85 research outputs found
Excitations and benchmark ensemble density functional theory for two electrons
A new method for extracting ensemble Kohn-Sham potentials from accurate
excited state densities is applied to a variety of two electron systems,
exploring the behavior of exact ensemble density functional theory. The issue
of separating the Hartree energy and the choice of degenerate eigenstates is
explored. A new approximation, spin eigenstate Hartree-exchange (SEHX), is
derived. Exact conditions that are proven include the signs of the correlation
energy components, the virial theorem for both exchange and correlation, and
the asymptotic behavior of the potential for small weights of the excited
states. Many energy components are given as a function of the weights for two
electrons in a one-dimensional flat box, in a box with a large barrier to
create charge transfer excitations, in a three-dimensional harmonic well
(Hooke's atom), and for the He atom singlet-triplet ensemble,
singlet-triplet-singlet ensemble, and triplet bi-ensemble.Comment: 15 pages, supplemental material pd
Warming Up Density Functional Theory
Density functional theory (DFT) has become the most popular approach to
electronic structure across disciplines, especially in material and chemical
sciences. Last year, at least 30,000 papers used DFT to make useful predictions
or give insight into an enormous diversity of scientific problems, ranging from
battery development to solar cell efficiency and far beyond. The success of
this field has been driven by usefully accurate approximations based on known
exact conditions and careful testing and validation. In the last decade,
applications of DFT in a new area, warm dense matter, have exploded. DFT is
revolutionizing simulations of warm dense matter including applications in
controlled fusion, planetary interiors, and other areas of high energy density
physics. Over the past decade or so, molecular dynamics calculations driven by
modern density functional theory have played a crucial role in bringing
chemical realism to these applications, often (but not always) with excellent
agreement with experiment. This chapter summarizes recent work from our group
on density functional theory at non-zero temperatures, which we call thermal
DFT. We explain the relevance of this work in the context of warm dense matter,
and the importance of quantum chemistry to this regime. We illustrate many
basic concepts on a simple model system, the asymmetric Hubbard dimer
Melting a Hubbard dimer: benchmarks of 'ALDA' for quantum thermodynamics
The competition between evolution time, interaction strength, and temperature
challenges our understanding of many-body quantum systems out-of-equilibrium.
Here we consider a benchmark system, the Hubbard dimer, which allows us to
explore all the relevant regimes and calculate exactly the related average
quantum work. At difference with previous studies, we focus on the effect of
increasing temperature, and show how this can turn competition between
many-body interactions and driving field into synergy. We then turn to use
recently proposed protocols inspired by density functional theory to explore if
these effects could be reproduced by using simple approximations. We find that,
up to and including intermediate temperatures, a method which borrows from
ground-state adiabatic local density approximation improves dramatically the
estimate for the average quantum work, including, in the adiabatic regime, when
correlations are strong. However at high temperature and at least when based on
the pseudo-LDA, this method fails to capture the counterintuitive qualitative
dependence of the quantum work with interaction strength, albeit getting the
quantitative estimates relatively close to the exact results
The role of prefrontal cortex in working-memory capacity, executive attention, and general fluid intelligence: An individual-differences perspective
Exact thermal density functional theory for a model system: Correlation components and accuracy of the zero-temperature exchange-correlation approximation
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