Time-independent Density Research Articles

An analytic description of electron thermalization in kilonovae ejecta

ABSTRACT A simple analytic description is provided of the rate of energy deposition by β-decay electrons in the homologously expanding radioactive plasma ejected in neutron star mergers, valid for a wide range of ejecta parameters – initial entropy, electron fraction {s0, Ye}, and scaled (time-independent) density ρt3. The formulae are derived using detailed numerical calculations following the time-dependent composition and β-decay emission spectra (including the effect of delayed deposition). The deposition efficiency depends mainly on ρt3 and only weakly on {s0, Ye}. The time te at which the ratio between the rates of electron energy deposition and energy production drops to 1 − e−1, is given by $t_e=t_{0e}\Big (\frac{\rho t^3}{0.5(\rho t^3)_0}\Big)^a$, where $(\rho t^3)_0=\frac{0.05\, {\rm M}_{\odot }}{4\pi (0.2c)^3}$, t0e(s0, Ye) ≈ 17 d, and 0.4 ≤ a(s0, Ye) ≤ 0.5. The fractional uncertainty in te due to nuclear physics uncertainties is $\approx 10~{{\ \rm per\ cent}}$. The result a ≤ 0.5 reflects the fact that the characteristic β-decay electron energies do not decrease with time (largely due to ‘inverted decay chains’ in which a slowly decaying isotope decays to a rapidly decaying isotope with higher end-point energy). We provide an analytic approximation for the time-dependent electron energy deposition rate, reproducing the numerical results to better than 50 per cent (typically $\lt 30~{{\ \rm per\ cent}}$, well within the energy production rate uncertainty due to nuclear physics uncertainties) over a 3–4 orders-of-magnitude deposition rate decrease with time. Our results may be easily incorporated in calculations of kilonovae light curves (with general density and composition structures), eliminating the need to numerically follow the time-dependent electron spectra. Identifying te, e.g. in the bolometric light curve, will constrain the ejecta density distribution.

AFM-based adhesive indentation of polymer brushes: A phenomenological approach

Indentation-based mechanical characterization of ultra-thin polymer layers is widely performed by means of atomic force microscopy (AFM) using both conventional indenters and spherical colloidal probes. While there exists an extensive theoretical framework for extracting the material properties of bulk samples from the depth-sensing indentation data, in regard to a polymer brush, it is still challenging to interpret the contact force/indenter displacement curve in terms of its time-independent interaction potential density. The indentation process is assumed to be quasistatic, and the effects of viscoelastic deformation and time-dependent adhesion have been neglected. The novelty of this study lies in the Griffith’s energy balance-based model of adhesive contact between an AFM probe and a polymer brush grafted onto a rigid substrate, which is applicable for finite deformations of polymer chains. A fitting-based analysis of experimental force/displacement curves available in the literature is presented.

On the motion of a pendulum with a cavity filled with a compressible fluid

We study the motion of the coupled system, S, constituted by a physical pendulum, B, with an interior cavity entirely filled with a viscous, compressible fluid, F. The system is constrained to rotate about a horizontal axis. The presence of the fluid may strongly affect the motion of B. In fact, we prove that, under appropriate assumptions, the fluid acts as a damper, namely, S must eventually reach a rest-state. Such a state is characterized by a suitable time-independent density distribution of F and a corresponding equilibrium position of the center of mass of S. These results are proved in the very general class of weak solutions and do not require any restriction on the initial data, other than having a finite energy. We complement our findings with some numerical tests. The latter show, among other things, the interesting property that “large” compressibility favors the damping effect, since it drastically reduces the time that S takes to go to rest.

