2,393 research outputs found
The limitations of Slater's element-dependent exchange functional from analytic density functional theory
Our recent formulation of the analytic and variational Slater-Roothaan (SR)
method, which uses Gaussian basis sets to variationally express the molecular
orbitals, electron density and the one body effective potential of density
functional theory, is reviewed. Variational fitting can be extended to the
resolution of identity method,where variationality then refers to the error in
each two electron integral and not to the total energy. It is proposed that the
appropriate fitting functions be charge neutral and that all ab initio energies
be evaluated using two-center fits of the two-electron integrals. The SR method
has its root in the Slater's Xalpha method and permits an arbitrary scaling of
the Slater-Gaspar-Kohn-Sham exchange-correlation potential around each atom in
the system. Of several ways of choosing the scaling factors (Slater's exchange
parameters), two most obvious are the Hartree-Fock (HF), alpha_HF, values and
the exact atomic, alpha_EA, values. The performance of this simple analytic
model with both sets for atomization energies of G2 set of 148 molecules is
better than the local density approximation or the HF theory, although the
errors in atomization energy are larger than the target chemical accuracy.
To improve peformance for atomization energies, the SR method is
reparametrized to give atomization energies of 148 molecules to be comparbale
to those obtained by one of the most widely used generalized gradient
approximations. The mean absolute error in ionization potentials of 49 atoms
and molecules is about 0.5 eV and that in bond distances of 27 molecules is
about 0.02 Angstrom. The overall good performance of the computationally
efficient SR method using any reasonable set of alpha values makes it a
promising method for study of large systems.Comment: 33 pages, Uses RevTex, to appear in The Journal of Chemical Physic
Accurate molecular energies by extrapolation of atomic energies using an analytic quantum mechanical model
Using a new analytic quantum mechanical method based on Slater's Xalpha
method, we show that a fairly accurate estimate of the total energy of a
molecule can be obtained from the exact energies of its constituent atoms. The
mean absolute error in the total energies thus determined for the G2 set of 56
molecules is about 16 kcal/mol, comparable to or better than some popular pure
and hybrid density functional models.Comment: 5 pages, REVTE
Distribution of averages in a correlated Gaussian medium as a tool for the estimation of the cluster distribution on size
Calculation of the distribution of the average value of a Gaussian random
field in a finite domain is carried out for different cases. The results of the
calculation demonstrate a strong dependence of the width of the distribution on
the spatial correlations of the field. Comparison with the simulation results
for the distribution of the size of the cluster indicates that the distribution
of an average field could serve as a useful tool for the estimation of the
asymptotic behavior of the distribution of the size of the clusters for "deep"
clusters where value of the field on each site is much greater than the rms
disorder.Comment: 15 pages, 6 figures, RevTe
Electronic and magnetic properties of some rare-earth dihydrides and dideuterides
Mössbauer spectroscopy has been used to study the electronic and magnetic properties of a number of rare-earth dihydrides and dideuterides. In stoichiometric ErH2 and DyH2, magnetic transitions and crystal field ground states have been established. In non-stoichiometric compounds DyH2+x and (Er)HoH2+x changes of the rare-earth point symmetry due to distributions in hydrogen site occupations are seen. This results in increases in the magnetic transition temperatures and distributions in the magnetic moments
Evidence from K2 for rapid rotation in the descendant of an intermediate-mass star
Using patterns in the oscillation frequencies of a white dwarf observed by
K2, we have measured the fastest rotation rate, 1.13(02) hr, of any isolated
pulsating white dwarf known to date. Balmer-line fits to follow-up spectroscopy
from the SOAR telescope show that the star (SDSSJ0837+1856, EPIC 211914185) is
a 13,590(340) K, 0.87(03) solar-mass white dwarf. This is the highest mass
measured for any pulsating white dwarf with known rotation, suggesting a
possible link between high mass and fast rotation. If it is the product of
single-star evolution, its progenitor was a roughly 4.0 solar-mass
main-sequence B star; we know very little about the angular momentum evolution
of such intermediate-mass stars. We explore the possibility that this rapidly
rotating white dwarf is the byproduct of a binary merger, which we conclude is
unlikely given the pulsation periods observed.Comment: 5 pages, 4 figure, 1 table; accepted for publication in The
Astrophysical Journal Letter
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Magnetic relaxation in highly anisotropic systems
Using the Mori formalism, a general theory is developed for the calculation of paramagnetic relaxation effects in a general crystal field doublet. For systems with extreme magnetic anisotropy, the relaxation rates show a strong angular dependence, being severely peaked in a direction perpendicular to the magnetic easy axis. Application to the RRh/sub 4/B/sub 4/ (R = rare-earth) compounds is discussed. 5 refs
Localization Properties of Electronic States in Polaron Model of poly(dG)-poly(dC) and poly(dA)-poly(dT) DNA polymers
We numerically investigate localization properties of electronic states in a
static model of poly(dG)-poly(dC) and poly(dA)-poly(dT) DNA polymers with
realistic parameters obtained by quantum-chemical calculation. The randomness
in the on-site energies caused by the electron-phonon coupling are completely
correlated to the off-diagonal parts. In the single electron model, the effect
of the hydrogen-bond stretchings, the twist angles between the base pairs and
the finite system size effects on the energy dependence of the localization
length and on the Lyapunov exponent are given. The localization length is
reduced by the influence of the fluctuations in the hydrogen bond stretchings.
