7,887 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
Catalogue of Be/X-ray binary systems in the Small Magellanic Cloud: X-ray, optical & IR properties
This is a catalogue of approximately 70 X-ray emitting binary systems in the
Small Magellanic Cloud (SMC) that contain a Be star as the mass donor in the
system and a clear X-ray pulse signature from a neutron star. The systems are
generally referred to as Be/Xray binaries. It lists all their known binary
characteristics (orbital period, eccentricity), the measured spin period of the
compact object, plus the characteristics of the Be star (spectral type, size of
the circumstellar disk, evidence for NRP behaviour). For the first time data
from the Spitzer Observatory are combined with ground-based data to provide a
view of these systems out into the far-IR. Many of the observational parameters
are presented as statistical distributions and compared to other similar
similar populations (eg isolated Be & B stars) in the SMC, and to other
Be/X-ray systems in the Milky Way. In addition previous important results are
re-investigated using this excellently homogeneous sample. In particular, the
evidence for a bi-modality in the spin period distribution is shown to be even
stronger than first proposed, and the correlation between orbital period and
circumstellar disk size seen in galactic sources is shown to be clearly present
in the SMC systems and quantised for the first time.Comment: 10 pages, 10 figures, 4 tables. Accepted for publication in MNRA
OGLE observations of four X-ray binary pulsars in the SMC
This paper presents analysis and interpretation of OGLE photometric data of
four X-ray binary pulsar systems in the Small Magellanic Cloud: 1WGA
J0054.9-7226, RX J0050.7-7316, RX J0049.1-7250, and 1SAX J0103.2-7209. In each
case, the probable optical counterpart is identified on the basis of its
optical colours. In the case of RX J0050.7-7316 the regular modulation of its
optical light curve appears to reveal an ellipsoidal modulation with a period
of 1.416 days. Using reasonable masses for the neutron star and the B star, we
show that the amplitude and relative depths of the minima of the I-band light
curve of RX J0050.7-7316 can be matched with an ellipsoidal model where the B
star nearly fills its Roche lobe. For mass ratios in the range of 0.12 to 0.20,
the corresponding best-fitting inclinations are about 55 degrees or larger. The
neutron star would be eclipsed by the B star at inclinations larger than 60
degrees for this particular mass ratio range. Thus RX J0050.7-7316 is a good
candidate system for further study. In particular, we would need additional
photometry in several colours, and most importantly, radial velocity data for
the B star before we could draw more quantitative conclusions about the
component masses
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
Calculations of Potential Energy Surfaces Using Monte Carlo Configuration Interaction
We apply the method of Monte Carlo configuration interaction (MCCI) to
calculate ground-state potential energy curves for a range of small molecules
and compare the results with full configuration interaction. We show that the
MCCI potential energy curve can be calculated to relatively good accuracy, as
quantified using the non-parallelity error, using only a very small fraction of
the FCI space. In most cases the potential curve is of better accuracy than its
constituent single-point energies. We finally test the MCCI program on systems
with basis sets beyond full configuration interaction: a lattice of fifty
hydrogen atoms and ethylene. The results for ethylene agree fairly well with
other computational work while for the lattice of fifty hydrogens we find that
the fraction of the full configuration interaction space we were able to
consider appears to be too small as, although some qualitative features are
reproduced, the potential curve is less accurate.Comment: 14 pages, 22 figure
Monte Carlo configuration interaction applied to multipole moments, ionisation energies and electron affinities
The method of Monte Carlo configuration interaction (MCCI) [1,2] is applied
to the calculation of multipole moments. We look at the ground and excited
state dipole moments in carbon monoxide. We then consider the dipole of NO, the
quadrupole of the nitrogen molecule and of BH. An octupole of methane is also
calculated. We consider experimental geometries and also stretched bonds. We
show that these non-variational quantities may be found to relatively good
accuracy when compared with FCI results, yet using only a small fraction of the
full configuration interaction space. MCCI results in the aug-cc-pVDZ basis are
seen to generally have reasonably good agreement with experiment. We also
investigate the performance of MCCI when applied to ionisation energies and
electron affinities of atoms in an aug-cc-pVQZ basis. We compare the MCCI
results with full configuration-interaction quantum Monte Carlo [3,4] and
`exact' non-relativistic results [3,4]. We show that MCCI could be a useful
alternative for the calculation of atomic ionisation energies however electron
affinities appear much more challenging for MCCI. Due to the small magnitude of
the electron affinities their percentage errors can be high, but with regards
to absolute errors MCCI performs similarly for ionisation energies and electron
affinities.Comment: 12 pages, 20 figure
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