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A new algorithm for calculating the Hamiltonian matrix elements with all-electron explicitly correlated Gaussian functions for quantum-mechanical calculations of atoms with arbitrary angular momentum is presented. The calculations are checked on several excited states of three and four electron systems. The presented formalism can be used as unified framework for high accuracy calculations of properties of small atoms and molecules. [ABSTRACT FROM AUTHOR]

In the framework of real time real space time-dependent density functional theory we have studied the electron-ion dynamics of a hydrogen-terminated silicon surface H-Si(111) subjected to intense laser irradiation. Two surface fragments of different sizes have been used in the simulations. When the intensity and duration of the laser exceed certain levels (which depend on the wavelength) we observe the desorption of the hydrogen atoms, while the underlying silicon layer remains essentially undamaged. Upon further increase of the laser intensity, the chemical bonds between silicon atoms break as well. The results of the simulations suggest that with an appropriate choice of laser parameters it should be possible to remove the hydrogen layer from the H-Si(111) surface in a matter of a few tens of femtoseconds. We have also observed that at high laser field intensities (2-4 V/Å in this work) the desorption occurs even when the laser frequency is smaller than the optical gap of the silicon surface fragments. Therefore, nonlinear phenomena must play an essential role in such desorption processes. [ABSTRACT FROM AUTHOR]

The time-dependent Schrödinger equation is solved by using an explicitly time-dependent basis. This approach allows efficient reflection-free time propagation of the wave function. The applicability of the method is illustrated by solving various time-dependent problems including the calculation of the above threshold ionization of a model atom and the optical absorption spectrum of a sodium dimer. [ABSTRACT FROM AUTHOR]

Variational calculations of the ground state of positronium hydride (HPs) are reported, including various expectation values, electron-positron annihilation rates, and leading relativistic corrections to the total and dissociation energies. The calculations have been performed using a basis set of 4000 thoroughly optimized explicitly correlated Gaussian basis functions. The relative accuracy of the variational energy upper bound is estimated to be of the order of 2×10-10, which is a significant improvement over previous nonrelativistic results. [ABSTRACT FROM AUTHOR]

Electron transport in nanostructures is calculated and compared using a time-independent and a time-dependent first-principles framework. The time-independent approach uses the nonequilibrium Green's-function technique to calculate the current, while the time-dependent method extracts the current from the time propagated wave function. The approaches have been tested using gold-benzene-dithiolate-gold and gold-bipyridine-gold molecular junctions. The reasons for the differences in the current-voltage curves predicted by the two methods are discussed. [ABSTRACT FROM AUTHOR]

The stochastic variational method is a powerful approach to solve few-body problems. The application of the stochastic variational approach to few-body problems in condensed matter physics is presented. The examples include calculation of energy spectra of atoms in magnetic field, confined atoms and trapped Fermi gases. [ABSTRACT FROM AUTHOR]

A multidomain decomposition method is presented for time-dependent density functional calculations. The multidomain decomposition approach divides the coordinate space into subdomains where local solutions are generated. The subdomain solutions are used as basis states to describe the time propagation of electronic wave functions in the whole system. The applicability and flexibility of the approach are demonstrated on various test systems including the calculation of the photoabsorption cross section of a graphene ribbon and a light-harvesting triad. [ABSTRACT FROM AUTHOR]

The binding energy and structure of biexcitons in strong magnetic field is investigated using the stochastic variational method. The magnetic field confines the electrons and positrons in a small volume leading to Wigner-crystal like states of particles. [ABSTRACT FROM AUTHOR]