Correction: Electronic excitations of the charged nitrogen-vacancy center in diamond obtained using time-independent variational density functional calculations

Correction: Electronic excitations of the charged nitrogen-vacancy center in diamond obtained using time-independent variational density functional calculations

Electronic excitations of the charged nitrogen-vacancy center in diamond obtained using time-independent variational density functional calculations

Elucidation of the mechanism for optical spin initialization of point defects in solids in the context of quantum applications requires an accurate description of the excited electronic states involved. While variational density functional calculations have been successful in describing the ground state of a great variety of systems, doubts have been expressed in the literature regarding the ability of such calculations to describe electronic excitations of point defects. A direct orbital optimization method is used here to perform time-independent, variational density functional calculations of a prototypical defect, the negatively charged nitrogen-vacancy center in diamond. The calculations include up to 511 atoms subject to periodic boundary conditions and the excited state calculations require similar computational effort as ground state calculations. Contrary to some previous reports, the use of local and semilocal density functionals gives the correct ordering of the low-lying triplet and singlet states, namely {}^{3}A_2 < {}^{1}E < {}^{1}A_1 < {}^{3}E3A2<1E<1A1<3E. Furthermore, the more advanced meta generalized gradient approximation functionals give results that are in remarkably good agreement with high-level, many-body calculations as well as available experimental estimates, even for the excited singlet state which is often referred to as having multireference character. The lowering of the energy in the triplet excited state as the atom coordinates are optimized in accordance with analytical forces is also close to the experimental estimate and the resulting zero-phonon line triplet excitation energy is underestimated by only 0.15 eV. The approach used here is found to be a promising tool for studying electronic excitations of point defects in, for example, systems relevant for quantum technologies.

Exponential galaxy discs as the quasi-stationary distribution in a Markov chain model simulating stellar scattering

ABSTRACT Previous models have shown that stochastic scattering of stars in a two-dimensional galaxy disc can generate a time-independent surface density distribution that is an exponential divided by radius when a constant inward scattering bias is present. Here we show, using a Markov chain model, that similar profiles result from an outward scattering bias, although the disc surface density decreases slowly with time because of a net stellar outflow. The trend towards a near-exponential surface profile is robust, as it exists even if the scattering intensity has moderate radial and time dependences, subject to some limitations on the scattering rates discussed in the text. The exponential scale length of the pseudo-equilibrium disc depends on the scattering bias, the scattering length, and the size of the disc where scattering is important.

Synthesis and investigations of reactive properties, photophysical properties and biological activities of a pyrazole-triazole hybrid molecule

In this work, we report the synthesis, spectroscopic characterization, reactivity study, photophysical properties and evaluation of α-glucosidase inhibitory activity pyrazole-triazole hybrid molecule. Reactive properties of the title compound have been investigated using density functional theory (DFT) calculations. Local reactive descriptors such as condensed versions of Fukui functions and Molecular electrostatic potential were used to predict the chemical reactivity. Further, the influence of solvents on the photophysical properties was investigated by using Time independent Density Functional Theory (TDDFT) calculations. In addition, the in vitro anti-diabetic activity against α-glucosidase enzyme revealed that the title compound has α-glucosidase inhibitory activity with IC50 value of 43.92 µg/mL. Molecular docking studies were carried out to understand the binding mechanism of the title compound in the active site of the α-glucosidase enzyme. The stability of the protein-ligand complex was validated by Molecular Dynamics (MD) simulations and Molecular Mechanics Poisson-Boltzmann Surface Area (MMPBSA) analysis.

Variational Density Functional Calculations of Excited States: Conical Intersection and Avoided Crossing in Ethylene Bond Twisting.

Theoretical studies of photochemical processes require a description of the energy surfaces of excited electronic states, especially near degeneracies, where transitions between states are most likely. Systems relevant to photochemical applications are typically too large for high-level multireference methods, and while time-dependent density functional theory (TDDFT) is efficient, it can fail to provide the required accuracy. A variational, time-independent density functional approach is applied to the twisting of the double bond and pyramidal distortion in ethylene, the quintessential model for photochemical studies. By allowing for symmetry breaking, the calculated energy surfaces exhibit the correct topology around the twisted-pyramidalized conical intersection even when using a semilocal functional approximation, and by including explicit self-interaction correction, the torsional energy curves are in close agreement with published multireference results. The findings of the present work point to the possibility of using a single determinant time-independent density functional approach to simulate nonadiabatic dynamics, even for large systems where multireference methods are impractical and TDDFT is often not accurate enough.

