323,169 research outputs found
Relativistic quantum theory and algorithms: A toolbox for modeling many-fermion systems in different scenarios☆
No full textIn this chapter we focus first on the theoretical methods and relevant computational approaches to calculate the electronic structure of atoms, molecules, and clusters containing heavy elements for which relativistic effects become significant. In particular, we discuss the mean-field approximation of the Dirac equation for many-electron systems, and its self-consistent numerical solution by using either radial mesh or Gaussian basis sets. The former technique is appropriate for spherical symmetric problems, such as atoms, while the latter approach is better suited to study nonspherical nonperiodic polycentric systems, such as molecules and clusters. We also outline the pseudopotential approximation in relativistic context to deal with the electron-ion interaction in extended systems, where the unfavorable computational scaling with system size makes it necessary. As test cases we apply our theoretical and numerical schemes to the calculation of the electronic structure (i) of the gold atom and (ii) of the superatom W@Au12, where the inclusion of spin-orbit effects is crucial to the accurate understanding of the electronic properties. Furthermore, we describe the extension of our relativistic approach to deal with nuclear reactions driven by the weak force, such as the electron capture and β-decay, also at finite temperature in astrophysical scenarios, using the Fermi–Dirac statistics. The latter processes are indeed major drivers of the nucleosynthesis of the elements in stars and, thus, their understanding is crucial to model the chemical evolution of the Universe. Finally, we show the application of our relativistic quantum mechanical framework to the assessment of the elastic differential scattering cross section of electrons impinging on molecular targets, notably liquid water. The latter process, together with several inelastic scattering collisions by which secondary electrons deposit their energy, represents a fundamental event of the chain of the physico-chemical mechanisms initiated by the passage of fast ion beams through a bio-medium. This technique is used in hadrontherapy for cancer cure
The resonant and normal auger spectra of ozone
No full textIn this work, we outline a general method for calculating Auger spectra in molecules, which accounts for the underlying symmetry of the system. This theory starts from Fano’s formulation of the interaction between discrete and continuum states, and it generalizes this formalism to deal with the simultaneous presence of several intermediate quasi-bound states and several non-interacting decay channels. Our theoretical description is specifically tailored to resonant autoionization and Auger processes, and it explicitly includes the incoming wave boundary conditions for the continuum states and an accurate treatment of the Coulomb repulsion. This approach is implemented and applied to the calculation of the K - LL Auger and autoionization spectra of ozone, which is a C2v symmetric molecule, whose importance in our atmosphere to filter out radiation has been widely confirmed. We also show the effect that the molecular point group and, in particular, the localization of the core-hole in the oxygen atoms related by symmetry operations, has on the electronic structure of the Auger states and on the spectral lineshape by comparing our results with the experimental data
Zeolitic imidazolate frameworks for separation of binary mixtures of CO2, CH4,N2 and H2 : A computer simulation investigation
No full textLithium abundances in AGB stars and a new estimate for the7Be life-time
No full textIn most cases RGB and AGB stars with M <= 2M(circle dot) destroy Li (which is instead synthesized trough electron-captures on Be-7). This occurs through the combined operation of mixing processes and proton captures, when H-burning operates close to the envelope. Observed Li abundances are however difficult to explain, as they cover a wide spread. Various uncertainties affect model attempts, but so far the largest one concerns the processes of bound and free e-captures on Be-7, hence its life-time, whose known estimates are valid only for solar conditions. RGB and AGB stages have temperatures and densities below the envelope covering a wide range and differing from solar by up to a factor of five for T and up to five orders of magnitudes for rho, hence extrapolations are unreliable. Recently, we presented an estimate of the Be-7 half-life based on a fully quantistic method that goes beyond the Debye-Huckel approximation. Here we discuss its consequences on Li nucleosynthesis in low mass AGB stars
THEORETICAL ESTIMATES OF STELLARe–CAPTURES. I. THE HALF-LIFE OF7Be IN EVOLVED STARS