It is also shown that the helical twist angle affects the localization length
in the poly(dG)-poly(dC) DNA polymer more strongly than in the
poly(dA)-poly(dT) one. Furthermore, we show resonance structures in the energy
dependence of the localization length when the system size is relatively small.Comment: 6 pages, 6 figure
Quantitative analysis of regulatory flexibility under changing environmental conditions
The circadian clock controls 24-h rhythms in many biological processes, allowing appropriate timing of biological rhythms relative to dawn and dusk. Known clock circuits include multiple, interlocked feedback loops. Theory suggested that multiple loops contribute the flexibility for molecular rhythms to track multiple phases of the external cycle. Clear dawn- and dusk-tracking rhythms illustrate the flexibility of timing in Ipomoea nil. Molecular clock components in Arabidopsis thaliana showed complex, photoperiod-dependent regulation, which was analysed by comparison with three contrasting models. A simple, quantitative measure, Dusk Sensitivity, was introduced to compare the behaviour of clock models with varying loop complexity. Evening-expressed clock genes showed photoperiod-dependent dusk sensitivity, as predicted by the three-loop model, whereas the one- and two-loop models tracked dawn and dusk, respectively. Output genes for starch degradation achieved dusk-tracking expression through light regulation, rather than a dusk-tracking rhythm. Model analysis predicted which biochemical processes could be manipulated to extend dusk tracking. Our results reveal how an operating principle of biological regulators applies specifically to the plant circadian clock
Solution of the relativistic Dirac-Hulthen problem
The one-particle three-dimensional Dirac equation with spherical symmetry is
solved for the Hulthen potential. The s-wave relativistic energy spectrum and
two-component spinor wavefunctions are obtained analytically. Conforming to the
standard feature of the relativistic problem, the solution space splits into
two distinct subspaces depending on the sign of a fundamental parameter in the
problem. Unique and interesting properties of the energy spectrum are pointed
out and illustrated graphically for several values of the physical parameters.
The square integrable two-component wavefunctions are written in terms of the
Jacobi polynomials. The nonrelativistic limit reproduces the well-known
nonrelativistic energy spectrum and results in Schrodinger equation with a
"generalized" three-parameter Hulthen potential, which is the sum of the
original Hulthen potential and its square.Comment: 13 pages, 3 color figure
Time evolution of models described by one-dimensional discrete nonlinear Schr\"odinger equation
The dynamics of models described by a one-dimensional discrete nonlinear
Schr\"odinger equation is studied. The nonlinearity in these models appears due
to the coupling of the electronic motion to optical oscillators which are
treated in adiabatic approximation. First, various sizes of nonlinear cluster
embedded in an infinite linear chain are considered. The initial excitation is
applied either at the end-site or at the middle-site of the cluster. In both
the cases we obtain two kinds of transition: (i) a cluster-trapping transition
and (ii) a self-trapping transition. The dynamics of the quasiparticle with the
end-site initial excitation are found to exhibit, (i) a sharp self-trapping
transition, (ii) an amplitude-transition in the site-probabilities and (iii)
propagating soliton-like waves in large clusters. Ballistic propagation is
observed in random nonlinear systems. The effect of nonlinear impurities on the
superdiffusive behavior of random-dimer model is also studied.Comment: 16 pages, REVTEX, 9 figures available upon request, To appear in
Physical Review
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