ESTIMATING THE PROPAGATION OF A UNIFORMLY ACCELERATED JET

We study the problem of a Herbig-Haro jet with a uniformly accelerating ejection velocity, travelling into a uniform environment. For the ejection density we consider two cases: a time-independent density, and a time-independent mass loss rate. For these two cases, we obtain analytic solutions for the motion of the jet head using a ram-pressure balance and a center of mass equation of motion. We also compute axisymmetric numerical simulations of the same flow, and compare the time-dependent positions of the leading working surface shocks with the predictions of the two analytic models. We find that if the jet is over-dense and over-pressured (with respect to the environment) during its evolution, a good agreement is obtained with the analytic models, with the flow initially following the center of mass analytic solution, and (for the constant ejection density case) at later times approaching the ram-pressure balance solution.

Can small-scale baryon inhomogeneities resolve the Hubble tension? An investigation with ACT DR4

Small-scale inhomogeneities in the baryon density around recombination have been proposed as a solution to the tension between local and global determinations of the Hubble constant. These baryon clumping models make distinct predictions for the cosmic microwave background anisotropy power spectra on small angular scales. We use recent data from the Atacama Cosmology Telescope to test these predictions. No evidence for baryon clumping is found, assuming a range of parameterizations for time-independent baryon density probability distribution functions. The inferred Hubble constant remains in significant tension with the SH0ES measurement.

Dehydrogenation and dehydration of formic acid over orthorhombic molybdenum carbide

The dehydrogenation and dehydration of formic acid is investigated on the β-Mo2C (100) catalyst surface using time independent density functional theory. The energetics of the two mechanisms are calculated, and the thermochemistry and kinetics are discussed using the transition state theory. Subsequently, microkinetic modelling of the system is conducted, considering the batch reactor model. The potential energy landscape of the reaction shows a thermodynamically favourable cleavage of HCOOH to form CO; however, the kinetics show that the dehydrogenation mechanism is faster and CO2 is continuously formed. The effect of HCOOH adsorption on the surface is also analysed, in a temperature-programmed desorption, with the conversion proceeding at under 350 K and desorption of CO2 is observed with a selectivity of about 100 %, in line with the experimental reports.

Spectral, DFT—TDDFT computational investigation and biological studies of transition metal complexes of dehydroacetic acid schiff base

Schiff base of dehydroacetic acid ligand (HL) and its transition metal complexes were prepared. The complexes were described by elemental analyses, molar conductivities, magnetic moments and spectral analyses. From the IR spectra, the ligand (HL) behaves as monobasic di- or tridentate ligand coordinated to the metal ions via azomethine N and O. UV spectra and magnetic moment show that the Cu(II), Co(II) and Ru(III) complexes have octahedral, Fe(III), Mn(II) and Ni(II) complexes have tetrahedral geometry and Pd(II) and Ag(I) complexes show square planar geometry. The complexes [Pd2(HL) Cl4] and [Ru(L)Cl2. 2H2O] 3H2O have antibacterial activities toward all the gram-positive, negative bacteria and fungi. The compounds have antitumor activity arrangement against HEPG2 liver cell [Pd2(HL) Cl4] > [Ru(L)Cl2. 2H2O]. 3H2O > HL and have the following arrangement against MCF7 breast cell HL > [Pd2(HL) Cl4] > [Ru(L)Cl2. 2H2O]. 3H2O. The geometries, bonding characteristics, frontier orbital energy gaps, NBO analysis dipole moments, hyperpolarizabilities, thermochemical properties, PDOS plot and simulated absorption spectra (IR and UV–visible spectra) of the ligand and its Cu (II) and Co(II) complexes have been examined by means of the time-independent density functional theory (B3LYP and CAM-B3LYP) as well as the time-dependent density functional theory (TD-B3LYP and TD-CAM-B3LYP)

Pristine and holey graphene quantum dots: Optical properties using time independent and dependent density functional theory

