculated using the Wang-Landau algorithm [Wang, F.; Landau, D. P. Phys. Rev. Lett. 2001, 86, 2050]. We apply this method to a 2D lattice model of a hydrophobic homopolymer in water that exhibits protein-like cold, pressure, and thermal unfolding. The separate computation of the protein and water density of states contributions is faster and more accurate than the combined simulation of both components and allows for the investigation of larger systems.     

Incompletely symmetric non-bloch electron states in perfect cubic crystals. II. Density of states in the FCC lattice

The electron density of both the perturbed and unperturbed crystal can be made up of individual terms described by the basis functions of irreducible representations of the crystal point group. For the perfect FCC lattice, a detailed comparison was made between the density of states, calculated in terms of the LCAO wave functions classified according to representations of the crystal point group, and the density of states, provided by the Bloch theory.     

Structures and Surface States of Polymer Brushes in Good Solvents: Effects of Surface Interactions

The influence of the surface interaction on the mesoscopic structure of grafted polymers in good solvents has been examined. At high surface coverage, tethered polymers are in the brush state and the parabolic segment density profile is confirmed by self-consistent field theory (SCFT) calculations. It is found that this is a universal behavior for a whole range of surface interactions from complete repulsion to strong attraction. More interestingly, finite surface repulsion may lead to the maximum in the proximal layer of its segment density profile, which is significantly different from both the depletion layer of pure repulsion and the adsorbing layer of attraction. In addition to the brush state on both repulsive and attractive surfaces, three additional surface states were identified by analyzing the scaling behavior of the layer thickness of polymer brushes: the mushroom state on repulsive substrates, the dilute and the semidilute surface states on attractive substrates.     

Long-range corrected time-dependent density functional study on fluorescence of 4,4'-dimethylaminobenzonitrile

Dual fluorescence of 4,4(')-dimethylaminobenzonitrile (DMABN) was theoretically investigated on the basis of long-range corrected time-dependent density functional theory. Excited-state geometry optimization states and single-point energy calculations with and without solvent effect were carried out. It has been explained that DMABN emits dual fluorescence only in polar solvents through locally excited (LE) and charge transfer (CT) states. It was, however, concluded from this study that although the main spectrum of dual fluorescence in acetonitrile solvent is clearly due to twisted intramolecular CT fluorescence, small secondary fluorescence in acetonitrile may also emanate from CT fluorescence during the DMABN twisting process. This conclusion is supported by an experimental interpretation on polarization spectroscopy. It was also found that the optimized DMABN geometries have certain wagging angles for the CT state and no wagging angle for the LE state. This may support an early experimental hypothesis that the dual fluorescence of DMABN is induced by the wagging mode due to vibronic coupling between LE and CT states. Consequently, the authors propose a fluorescence mechanism of DMABN in gas phase and in acetonitrile solvent: the main absorption proceeds to the CT state in both situations. In gas phase, single fluorescence is chiefly emitted from the LE state through the internal conversion from CT to LE states. Dual fluorescence in acetonitrile solvent may only be emitted from the CT state.     

稀土对ZA27合金晶间腐蚀影响的电子理论研究

通过自行开发的计算机软件构造了ZA27合金中α相大角度晶界析出η相及稀土元素的原子集团模型. 采用递归法计算了Al, Zn, La, Y的局域态密度, 计算并分析了α相、η相的总态密度和费米能级, 及稀土对态密度和费米能级的影响. 计算表明: 稀土元素的局域态密度形状与Zn相近, 其与Zn结合的能力大; 稀土元素降低η相的费米能级, 减小Zn, Al电极电位差, 具有抑制晶间腐蚀的作用; 稀土元素不改变α相、η相的总态密度形状, 但使η相的态密度增大, 改变η相的电子结构; 晶格结构对原子的态密度有一定的影响, 掺杂原子的态密度趋于与基体原子态密度相同.     

Understanding Chemical Reactivity in Extended Systems:Exploring Models of Chemical Softness in Carbon Nanotubes

Chemical reactivity towards electron transfer is captured by the Fukui function.However,this is not well defined when the system or its ions have degenerate or pseudo-degenerate ground states.In such a case,the first-order chemical response is not independent of the perturbation and the correct response has to be computed using the mathematical formalism of perturbation theory for degenerate states.Spatialpseudo-degeneracy is ubiquitous in nanostructures with high symmetry and totally extended systems.Given the size of these systems,using degenerate-state perturbation theory is impractical because it requires the calculation of many excited states.Here we present an alternative to compute the chemical response of extended systems using models of local softness in terms of the local density of states.The local softness is approximately equal to the density of states at the Fermi level.However,such approximation leaves out the contribution of inner states.In order to include and weight the contribution of the states around the Fermi level,a model inspired by the long-range behavior of the local softness is presented.Single wall capped carbon nanotubes(SWCCNT) illustrate the limitation of the frontier orbital theory in extended systems.Thus,we have used a C360 SWCCNT to test the proposed model and how it compares with available models based on the local density of states.Interestingly,a simple Hü ckel approximation captures the main features of chemical response of these systems.Our results suggest that density-of-states models of the softness along simple tight binding Hamiltonians could be used to explore the chemical reactivity of more complex system,such a surfaces and nanoparticles.