Abstract: This paper is focused on the in situ radiotracer and voltammetric studies of the induced HSO4 −/SO4 2− adsorption at Pt(poly) and Pt(111) surfaces in 0.1moldm−3 HClO4 solution in the course of Cr(VI) electroreduction. Besides this, the sorption behavior of HSO4 −/SO4 2− ions on bare Pt(poly) and Pt(111) electrodes is compared and discussed. From the experimental results it can be stated that: (i) although the extent of bisulfate/sulfate adsorption is strongly dependent upon the crystallographic orientation of Pt surfaces, the maximum coverage on the Pt(111) does not exceed 0.2 monolayer; (ii) the Cr(VI) electroreduction on both poly- and (111) oriented platinum proceeds via a ce (chemical-electron-transfer) mechanism to yield Pt surfaces covered with intermediate surface adlayers containing Cr(VI) particles (and reduced Cr-containing adspecies) and ‘strongly bonded’ HSO4 −/SO4 2− ions; (iii) while the coverage of platinum surfaces by the intermediate complexes formed in the course of Cr(VI) electroreduction at E >0.20V is basically independent of the crystallographic orientation of the Pt electrode, the onset for rapid Cr(VI) reduction is highly affected by the nature and crystallographic orientation of the electrode. [Copyright &y& Elsevier]

The electroreduction of Cr(VI) species to Cr(III) at noble metal electrodes in acidic solutions takes place via a ce (chemical-electron-transfer) mechanism where the heterogeneous chemical step which precedes the charge transfer involves anions (e.g. HSO4−/SO42−) leading to the formation of intermediate surface complexes. In this paper, we report some new findings obtained by in situ radiotracer and voltammetric studies of the simultaneous adsorption of anions (HSO4−/SO42− labeled with 35S, Cl− labeled with 36Cl) and Cr-containing species (labeled with 51Cr) on a gold electrode in the course of dichromate reduction in 1 mol dm−3 HClO4 supporting electrolyte. Moreover, special attention is paid to present a detection and calculation method elaborated for the quantitative evaluation of the surface excess of radiolabeled Cr species via measurement of the intensity of low energy X-rays (E=4.90 keV) emitted by 51Cr. From our experimental results it can be stated that: (i) at E<1.20 V the electroreduction of Cr(VI) particles presumably proceeds via a ce mechanism to yield a gold surface covered with an intermediate surface adlayer containing Cr(VI) species and added anions (HSO4−/SO42−, ClO4−); (ii) no data indicating the embedding of Cl− ions into the surface film could be detected; (iii) the maximum surface excess of Cr-containing species (Γmax=1.2×10−9 mol cm−2) as well as the molar ratio between the Cr species and HSO4−/SO42− ions (m&ap;6 at E=0.8 V) attests that the coverage of the gold surface with intermediate complexes does not exceed one monolayer. [Copyright &y& Elsevier]

A complex potential is constructed to inject electrons into the conduction band, mimicking electron currents in nanoscale systems. The injected electrons are time propagated until a steady state is reached. The local current density can then be calculated to show the path of the conducting electrons on an atomistic level. The method allows for the calculation of the current density vectors within the medium as a function of energy of the conducting electron. Using this method, we investigate the electron pathway of graphene nanoribbons in various structures, molecular junctions, and black phosphorus nanoribbons. By analyzing the current flow through the structures, we find strong dependence on the structural geometry and the energy of the injected electrons. This method may be of general use in the study of nano-electronic materials and interfaces. [ABSTRACT FROM AUTHOR]

The dynamics of hydrogen desorption from H-terminated silicon surface clusters has been simulated in the framework of real space time-dependent density functional theory complemented with molecular dynamics for ions. It has been demonstrated that by choosing an appropriate frequency and intensity of the laser it is possible to remove the hydrogen layer from the surface without destroying the structure of underlying silicon. At the laser field intensities used in the current study (0.5-2.0 V/Å) the desorption process is notably nonlinear. [ABSTRACT FROM AUTHOR]

The quantum-electrodynamical time-dependent density functional theory equations are solved by time propagating the wave function on a tensor product of a Fock-space and real-space grid. Applications for molecules in cavities show the accuracy of the approach. Examples include the coupling strength and light frequency dependence of the energies, wave functions, optical absorption spectra, and Rabi splitting magnitudes in cavities, as well as a description of high harmonic generation in cavities. [ABSTRACT FROM AUTHOR]