The zero bandgap of graphene limits its utilization in optoelectronics. Quantum confinement in finite-size graphene structures, such as graphene quantum dots (GQDs) and holey GQDs, may offer a path for generating a wide range of HOMO-LUMO gaps. Edge terminations and pore passivations in these structures provide further means to alter their optical properties. In this work, we study the structural stabilities and optical properties of GQDs and holey GQDs with different sizes, edge terminations, and pore passivations, using density functional theory (DFT) as well as time dependent DFT. We find that the optical spectra of GQDs depend primarily on their size. Edge termination has a small influence on the spectra; shifting the absorption peaks by ~ 0.2 eV, which gets smaller for larger GQDs. Creation of pores in GQDs lead to the emergence of new peaks in their absorption spectra. Pore passivation seems to have the biggest effect on the absorption spectra of holey GQDs, while their termination leads to slight shifts in their peaks. Our results can be used to develop promising materials for many applications, such as biological sensors, and optoelectronic devices. • DFT and TDDFT are applied to study the optical properties of GQDs and holey GQDs. • The pore passivations and pore size affect the optical properties of GQDs. • There is a blueshift in the spectrum of TDDFT calculations as compared to the corresponding DFT calculations. • The spectral range and the broadening of the absorption are larger for holey GQDs than for GQDs. • Our results can be used to develop promising materials for many applications biological sensors, and optoelectronic devices.

Studying the Effect of Halogen Atoms and the Cyano Groups on Optical, Electronic and Charge Transfer Properties of Hexasene Molecule: Molecular Design Methods, Structure-Property Relationship

Bu çalışmada, π-konjugasyon yapısına sahip hekzasen molekülü, farklı halojen atomlar (Flor, Klor, Brom) ve molekülde farklı konumlandırılmış siyano grupları ile dizayn edilmiş, elektronik, optik ve yük transfer özellikleri zamana bağlı ve zamandan bağımsız Yoğunluk Fonksiyon Teorisi (YFT) ile incelenmiştir. Dizayn edilen beş hekzasen türevinin yapı ve özellik arasındaki ilişkiyi kurabilmek için, moleküler geometri, reorganizasyon enerji, HOMO-LUMO orbital enerjileri, iyonizasyon potansiyeli (IP), elektron afinitesi (EA), geometriden dolayı nötr ve yüklü durumlara bağlı olarak bağ uzunluğu değişimi (BUD) üzerine substitüe etkisi incelenmiş ve etkiyi daha iyi anlayabilmek için sonuçlar hekzasen ve antrasen moleküllerin sonuçlarıyla karşılaştırılmıştır. Moleküllerin, elektron enjeksiyon bariyerinin hekzasen molekülünden daha düşük olduğu ve iyi bir boşluk taşıyıcısı olarak bilinen antrasenle kıyaslanabilir derecede etkili boşluk yük transfer oranına sahip olduğu belirtilmiştir. Dizayn edilen hekzasen türevlerinin, optoelektronik alanda potansiyel uygulamalar için ilgi çekebileceğini ve malzeme biliminde umut vaad edeceğini umuyorum.

Theoretical Investigation on Reaction Pathway, Biological Activity, Toxicity and NLO Properties of Diclofenac Drug and Its Ionic Carriers

The present study included the use of the approximate semi-experimental method, the time-independent density function theory (unrestricted), the time-dependent density function theory, and Hartree-Fock method to calculate the reaction pathway of the anti-inflammatory drug diclofenac with its common ionic carriers (sodium and potassium). The basis sets used were STO-3G, 3-21G, 6-31G, and 6-311G. The drug was studied with two new proposed carrier ions (lithium and calcium) which were compared with common carriers. The calculations included the optimized geometrical structure and some physical properties such as standard heat of formation, dipole moment, total energies, and analytical spectra of IR, UV-VIS and 1H NMR. The biological and toxicological activities and the nonlinear optical (NLO) properties were also studied theoretically for the drug and for its proposed and common carriers. All calculations were performed using Gaussian-09 program. The results of the proposed carriers were compared with the common carriers in terms of activation energies, transition states, and products. This study is considered as a step to develop diclofenac prodrugs and find new carriers for diclofenac. The proposed lithium showed a good result and a potential for use as a drug carrier. The results also showed the convergence of the values of the common carriers (Na, K) and those of the proposed carrier (Ca), with their preference over it.

