Results 1 - 10 of 21427
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[en] It is shown that the conditions claimed to transform the algebraic version of the Resonating-Group Model, originally invented for scattering problems, into a complex eigenvalue problem corresponding to resonant states are necessary but not sufficient. This can be concluded from the fact that false resonances are produced along with true ones. They can be distinguished and discarded by introducing an arbitrary non-linear paramenter. The true solutions are invariant against this parameter but the false ones can be swept out even into non-physical regions of the energy. (authors)
[en] The traditional method of applying two-body methods for the study of nuclear reactions is briefly reviewed. The recent developments in the N particle scattering theory are described in detail. The application of the methods in the study of effective two and few-body problems is also considered. (P.L.)
[en] Variational Matrix Pade Approximants are studied as a nonlinear variational problem. It is shown that although a stationary value of the Schwinger functional is a stationary value of VMPA, the latter has also another stationary value. It is therefore proposed that instead of looking for a stationary point of VMPA, one minimizes some non-negative functional and then one calculates VMPA at the point where the former has the absolute minimum. This approach, which we call the Method of the Variational Gradient (MVG) gives unambiguous results and is also shown to minimize a distance between the approximate and the exact stationary values of the Schwinger functional
[en] We prescribe a formulation of the particle production with real-time Stochastic Quantization. To construct the retarded and the time-ordered propagators we decompose the stochastic variables into positive- and negative-energy parts. In this way we demonstrate how to derive a standard formula for the Schwinger mechanism under time-dependent electric fields. We discuss a mapping to the Schwinger–Keldysh formalism and a relation to the classical statistical simulation
[en] Ab initio electronic structure calculations for condensed matter physics are performed in the frame of the Density Functional Theory (DFT), that has become nowadays a standard model for such calculations. The practical solving of the DFT equations relies on the Kohn and Sham method and a standard approximation of DFT is the local density approximation (LDA). LDA has led to noticeable results and is able to account for many experimental results but one of its weaknesses is that the potential felt by the electrons is assumed not to vary with the occupation rate of localized electronic orbitals. It is necessary to go beyond this approximation and to develop new models to describe specific highly correlated compounds such as actinides and their oxides. These developments, available thanks to the evolution of computers, must be supported by a constant adaptation of the calculation codes within multidisciplinary teams
[en] Solving the electronic structure from a generalized or standard eigenproblem is often the bottleneck in large scale calculations based on Kohn-Sham density-functional theory. This problem must be addressed by essentially all current electronic structure codes, based on similar matrix expressions, and by high-performance computation. We here present a unified software interface, ELSI, to access different strategies that address the Kohn-Sham eigenvalue problem. Currently supported algorithms include the dense generalized eigensolver library ELPA, the orbital minimization method implemented in libOMM, and the pole expansion and selected inversion (PEXSI) approach with lower computational complexity for semilocal density functionals. The ELSI interface aims to simplify the implementation and optimal use of the different strategies, by offering (a) a unified software framework designed for the electronic structure solvers in Kohn-Sham density-functional theory; (b) reasonable default parameters for a chosen solver; (c) automatic conversion between input and internal working matrix formats, and in the future (d) recommendation of the optimal solver depending on the specific problem. As a result, comparative benchmarks are shown for system sizes up to 11,520 atoms (172,800 basis functions) on distributed memory supercomputing architectures.
[en] A simple method for non-empirical ligand field multiplet calculations for transition metal L-edge spectra is presented. Ligand field splittings and anisotropic scaling factors for Coulomb integrals are obtained from density functional theory. The method is applied to transition metal monoxide solids and nickel and cobalt phthalocyanines molecules and good agreement with experiment is obtained.
[en] Spatial symmetries of the densities appearing in the nuclear Density Functional Theory are discussed. General forms of the local densities are derived by using methods of construction of isotropic tensor fields. The spherical and axial cases are considered. (author)