5,892 research outputs found
Sensitivity of the r-process to nuclear masses
The rapid neutron capture process (r-process) is thought to be responsible
for the creation of more than half of all elements beyond iron. The scientific
challenges to understanding the origin of the heavy elements beyond iron lie in
both the uncertainties associated with astrophysical conditions that are needed
to allow an r-process to occur and a vast lack of knowledge about the
properties of nuclei far from stability. There is great global competition to
access and measure the most exotic nuclei that existing facilities can reach,
while simultaneously building new, more powerful accelerators to make even more
exotic nuclei. This work is an attempt to determine the most crucial nuclear
masses to measure using an r-process simulation code and several mass models
(FRDM, Duflo-Zuker, and HFB-21). The most important nuclear masses to measure
are determined by the changes in the resulting r-process abundances. Nuclei
around the closed shells near N=50, 82, and 126 have the largest impact on
r-process abundances irrespective of the mass models used.Comment: 5 pages, 4 figures, accepted in European Physical Journal
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
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