Influence of Current Redistribution in Switching Models for Perpendicular STT-MRAM

Simulation of switching in spin-transfer torque magnetoresistive random access memory is usually performed by assuming that the torque is created by a position- and time-independent current density. However, in real circuits the voltage is fixed, not the current density. The assumption of a fixed current density, especially in modern devices with a tunneling magnetoresistance up to 200%, becomes thus questionable. In this work we compare the switching time distribution obtained under the assumptions of fixed voltage and fixed current density for a wide range of tunneling magnetoresistance and surface area values. We demonstrate that the approximate fixed current density approach can reproduce the correct switching times, provided that the current value is appropriately adjusted. We show that the correction on the current depends on the switching speed, dictated by different system parameters.

Excitation transport in quasi-one-dimensional quantum devices: a multiscale approach with analytical time-dependent non-equilibrium Green’s function

Under the wide-band limit approximation for electrodes, this research proposes analytical time-dependent non-equilibrium Green’s function (TD-NEGF) formulae to investigate dynamical functionalities of quasi-one-dimensional quantum devices, especially for (microwave) photon-assisted transports. Together with a multiscale approach by lumped element model, we also study the effects of transiently-transferring charges to reflect the non-conservation of charges in open quantum systems, and implement numerical calculations in hetero-junction systems composed of functional quantum devices and electrode-contacts (to the environment). The results show that (i) the current calculation by the analytical algorithms, versus those by conventional numerical integrals, presents superior numerical stability on a large-time scale, (ii) the correction of charge transfer effects can better clarify non-physical transport issues, e.g. the blocking of AC signaling under the assumption of conventional constant hamiltonian, (iii) the current in the long-time limit validly converges to the steady value obtained by standard time-independent density functional calculations, and (iv) the occurrence of the photon-assisted transport is well-identified.

The physical structure and surface reactivity of graphene oxide

The structural configuration of graphene oxide and its surface reactivity were investigated experimentally and clarified on the bases of time-independent density functional calculations (DFT). The physical structure was examined by transmission electron microscopy, infrared absorption, and X-ray diffraction parallel to radial distribution analysis. Surface reactivity was examined using N2-adsorption at 77 K and the kinetic of the adsorption of Ni2+, Cd2+, and Pb2+ ions at 300 K. The lattice structure of the material was hexagonal with unit cell a = 2.455 Å and c = 8.64 Å and with layer-layer distance of 4.32 Å. This structure is undulated in response to existence of oxygen atoms that shape surface topological imperfections and breaks the planner regularity. The undulation distance comprises 0.3 Å. The specific surface area was found 60.3 m2/g with about 50% of this area were available to adsorption of 2+ ions due to surface polarization. The vibration frequencies observed in FTIR are identified by DFT calculations and showed that the structure acts as an extended quantum system for which Pauli blocking prevents existence of two modes of marginal hydrogen vibrations within the same sheet.

Coordinate Scaling in Time-Independent Excited-State Density Functional Theory for Coulomb Systems

A time-independent density functional theory for excited states of Coulomb systems has recently been proposed in a series of papers. It has been revealed that the Coulomb density determines not only its Hamiltonian, but the degree of excitation as well. A universal functional valid for any excited state has been constructed. The excited-state Kohn–Sham equations bear resemblance to those of the ground-state theory. In this paper, it is studied how the excited-state functionals behave under coordinate scaling. A few relations for the scaled exchange, correlation, exchange-correlation, and kinetic functionals are presented. These relations are expected to be advantageous for designing approximate functionals.

The accuracy of a popular quantum hydrodynamic plasma model

A new article investigates the accuracy of a popular hydrodynamic model based on time-independent density functional theory, finding that it should only be used for the high-frequency regime.