Deformed explicitly correlated Gaussian (DECG) basis functions are introduced, and their matrix elements are calculated. All matrix elements can be calculated analytically in a closed form, except the Coulomb one, which has to be approximated by a Gaussian expansion. The DECG basis functions can be used to solve problems with nonspherical potentials. One example of such potential is the dipole self-interaction term in the Pauli–Fierz Hamiltonian. Examples are presented showing the accuracy and necessity of deformed Gaussian basis functions to accurately solve light–matter coupled systems in cavity QED. [ABSTRACT FROM AUTHOR]

A dual time propagation approach is introduced to describe electron scattering and ionization. The space is divided into two regions, a central region with a full time-dependent Hamiltonian and an outer region where the kinetic operator and the laser field dominate. The two regions are connected by a source term. Time-dependent density functional theory calculations of wave packet scattering on molecules and photoelectron spectrum due to circularly polarized laser are presented to illustrate the efficiency and applicability of the approach. [ABSTRACT FROM AUTHOR]

In the framework of real-time real-space time-dependent density functional theory complemented with Ehrenfest molecular dynamics we have studied the response of small graphene and graphane fragments to intense femtosecond laser pulses. In particular, we have investigated how the response changes with laser pulses of different frequency (near IR, visible, and UV). The results of our simulations show that graphene has a very high immediate (i.e., within laser-pulse duration) damage threshold. They also suggest that, similar to the case of other hydrogenated surfaces, it should be possible to selectively desorb hydrogens from graphane without destroying the underlying carbon structure provided that the laser pulse parameters are properly chosen. [ABSTRACT FROM AUTHOR]

Erratum: "Real-space, real-time approach to quantum-electrodynamical time-dependent density functional theory" [J. Chem. Phys. In Eq. (2.22), a term, HT ht , is missing. [Extracted from the article]

Using first-principles calculations, it is shown that the work function of graphene on copper can be adjusted by varying the concentration of intercalated alkali metals. Using density functional theory, we calculate the modulation of work function when Li, Na, or K are intercalated between graphene and a Cu(111) surface. The physical origins of the change in work function are explained in terms of phenomenological models accounting for the formation and depolarization of interfacial dipoles and the shift in the Fermi-level induced via charge transfer. [ABSTRACT FROM AUTHOR]

Highlights: [•] Porous Pt layers were electroplated for radioelectrochemical measurements. [•] The theoretical relationships describing the porous electrode have been validated. [•] Surface excess of the chloride ions as a function of the potential could be measured. [•] Saturation time of the porous layers could be quantitatively estimated. [•] Mobility of the adsorbed chloride ions have been tested. [ABSTRACT FROM AUTHOR]

Highlights: [•] The basis of a new radielectrochemical method with porous electrode was described. [•] The relationship for the radiation intensity vs. surface excess function was given. [•] The parameters were optimized for the platinum black/36Cl chloride ion system. [•] The proper thickness of the infinitely thin porous layer was calculated. [ABSTRACT FROM AUTHOR]

Time-dependent density functional theory simulation of laser-induced ionization is presented. Various test systems including a small wire-like molecule, C12H14, as well as carbon nanotubes with varying diameter are studied. It has been demonstrated that significant ionization electron current is produced when a laser pulse is applied. Moreover, pulse-like patterns of the current have been observed, which suggests that short laser pulses can be used to create spatially and temporally localized electron sources. [ABSTRACT FROM AUTHOR]

A method to calculate the transmission coefficient in multiterminal systems is presented. By adding a complex absorbing potential to the Hamiltonian of the semi-infinite leads, the problem of inverting an infinite dimensional matrix is transformed into a finite dimensional eigenvalue problem. Using this approach transmission coefficients are calculated for all energies at once. The accuracy of the approach is demonstrated with an analytically solvable model system. Numerical examples of a four-terminal graphene cross junction and six-terminal carbon nanotube junction are presented. [ABSTRACT FROM AUTHOR]

