132 research outputs found
Comparison of low-order multireference many-body perturbation theories
Tests have been made to benchmark and assess the relative accuracies of low-order multireference perturbation theories as compared to coupled cluster (CC) and full configuration interaction (FCI) methods. Test calculations include the ground and some excited states of the Be, H2, BeH2, CH2, and SiH2 systems. Comparisons with FCI and CC calculations show that in most cases the effective valence shell Hamiltonian (Hv) method is more accurate than other low-order multireference perturbation theories, although none of the perturbative methods is as accurate as the CC approximations. We also briefly discuss some of the basic differences among the multireference perturbation theories considered in this work
Variational Excitations in Real Solids: Optical Gaps and Insights into Many-Body Perturbation Theory
We present an approach to studying optical band gaps in real solids in which
quantum Monte Carlo methods allow for the application of a rigorous variational
principle to both ground and excited state wave functions. In tests that
include small, medium, and large band gap materials, optical gaps are predicted
with a mean-absolute-deviation of 3.5% against experiment, less than half the
equivalent errors for typical many-body perturbation theories. The approach is
designed to be insensitive to the choice of density functional, a property we
exploit in order to provide insight into how far different functionals are from
satisfying the assumptions of many body perturbation theory. We explore this
question most deeply in the challenging case of ZnO, where we show that
although many commonly used functionals have shortcomings, there does exist a
one particle basis in which perturbation theory's zeroth order picture is
sound. Insights of this nature should be useful in guiding the future
application and improvement of these widely used techniques.Comment: 8 pages, 5 figures, 2 table
Koopmans-compliant functionals and their performance against reference molecular data
Koopmans-compliant functionals emerge naturally from extending the constraint
of piecewise linearity of the total energy as a function of the number of
electrons to each fractional orbital occupation. When applied to approximate
density-functional theory, these corrections give rise to
orbital-density-dependent functionals and potentials. We show that the simplest
implementations of Koopmans' compliance provide accurate estimates for the
quasiparticle excitations and leave the total energy functional almost or
exactly intact, i.e., they describe correctly electron removals or additions,
but do not necessarily alter the electronic charge density distribution within
the system. Additional functionals can then be constructed that modify the
potential energy surface, including e.g. Perdew-Zunger corrections. These
functionals become exactly one-electron self-interaction free and, as all
Koopmans-compliant functionals, are approximately many-electron
self-interaction free. We discuss in detail these different formulations, and
provide extensive benchmarks for the 55 molecules in the reference G2-1 set,
using Koopmans-compliant functionals constructed from local-density or
generalized-gradient approximations. In all cases we find excellent performance
in the electronic properties, comparable or improved with respect to that of
many-body perturbation theories, such as GW and self-consistent GW, at
a fraction of the cost and in a variational framework that also delivers energy
derivatives. Structural properties and atomization energies preserve or
slightly improve the accuracy of the underlying density-functional
approximations (Note: Supplemental Material is included in the source)
A new view on the origin of zero-bias anomalies of Co atoms atop noble metal surfaces
Many-body phenomena are paramount in physics. In condensed matter, their
hallmark is considerable on a wide range of material characteristics spanning
electronic, magnetic, thermodynamic and transport properties. They potentially
imprint non-trivial signatures in spectroscopic measurements, such as those
assigned to Kondo, excitonic and polaronic features, whose emergence depends on
the involved degrees of freedom. Here, we address systematically zero-bias
anomalies detected by scanning tunneling spectroscopy on Co atoms deposited on
Cu, Ag and Au(111) substrates, which remarkably are almost identical to those
obtained from first-principles. These features originate from gaped
spin-excitations induced by a finite magnetic anisotropy energy, in contrast to
the usual widespread interpretation relating them to Kondo resonances. Resting
on relativistic time-dependent density functional and many-body perturbation
theories, we furthermore unveil a new many-body feature, the spinaron,
resulting from the interaction of electrons and spin-excitations localizing
electronic states in a well defined energy.Comment: Supplementary Information include
Origins and Impacts of High-Density Symmetry Energy
What is nuclear symmetry energy? Why is it important? What do we know about
it? Why is it so uncertain especially at high densities? Can the total symmetry
energy or its kinetic part be negative? What are the effects of three-body
and/or tensor force on symmetry energy? How can we probe the density dependence
of nuclear symmetry energy with terrestrial nuclear experiments? What
observables of heavy-ion reactions are sensitive to the high-density behavior
of nuclear symmetry energy? How does the symmetry energy affect properties of
neutron stars, gravitational waves and our understanding about the nature of
strong-field gravity? In this lecture, we try to answer these questions as best
as we can based on some of our recent work and/or understanding of research
done by others. This note summarizes the main points of the lecture.Comment: Invited lecture given at the Carpathian Summer School of Physics
2016, Exotic Nuclei and Nuclear Astrophysics (VI), Sinaia, Romania, June 26
to July 9, 201
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