15,164 research outputs found
Non-linear biases, stochastically-sampled effective Hamiltonians and spectral functions in quantum Monte Carlo methods
In this article we study examples of systematic biases that can occur in
quantum Monte Carlo methods due to the accumulation of non-linear expectation
values, and approaches by which these errors can be corrected. We begin with a
study of the Krylov-projected FCIQMC (KP-FCIQMC) approach, which was recently
introduced to allow efficient, stochastic calculation of dynamical properties.
This requires the solution of a sampled effective Hamiltonian, resulting in a
non-linear operation on these stochastic variables. We investigate the
probability distribution of this eigenvalue problem to study both stochastic
errors and systematic biases in the approach, and demonstrate that such errors
can be significantly corrected by moving to a more appropriate basis. This is
lastly expanded to include consideration of the correlation function QMC
approach of Ceperley and Bernu, showing how such an approach can be taken in
the FCIQMC framework.Comment: 12 pages, 7 figure
An orthologue of bacteroides fragilis NanH is the principal sialidase in tannerella forsythia
Sialidase activity is a putative virulence factor of the anaerobic periodontal pathogen Tannerella forsythia, but it is uncertain which genes encode this activity. Characterization of a putative sialidase, SiaHI, by others, indicated that this protein alone may not be responsible for all of the sialidase activity. We describe a second sialidase in T. forsythia (TF0035), an orthologue of Bacteroides fragilis NanH, and its expression in Escherichia coli. Sialidase activity of the expressed NanH was confirmed by using 2′-(4-methylumbelliferyl)-α-d-N-acetylneuraminic acid as a substrate. Biochemical characterization of the recombinant T. forsythia NanH indicated that it was active over a broad pH range, with optimum activity at pH 5.5. This enzyme has high affinity for 2′-(4-methylumbelliferyl)-α-d-N-acetylneuraminic acid (Km of 32.9 ± 10.3 μM) and rapidly releases 4-methylumbelliferone (Vmax of 170.8 ± 11.8 nmol of 4-methylumbelliferone min−1 mg of protein−1). E. coli lysates containing recombinant T. forsythia NanH cleave sialic acid from a range of substrates, with a preference for α2-3 glycosidic linkages. The genes adjacent to nanH encode proteins apparently involved in the metabolism of sialic acid, indicating that the NanH sialidase is likely to be involved in nutrient acquisition
Molecular clouds in the centers of galaxies: Constraints from HCN and CO-13 line emission
We have searched for HCN J=1-0 line emission in the centers of 12 galaxies and have detected it in 10 of them. We have obtained complementary data on J=1-0 and 2-1 transitions of CO-12 and CO-13 in these systems. The ratio of integrated intensities, I(CO 1-0)/I(HCN 1-0) = 25 +/- 11 for this sample. We find that HCN emission of this strength can be produced under conditions of subthermal excitation. In combination with the line ratios in CO and CO-13, HCN puts constraints on the mean conditions of molecular clouds and on the mix of cloud types within the projected beam
Energy-weighted density matrix embedding of open correlated chemical fragments
We present a multi-scale approach to efficiently embed an ab initio
correlated chemical fragment described by its energy-weighted density matrices,
and entangled with a wider mean-field many-electron system. This approach,
first presented in Phys. Rev. B, 98, 235132 (2018), is here extended to account
for realistic long-range interactions and broken symmetry states. The scheme
allows for a systematically improvable description in the range of correlated
fluctuations out of the fragment into the system, via a self-consistent
optimization of a coupled auxiliary mean-field system. It is discussed that the
method has rigorous limits equivalent to existing quantum embedding approaches
of both dynamical mean-field theory, as well as density matrix embedding
theory, to which this method is compared, and the importance of these
correlated fluctuations is demonstrated. We derive a self-consistent local
energy functional within the scheme, and demonstrate the approach for Hydrogen
rings, where quantitative accuracy is achieved despite only a single atom being
explicitly treated.Comment: 14 pages, 8 figure
A quasilocal calculation of tidal heating
We present a method for computing the flux of energy through a closed surface
containing a gravitating system. This method, which is based on the quasilocal
formalism of Brown and York, is illustrated by two applications: a calculation
of (i) the energy flux, via gravitational waves, through a surface near
infinity and (ii) the tidal heating in the local asymptotic frame of a body
interacting with an external tidal field. The second application represents the
first use of the quasilocal formalism to study a non-stationary spacetime and
shows how such methods can be used to study tidal effects in isolated
gravitating systems.Comment: REVTex, 4 pages, 1 typo fixed, standard sign convention adopted for
the Newtonian potential, a couple of lines added to the discussion of gauge
dependent term
Observation of blue-shifted ultralong-range Cs Rydberg molecules
We observe ultralong-range blue-shifted Cs molecular states near
Rydberg states in an optical dipole trap, where .
The accidental near degeneracy of and Rydberg states for in
Cs, due to the small fractional quantum defect, leads to non-adiabatic
coupling among these states, producing potential wells above the
thresholds. Two important consequences of admixing high angular momentum states
with states are the formation of large permanent dipole moments, Debye, and accessibility of these states via two-photon association.
The observed states are in excellent agreement with theory. Both projections of
the total angular momentum on the internuclear axis are visible in the
experiment
Unforeseen high temperature and humidity stability of FeCl intercalated few layer graphene
We present the first systematic study of the stability of the structure and
electrical properties of FeCl intercalated few-layer graphene to high
levels of humidity and high temperature. Complementary experimental techniques
such as electrical transport, high resolution transmission electron microscopy
and Raman spectroscopy conclusively demonstrate the unforeseen stability of
this transparent conductor to a relative humidity up to at room
temperature for 25 days, to a temperature up to 150\,^\circC in atmosphere
and up to a temperature as high as 620\,^\circC in vacuum, that is more than
twice higher than the temperature at which the intercalation is conducted. The
stability of FeCl intercalated few-layer graphene together with its unique
values of low square resistance and high optical transparency, makes this
material an attractive transparent conductor in future flexible electronic
applications.Comment: Scientific Reports, volume 5, article no. 760
Fundamental properties and applications of quasi-local black hole horizons
The traditional description of black holes in terms of event horizons is
inadequate for many physical applications, especially when studying black holes
in non-stationary spacetimes. In these cases, it is often more useful to use
the quasi-local notions of trapped and marginally trapped surfaces, which lead
naturally to the framework of trapping, isolated, and dynamical horizons. This
framework allows us to analyze diverse facets of black holes in a unified
manner and to significantly generalize several results in black hole physics.
It also leads to a number of applications in mathematical general relativity,
numerical relativity, astrophysics, and quantum gravity. In this review, I will
discuss the basic ideas and recent developments in this framework, and
summarize some of its applications with an emphasis on numerical relativity.Comment: 14 pages, 2 figures. Based on a talk presented at the 18th
International Conference on General Relativity and Gravitation, 8-13 July
2007, Sydney, Australi
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