No full textThe enrichment of Li in the universe is still unexplained, presenting various puzzles to astrophysics. One open issue is that of obtaining reliable estimates for the rate of e – captures on 7Be for T and ρ conditions that are different from the solar ones. This is of crucial importance for modeling the Galactic nucleosynthesis of Li. In this framework, we present here a new theoretical method for calculating the e – capture rate in typical conditions for evolved stars. Furthermore, we show how our approach compares with state-of-the-art techniques for solar conditions, where various estimates are available. Our computations include (1) "traditional" calculations of the electronic density at the nucleus, to which the e – capture rate for 7Be is proportional, for different theoretical approaches including the Thomas-Fermi, Poisson-Boltzmann, and Debye-Hückel (DH) models of screening; and (2) a new computation, based on a formalism that goes beyond the previous ones, adopting a mean-field "adiabatic" approximation to the scattering process. The results obtained with the new approach as well as with traditional ones and their differences are discussed in some detail, starting from solar conditions, where our approach and the DH model essentially converge to the same solution. We then analyze the applicability of both our method and the DH model to a rather broad range of T and ρ values, embracing those typical of red giant stars, where both bound and continuum states contribute to the capture. We find that over a wide region of the parameter space explored, the DH approximation does not really stand, so that the more general method we suggest should be preferred. As a first application, we briefly reanalyze the 7Li abundances in red giant branch and asymptotic giant branch stars of the Galactic disk in light of a revision in the Be decay only; however, we emphasize that the changes we find in the electron density at the nucleus would also induce effects on the electron screening (for p-captures on Li itself, as well as for other nuclei) so that our new approach might have rather wide astrophysical consequences
Elastic scattering of electrons by water: An ab initio study
No full textIn this work we devise a theoretical and computational method to compute the elastic scattering of electrons from a non-spherical potential, such as in the case of molecules and molecular aggregates. Its main feature is represented by the ability of calculating accurate wave functions for continuum states of polycentric systems via the solution of the Lippmann-Schwinger equation, including both the correlation effects and multi-scattering interference terms, typically neglected in widely used approaches, such as the Mott theory. Within this framework, we calculate the purely elastic scattering matrix elements. As a test case, we apply our scheme to the modelling of electron-water elastic scattering. The Dirac-Hartree-Fock self-consistent field method is used to determine the non-spherical molecular potential projected on a functional space spanned by Gaussian basis set. By adding a number of multi-centric radially-arranged s-type Gaussian functions, whose exponents are system-dependent and optimized to reproduce the properties of the continuum electron wave function in different energy regions, we are able to achieve unprecedented access to the description of the low energy range of the spectrum (0.001 < E < 10 eV) up to keV, finding a good agreement with experimental data and previous theoretical results. To show the potential of our approach, we also compute the total elastic scattering cross section of electrons impinging on clusters of water molecules and zundel cation. Our method can be extended to deal with inelastic scattering events and heavy-charged particles
Data for: Structural, Electronic and Mechanical Properties of all-sp Carbon Allotropes with Density Lower Than Graphene
No full textAb-initio molecular dynamics simulation of tilene at 500
Data for: Structural, Electronic and Mechanical Properties of all-sp Carbon Allotropes with Density Lower Than Graphene
No full textData report the stress-strain relationships for the materials proposed. Strain is reported in the first column (which must be multiplied by 2 to obtain the true strain). In the right column one may find the stress in Kbar. The cell in the direction orthogonal to the plane measures 20 Angstrom
Data for: Structural, Electronic and Mechanical Properties of all-sp Carbon Allotropes with Density Lower Than Graphene
No full textData report the x (second colum), y (third column), z (fourth column) cartesian coordinates of the atomic centers whose atomic species are specified in the first column (C = carbon). First line of the file reports the number of atoms in the unit cell, second line the cell size defining the periodic boundary conditions. Data are all in Angstrom
Finite-range effects in dilute Fermi gases at unitarity
No full textWe develop a theoretical method going beyond the contact-interaction approximation frequently used in mean-field theories of many-fermion systems, based on the low-energy T matrix of the pair potential to rigorously define the effective radius of the interaction. One of the main consequences of our approach is the possibility to investigate finite-density effects, which are outside the range of validity of approximations based on δ-like potentials. We apply our method to the calculation of density-dependent properties of an ultracold gas of 6Li atoms at unitarity, whose two-body interaction potential is calculated using ab initio quantum chemistry methods. We find that density effects will be significant in ultracold gases with densities 1 order of magnitude higher than those attained in current experiments
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