The spatial current distribution in H-terminated zigzag graphene nanoribbons (ZGNRs) under electrical bias is investigated using time-dependent density-functional theory solved on a real-space grid. A projected complex absorbing potential is used to minimize the effect of reflection at simulation cell boundary. The calculations show that the current flows mainly along the edge atoms in the hydrogen terminated pristine ZGNRs. When a vacancy is introduced to the ZGNRs, loop currents emerge at the ribbon edge due to electrons hopping between carbon atoms of the same sublattice. The loop currents hinder the flow of the edge current, explaining the poor electric conductance observed in recent experiments. [ABSTRACT FROM AUTHOR]

Kinetic, potential and overlap matrix elements of one dimensional correlated Gaussians multiplied by polynomial factors are presented. These matrix elements can be used to calculate energies of one dimensional cold atom systems, or to construct a tensorial product to calculate energies in 2 or 3 dimensional systems with a nonspherical potential. [ABSTRACT FROM AUTHOR]

A real-space, real-time implementation of time-dependent density functional theory is used to study electron field emission from nanostructures. Carbon nanotubes and graphene nanoribbons are used as model systems. The calculations show that carbon nanotubes with iron adsorbates have spin-polarized emission currents. Graphene nanoribbons are shown to be good field emitters with spatial variation of the emission current influenced by the presence of passivating hydrogen. [ABSTRACT FROM AUTHOR]

The integrating factor and exponential time differencing methods are implemented and tested for solving the time-dependent Kohn-Sham equations. Popular time propagation methods used in physics, as well as other robust numerical approaches, are compared to these exponential integrator methods in order to judge the relative merit of the computational schemes. We determine an improvement in accuracy of multiple orders of magnitude when describing dynamics driven primarily by a nonlinear potential. For cases of dynamics driven by a time-dependent external potential, the accuracy of the exponential integrator methods are less enhanced but still match or outperform the best of the conventional methods tested. [ABSTRACT FROM AUTHOR]

Abstract: Sum‐accelerated pseudospectral methods herein are used to describe the kinetic energy matrix within density functional theory descriptions of few‐atom systems. The resulting energies are compared to the conventional finite difference results. It is demonstrated that the sum‐acceleration allows for significantly improved scaling with respect to the grid step size and matrix bandwidth while maintaining a computational speed equivalent with that of the finite difference description. In addition, analytical expressions for the pseudopotential matrix elements are introduced to alleviate the eggbox effect—the dependence of the energy on the relative position of the atoms and grid points. [ABSTRACT FROM AUTHOR]

The stochastic variational method is applied to excitonic formations within semiconducting transition metal dichalcogenides using a correlated Gaussian basis. The energy and structure of two- to six-particle systems are investigated along with their dependence on the effective screening length of the two-dimensional Keldysh potential and the electron-hole effective mass ratio. Excited state biexcitons are shown to be bound, with binding energies of the L=0 state showing good agreement with experimental measurements of biexciton binding energies. Ground and newly discussed excited state exciton-trions are predicted to be bound and their structures are investigated. [ABSTRACT FROM AUTHOR]

Time-dependent density functional theory is utilized to simulate an asymmetrical jellium model, representing a nano-scale vacuum-tube diode comprised of bulk lithium. A sharp tip on one end of the jellium model allows for enhanced field emission upon interaction with an external laser field, leading to a preferential net current direction. This is verified by comparing the rate of electron transfer between the effective anode and cathode tips for both the diode jellium model and a symmetric cylinder jellium shape for various laser phase parameters. This rate of transfer is shown to significantly increase with smaller separation distances. With stronger laser intensities, this rate similarly increases but levels off as local near-field enhancements become negligible. [ABSTRACT FROM AUTHOR]

