582 research outputs found
Theorems on ground-state phase transitions in Kohn-Sham models given by the Coulomb density functional
Some theorems on derivatives of the Coulomb density functional with respect
to the coupling constant are given. Consider an electron density
given by a ground state. A model Fermion system with the
reduced coupling constant, , is defined to reproduce and the ground state energy. Fixing the charge density, possible phase
transitions as level crossings detected in a value of the reduced density
functional happen only at discrete points along the axis. If the
density is -representable also for , accumulation of phase
transition points is forbidden when . Relevance of the
theorems for the multi-reference density functional theory is discussed.Comment: 19 page
Nuclear Resonance Vibrational Spectroscopy of Iron Sulfur Proteins
Nuclear inelastic scattering in conjunction with density functional theory
(DFT) calculations has been applied for the identification of vibrational modes
of the high-spin ferric and the high-spin ferrous iron-sulfur center of a
rubredoxin-type protein from the thermophylic bacterium Pyrococcus abysii
DFT Calculations as a Tool to Analyse Quadrupole Splittings of Spin Crossover Fe(II) complexes
Density functional methods have been applied to calculate the quadrupole
splitting of a series of iron(II) spin crossover complexes. Experimental and
calculated values are in reasonable agreement. In one case spin-orbit coupling
is necessary to explain the very small quadrupole splitting value of 0.77 mm/s
at 293 K for a high-spin isomer
Separable Dual Space Gaussian Pseudo-potentials
We present pseudo-potential coefficients for the first two rows of the
periodic table. The pseudo potential is of a novel analytic form, that gives
optimal efficiency in numerical calculations using plane waves as basis set. At
most 7 coefficients are necessary to specify its analytic form. It is separable
and has optimal decay properties in both real and Fourier space. Because of
this property, the application of the nonlocal part of the pseudo-potential to
a wave-function can be done in an efficient way on a grid in real space. Real
space integration is much faster for large systems than ordinary multiplication
in Fourier space since it shows only quadratic scaling with respect to the size
of the system. We systematically verify the high accuracy of these
pseudo-potentials by extensive atomic and molecular test calculations.Comment: 16 pages, 4 postscript figure
Electron Correlation and the c-axis Dispersion of Cu d_z^2: a New Band Structure for High Temperature Superconductors
Previously we showed the major effect of electron correlation in the cuprate
superconductors is to lower the energy of the Cu d_x^2-y^2/O p_sigma (x^2-y^2)
band with respect to the Cu d_z^2/O' p_z (z^2) band. In our 2D Hubbard model
for La_1.85Sr_0.15CuO_4 (LaSCO), the z^2 band is narrow and crosses the
standard x^2-y^2 band just below the Fermi level. In this work, we introduce
c-axis dispersion to the model and find the z^2 band to have considerable
anisotropic 3D character. An additional hole-like surface opens up in the z^2
band at (0,0,2pi/c) which expands with doping. At sufficient doping levels, a
symmetry allowed x^2-y^2/z^2 band crossing along the (0,0)-(pi,pi) direction of
the Brillouin zone appears at the Fermi level. At this point, Cooper pairs
between the two bands (e.g. (k uparrow x^2-y^2/k downarrow z^2)) can form,
providing the basis for the Interband Pairing Theory of superconductivity in
these materials.Comment: submitted to Phys. Rev. Lett. Related publications: Phys. Rev. B 58,
12303 (1998); Phys. Rev. B 58, 12323 (1998); cond-mat/9903088;
cond-mat/990310
A natural orbital functional for the many-electron problem
The exchange-correlation energy in Kohn-Sham density functional theory is
expressed as a functional of the electronic density and the Kohn-Sham orbitals.
An alternative to Kohn-Sham theory is to express the energy as a functional of
the reduced first-order density matrix or equivalently the natural orbitals. In
the former approach the unknown part of the functional contains both a kinetic
and a potential contribution whereas in the latter approach it contains only a
potential energy and consequently has simpler scaling properties. We present an
approximate, simple and parameter-free functional of the natural orbitals,
based solely on scaling arguments and the near satisfaction of a sum rule. Our
tests on atoms show that it yields on average more accurate energies and charge
densities than the Hartree Fock method, the local density approximation and the
generalized gradient approximations
Relativistic separable dual-space Gaussian Pseudopotentials from H to Rn
We generalize the concept of separable dual-space Gaussian pseudopotentials
to the relativistic case. This allows us to construct this type of
pseudopotential for the whole periodic table and we present a complete table of
pseudopotential parameters for all the elements from H to Rn. The relativistic
version of this pseudopotential retains all the advantages of its
nonrelativistic version. It is separable by construction, it is optimal for
integration on a real space grid, it is highly accurate and due to its analytic
form it can be specified by a very small number of parameters. The accuracy of
the pseudopotential is illustrated by an extensive series of molecular
calculations
Understanding the decomposition reaction mechanism of chrysanthemic acid: a computational study
<p>Abstract</p> <p>Background</p> <p>Chrysanthemic acid (<b>CHA</b>) is a major product from the photodecomposition of pyrethrin which is an important class of pesticide compounds.</p> <p>In the following paper, Hybrid density functional theory (DFT) calculations of the potential energy surface (PES) for three possible channels decomposition of chrysanthemic acid <b>(</b>cis-trans isomerization, rearrangement and fragmentation) have been carried at the B3LYP/6-311+G** level of theory. DFT was employed to optimize the geometry parameters of the reactants, transition states, intermediates and products based on detailed potential energy surfaces (PES).</p> <p>Results</p> <p>Our results suggest that all three pathways of <b>CHA </b>are endothermic. DFT calculations revealed that the activation barriers for cis-trans isomerization are low, leading to a thermodynamically favorable process than other two pathways. We also investigated the solvent effect on the PES using the polarizable continuum model (PCM). In addition, time-dependent density functional theory (TDDFT) calculations showed that these reactions occur in the ground state rather than in an excited state.</p> <p>Conclusion</p> <p>The rearrangement process seems to be more favorable than the decomposition of <b>CHA </b>to carbene formation. The solvent effect calculations indicated no changes in the shape of the PES with three continua (water, ethanol and cyclohexane), although the solvents tend to stabilize all of the species.</p
Structures of tetrasilylmethane derivatives (XMe2Si)2C(SiMe3)2 (X = H, Cl, Br) in the gas phase, and their dynamic structures in solution
The structures of the molecules (XMe2Si)2C(SiMe3)2, where X = H, Cl, Br, have been determined by gas electron diffraction (GED) using the SARACEN method of restraints, with all analogues existing in the gas phase as mixtures of C1- and C2-symmetric conformers. Variable temperature 1H and 29Si solution-phase NMR studies, as well as 13C NMR and 1H/29Si NMR shift correlation and 1H NMR saturation transfer experiments for the chlorine and bromine analogues, are reported. At low temperatures in solution there appear to be two C1 conformers and two C2 conformers, agreeing with the isolated-molecule calculations used to guide the electron diffraction refinements. For (HMe2Si)2C(SiMe3)2 the calculations indicated six conformers close in energy, and these were modeled in the GED refinement
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