6,047 research outputs found
Machine Learning Configuration Interaction
We propose the concept of machine learning configuration interaction (MLCI)
whereby an artificial neural network is trained on-the-fly to predict important
new configurations in an iterative selected configuration interaction
procedure. We demonstrate that the neural network can discriminate between
important and unimportant configurations, that it has not been trained on, much
better than by chance. MLCI is then used to find compact wavefunctions for
carbon monoxide at both stretched and equilibrium geometries. We also consider
the multireference problem of the water molecule with elongated bonds. Results
are contrasted with those from other ways of selecting configurations:
first-order perturbation, random selection and Monte Carlo configuration
interaction. Compared with these other serial calculations, this prototype MLCI
is competitive in its accuracy, converges in significantly fewer iterations
than the stochastic approaches, and requires less time for the higher-accuracy
computations.Comment: This document is the unedited Author's version of a Submitted Work
that was subsequently accepted for publication in The Journal of Chemical
Theory and Computation, copyright American Chemical Society after peer
review. To access the final edited and published work see
https://pubs.acs.org/articlesonrequest/AOR-dANIFXJKzRAyR99E6hb
Exact and LDA entanglement of tailored densities in an interacting one-dimensional electron system
We calculate the `exact' potential corresponding to a one-dimensional
interacting system of two electrons with a specific, tailored density. We use
one-dimensional density-functional theory with a local-density approximation
(LDA) on the same system and calculate densities and energies, which are
compared with the `exact' ones. The `interacting-LDA system' corresponding to
the LDA density is then found and its potential compared with the original one.
Finally we calculate and compare the spatial entanglement of the electronic
systems corresponding to the interacting-LDA and original interacting system.Comment: 7 pages, 4 figure
Financial conditions and the risks to economic growth in the United States since 1875
We explore the historical relationship between financial conditions and real economic growth for quarterly U.S. data from 1875 to 2017 with a flexible empirical copula modelling methodology. We compare specifications with both linear and non-linear dependence, and with both Gaussian and non-Gaussian marginal distributions. Our results indicate strong statistical support for models that are both non-Gaussian and nonlinear for our historical data, with considerable heterogeneity across sub-samples. We demonstrate that ignoring the contribution of financial conditions typically understates the conditional downside risks to economic growth in crises. For example, accounting for financial conditions more than doubles the probability of negative growth in the year following the 1929 stock market crash
Characterizing a configuration interaction excited state using natural transition geminals
We introduce natural transition geminals as a means to qualitatively
understand a transition where double excitations are important. The first two
singlet states of the CH cation are used as an initial example. We
calculate these states with configuration interaction singles (CIS) and
state-averaged Monte Carlo configuration interaction (SA-MCCI). For each method
we compare the important natural transition geminals with the dominant natural
transition orbitals. We then compare SA-MCCI and full configuration interaction
(FCI) with regards to the natural transition geminals using the beryllium atom.
We compare using the natural transition geminals with analyzing the important
configurations in the CI expansion to give the dominant transition for the
beryllium atom and the carbon dimer. Finally we calculate the natural
transition geminals for two electronic excitations of formamide.Comment: 15 pages, 2 figures. Mol. Phys. (2013
Approaching exact hyperpolarizabilities via sum-over-states Monte Carlo configuration interaction
We propose using sum-over-states calculations with the compact wavefunctions
of Monte Carlo configuration interaction to approach accurate values for
higher-order dipole properties up to second hyperpolarizabilities in a
controlled way. We apply the approach to small systems that can generally be
compared with full configuration interaction (FCI) results. We consider
hydrogen fluoride with a 6-31g basis and then look at results, including
frequency dependent properties, in an aug-cc-pVDZ basis. We extend one
calculation beyond FCI by using an aug-cc-pVTZ basis. The properties of an
H molecule with multireference character are calculated in an aug-cc-pVDZ
basis. We then investigate this method on a strongly multireference system with
a larger FCI space by modelling the properties of carbon monoxide with a
stretched geometry. The behavior of the approach with increasing basis size is
considered by calculating results for the neon atom using aug-cc-pVDZ to
aug-cc-pVQZ. We finally test if the unusual change in polarizability between
the first two states of molecular oxygen can be reproduced by this method in a
6-31g basis.Comment: 11 pages, 14 figure
Applying Monte Carlo configuration interaction to transition metal dimers: exploring the balance between static and dynamic correlation
We calculate potential curves for transition metal dimers using Monte Carlo
configuration interaction (MCCI). These results, and their associated
spectroscopic values, are compared with experimental and computational studies.
The multireference nature of the MCCI wavefunction is quantified and we
estimate the important orbitals. We initially consider the ground state of the
chromium dimer. Next we calculate potential curves for Sc where we
contrast the lowest triplet and quintet states. We look at the molybdenum dimer
where we compare non-relativistic results with the partial inclusion of
relativistic effects via effective core potentials, and report results for
scandium nickel.Comment: 9 pages and 8 figure
Effects of wing leading-edge deflection on low-speed aerodynamic characteristics of a low-aspect-ratio highly swept arrow-wing configuration
Static force tests were conducted in the Langley V/STOL tunnel at a Reynolds number (based on the mean aerodynamic chord) of about 2.0 x 10 to the 6th power for an angle-of-attack range from about - 10 deg to 17 deg and angles of sideslip of 0 and + or - 5 deg. Limited flow visualization studies were also conducted in order to provide a qualitative assessment of leading-edge upwash characteristics
Distributed upper-surface blowing concept
A low speed investigation was conducted in the Langley V/STOL tunnel to determine the powered lift aerodynamic performance of a distributed upper surface blown propulsive lift transport model. The model used blowing slots across the span of the wing to produce a thin jet efflux near the leading edge and at the knee of the trailing edge flap (internally blown jet flap). Results indicate that these concepts have both good propulsive related lift and low drag due to lift characteristics because of uniform spanwise propulsive thrust. The leading edge blowing concept provides low speed lift characteristics which are competitive with the flap-hinge-line blowing concept and does not require additional leading edge treatment for prevention of abrupt stall
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