Abstract: The so-called in situ thin gap method for the measurement of the surface excess of the radiolabelled species was originally designed for the application of noble metal electrodes with low surface roughness. Therefore, the observation of adsorbed species containing radioisotopes of high specific activity (i.e., 35S and 14C) was only possible. A recent extension of the original thin gap method to metals with a surface roughness factor higher than 10, however, made it possible to follow the adsorption of species with low specific activity (i.e., 36Cl). Based on the validation of the improved thin gap method (Horváth et al., 2013), this study aims at the comparison of the sulfate/bisulfate and chloride adsorption on both smooth (Pt and Au) and rough (platinized platinum) noble metal electrodes. It was found that the radiation signal of the adsorbed chloride ions can also be detected, and the surface excess of both radiolabelled bisulfate and chloride ions can be measured. The accumulation of the adsorbed species on rough electrodes took place at a much longer time scale than the removal of the same species. No such difference was found for smooth electrodes. The apparent asymmetry in the temporal behaviour of the adsorption and desorption processes on rough electrodes could be understood by the difference in the concentration gradient in the solution near or within the porous structures during the respective processes. [Copyright &y& Elsevier]

In this work, we present a stochastic variational calculation (SVM) of energies and wave functions of few particle systems coupled to quantum fields in cavity QED. The spatial wave function and the photon spaces are optimized by a random selection process. Using correlated basis functions, the SVM approach solves the problem accurately and opens the way to the same precision that is reached the nonlight coupled quantum systems. Examples for a two-dimensional trion and confined electrons as well as for the He atom and the H2 molecule are presented showing that the light-matter coupling drastically changes the electronic states. [ABSTRACT FROM AUTHOR]

The properties of carbon nanotube-graphene junctions are investigated with first-principles electronic structure and electron transport calculations. Contact properties are found to be key factors in determining the performance of nanotube based electronic devices. In a typical single-walled carbon nanotube-metal junction, there is a p-type Schottky barrier of up to ∼0.4 eV which depends on the nanotube diameter. Calculations of the Schottky barrier height in carbon nanotube-graphene contacts indicate that low barriers of 0.09 eV and 0.04 eV are present in nanotube-graphene contacts ((8,0) and (10,0) nanotubes, respectively). Junctions with a finite contact region are investigated with simulations of the current-voltage characteristics. The results suggest the suitability of the junctions for applications and provide insight to explain recent experimental findings. [ABSTRACT FROM AUTHOR]

The collision of energetic ions and graphene fragments is studied in the framework of real-space finite-difference time-dependent density functional theory (TDDFT) coupled with classical molecular dynamics for nuclei. The amount of energy transferred from the projectile to the target is calculated to explore the defect formation mechanisms as a function of the projectile's energy. It is found that creation of defects in graphene due to the interaction of a fast proton with valence electrons is unlikely. In the case of projectiles with higher charges, the transferred energy increases significantly, leading to higher probability of bond breaking. [ABSTRACT FROM AUTHOR]

Decontamination technologies are generally developed to reduce the collective dose of the maintenance and operation personnel at nuclear power plants (NPP). The highest efficiency (i.e., the highest decontamination factors) available without detrimental modification of the treated surface of structural material is the most important goal in the course of the application of a decontamination technology. At the Paks NPP the AP-CITROX procedure has been utilized for the decontamination of the primary coolant circuit's components (e.g., main circulating pump (MCP) and steam generators (SGs)). Our previous studies have revealed that a 'hybrid' structure of the amorphous and crystalline phases was formed in the outermost surface region of the austenitic stainless steel tubes of SGs as an undesired consequence of the industrial application of the AP-CITROX decontamination technology during the period of 1993-2001. In this paper, we report some comparative findings on the corrosion and surface chemical effects of the AP-CITROX procedure and the novel decontamination technology elaborated at our institution. On optimizing the operational parameters the latter technology may become suitable for the effective decontamination of both dismountable (e.g., MCP swivel) and separable (e.g., SGs) equipments. For this purpose experiments were performed. In this laboratory scale experiments, the passivity, morphology and chemical compositions of the treated surfaces of tube specimens were investigated by voltammetry, and SEM-EDX methods, respectively. The SEM-EDX results have revealed that the oxide removal is surprisingly uniform even after 2 or 3 consecutive cycles. The electrochemical studies have provided evidences that no unfavorable tendencies in the general corrosion state of the tube samples can be detected in the course of the chemical treatments. [ABSTRACT FROM AUTHOR]

Abstract: The surface contamination by uranium in the primary circuit of PWR type nuclear reactors is a fairly complex problem as (i) different chemical forms (molecular, colloidal and/or disperse) of the uranium atoms can be present in the boric acid coolant, and (ii) only limited pieces of information about the extent, kinetics and mechanism of uranium accumulation on constructional materials are available in the literature. A comprehensive program has been initiated in order to gain fundamental information about the uranium accumulation onto the main constituents of the primary cooling circuit (i.e., onto austenitic stainless steel type 08X18H10T (GOSZT 5632-61) and Zr(1%Nb) alloy). In this paper, some experimental findings on the time and pH dependences of U accumulation obtained in a pilot plant model system are presented and discussed. The surface excess, oxidation state and chemical forms of uranium species sorbed on the inner surfaces of the stainless steel tubes of steam generators have been detected by radiotracer (alpha spectrometric), ICP-OES and XPS methods. In addition, the passivity, morphology and chemical composition of the oxide-layers formed on the studied surfaces of steel specimens have been analyzed by voltammetry and SEM-EDX. The experimental data imply that the uranium sorption is significant in the pH range of 4–8 where the intense hydrolysis of uranyl cations in boric acid solution can be observed. Some specific adsorption and deposition of (mainly colloidal and disperse) uranyl hydroxide to be formed in the solution prevail over the accumulation of other U(VI) hydroxo complexes. The maximum surface excess of uranium species measured at pH 6 (Γ sample =1.22μgcm−2 U ≅4×10−9 molcm−2 UO2(OH)2) exceeds a monolayer coverage. [Copyright &y& Elsevier]

The coupled Maxwell and time-dependent Kohn-Sham equations are solved using the Riemann-Silberstein vectors to represent the electromagnetic fields. Momentum-space time propagation of the Riemann-Silberstein vectors are proposed and test calculations are presented to show the accuracy of the approach. [ABSTRACT FROM AUTHOR]

An efficient and accurate basis comprised of Volkov states is implemented and tested for time-dependent simulations of interactions between strong laser pulses and crystalline solids. The Volkov states are eigenstates of the free electron Hamiltonian in an electromagnetic field and analytically represent the rapidly oscillating time-dependence of the orbitals, allowing significantly faster time propagation than conventional approaches. The Volkov approach can be readily implemented in plane-wave codes by multiplying the potential energy matrix elements with a simple time-dependent phase factor. [ABSTRACT FROM AUTHOR]

The existence of polyexcitons, the N-body complexes of excitons for N>2 in three-dimensional bulk systems, has been controversial for more than 40 years since its first theoretical suggestion. We investigated the stability of fundamental excitonic complexes in diamond numerically with the stochastic variational method and an explicitly correlated Gaussian basis. The electron-hole many-body system is described by an effective mass Hamiltonian. Our model includes the effective mass anisotropy and multiple valley and band degrees of freedom. We show that the excitons, trions, biexcitons, charged biexcitons, and triexcitons are stable in diamond. Numerical calculations reproduce from 81% to 86% of the experimentally reported binding energies for neutral bound states. [ABSTRACT FROM AUTHOR]

An accurate method for time propagation of the coupled Maxwell and time-dependent Kohn-Sham (TDKS) equation is presented. The new approach uses a simultaneous fourth-order Runge-Kutta-based propagation of the vector potential and the Kohn-Sham orbitals. The approach is compared to the conventional fourth-order Taylor propagation and predictor-corrector methods. The calculations show several computational and numerical advantages, including higher computational performance, greater stability, better accuracy, and faster convergence. [ABSTRACT FROM AUTHOR]