2,408 research outputs found
Rigorous Screened Interactions for Realistic Correlated Electron Systems
We derive a widely-applicable first principles approach for determining two-body, static effective interactions for low-energy Hamiltonians with quantitative accuracy. The algebraic construction rigorously conserves all instantaneous two-point correlation functions in a chosen model space at the level of the random phase approximation, improving upon the traditional uncontrolled static approximations. Applied to screened interactions within a quantum embedding framework, we demonstrate these faithfully describe the relaxation of local subspaces via downfolding high-energy physics in molecular systems, as well as enabling a systematically improvable description of the long-range plasmonic contributions in extended graphene
Full configuration interaction quantum Monte Carlo for coupled electron--boson systems and infinite spaces
We extend the scope of full configuration interaction quantum Monte Carlo
(FCIQMC) to be applied to coupled fermion-boson hamiltonians, alleviating the a
priori truncation in boson occupation which is necessary for many other wave
function based approaches to be tractable. Detailing the required algorithmic
changes for efficient excitation generation, we apply FCIQMC in two contrasting
settings. The first is a sign-problem-free Hubbard--Holstein model of local
electron-phonon interactions, where we show that with care to control for
population bias via importance sampling and/or reweighting, the method can
achieve unbiased energies extrapolated to the thermodynamic limit, without
suffering additional computational overheads from relaxing boson occupation
constraints. Secondly, we apply the method as a `solver' within a quantum
embedding scheme which maps electronic systems to local electron-boson
auxiliary models, with the bosons representing coupling to long-range
plasmonic-like fluctuations. We are able to sample these general electron-boson
hamiltonians with ease despite a formal sign problem, including a faithful
reconstruction of converged reduced density matrices of the system
A 'moment-conserving' reformulation of GW theory
We show how to construct an effective Hamiltonian whose dimension scales
linearly with system size, and whose eigenvalues systematically approximate the
excitation energies of theory. This is achieved by rigorously expanding
the self-energy in order to exactly conserve a desired number of
frequency-independent moments of the self-energy dynamics. Recasting in
this way admits a low-scaling approach to build this
Hamiltonian, with a proposal to reduce this further to . This
relies on exposing a novel recursive framework for the density response moments
of the random phase approximation (RPA), where the efficient calculation of its
starting point mirrors the low-scaling approaches to compute RPA correlation
energies. The frequency integration of which distinguishes so many
different variants can be performed directly and cheaply in this moment
representation. Furthermore, the solution to the Dyson equation can be
performed exactly, avoiding analytic continuation, diagonal approximations or
iterative solutions to the quasiparticle equation, with the full-frequency
spectrum of all solutions obtained in a complete diagonalization of this
effective static Hamiltonian. We show how this approach converges rapidly with
respect to the order of the conserved self-energy moments, and is applied
across the benchmark dataset to obtain accurate spectra in
comparison to traditional implementations. We also show the ability to
systematically converge all-electron full-frequency spectra and high-energy
features beyond frontier excitations, as well as avoiding discontinuities in
the spectrum which afflict many other approaches
Experimental Validation of a Fundamental Model for PCR Efficiency
Recently a theoretical analysis of PCR efficiency has been published by Booth et al., (2010). The PCR yield is the product of three efficiencies: (i) the annealing efficiency is the fraction of templates that form binary complexes with primers during annealing, (ii)the polymerase binding efficiency is the fraction of binary complexes that bind to polymerase to form ternary complexes and (iii)the elongation efficiency is the fraction of ternary complexes that extend fully. Yield is controlled by the smallest of the three efficiencies and control could shift from one type of efficiency to another over the course of a PCR experiment. Experiments have been designed that are specifically controlled by each one of the efficiencies and the results are consistent with the mathematical model. The experimental data has also been used to quantify six key parameters of the theoretical model. An important application of the fully characterized model is to calculate initial template concentration from real-time PCR data. Given the PCR protocol, the midpoint cycle number (where the template concentration is half that of the final concentration) can be theoretically determined and graphed for a variety of initial DNA concentrations. Real-time results can be used to calculate the midpoint cycle number and consequently the initial DNA concentration, using this graph. The application becomes particularly simple if a conservative PCR protocol is followed where only the annealing efficiency is controlling
Efficiency of the Polymerase Chain Reaction
The polymerase chain reaction (PCR) has found wide application in biochemistry and molecular biology such as gene expression studies, mutation detection, forensic analysis and pathogen detection. Increasingly quantitative real time PCR is used to assess copy numbers from overall yield. In this study the yield is analyzed as a function of several processes: (1) thermal damage of the template and polymerase occurs during the denaturing step, (2) competition exists between primers and templates to either anneal or form dsDNA, (3) polymerase binding to annealed products (primer/ssDNA) to form ternary complexes and (4) extension of ternary complexes. Explicit expressions are provided for the efficiency of each process, therefore reaction conditions can be directly linked to the overall yield. Examples are provided where different processes play the yield-limiting role. The analysis will give researchers a unique understanding of the factors that control the reaction and will aid in the interpretation of experimental results
Undertaking a randomised controlled trial in the police setting : methodological and practical challenges
BACKGROUND: There has been an increased drive towards Evidence Based Policing in recent years. Unlike in other public sector services, such as health and education, randomised controlled trials in the police setting are relatively rare. This paper discusses some of the methodological and practical challenges of conducting a randomised controlled trial in the police setting in the UK, based on our experience of the Connect trial. This pragmatic, cluster-randomised controlled trial investigated the effectiveness of a face-to-face training intervention for frontline officers in comparison to routine training. The primary outcome was the number of incidents which resulted in a police response reported to North Yorkshire Police control room in a 1-month period up to 6 months after delivery of training. MAIN TEXT: The methodological and practical challenges that we experienced whilst conducting the Connect trial are discussed under six headings: establishing the unit of randomisation; population of interest and sample size; co-production of evidence; time frame; outcomes; and organisational issues. CONCLUSION: Recommendations on the conduct of future randomised controlled trials in the police setting are made. To understand the context in which research is undertaken, collaboration between police and academia is needed and police officers should be embedded within trial management groups. Engagement with police data analysts to understand what data is available and facilitate obtaining trial data is also recommended. Police forces may wish to review their IT systems and recording practices. Pragmatic trials are encouraged and time frames need to allow for trial set-up and obtaining relevant ethical approvals. TRIAL REGISTRATION: ISRCTN Registry, ID: ISRCTN11685602 . Retrospectively registered on 13 May 2016
Fast electron transport patterns in intense laser-irradiated solids diagnosed by modeling measured multi-MeV proton beams
The measured spatial-intensity distribution of the beam of protons accelerated from the rear side of a solid target irradiated by an intense (>10 Wcm) laser pulse provides a diagnostic of the two-dimensional fast electron density profile at the target rear surface and thus the fast electron beam transport pattern within the target. An analytical model is developed, accounting for rear-surface fast electron sheath dynamics, ionization and projection of the resulting beam of protons. The sensitivity of the spatial-intensity distribution of the proton beam to the fast electron density distribution is investigated. An annular fast electron beam transport pattern with filamentary structure is inferred for the case of a thick diamond target irradiated at a peak laser intensity of 6 × 10 Wcm
PuPt2In7: a computational and experimental investigation
Flux-grown single crystals of PuPtIn are characterized and found to
be both non-superconducting and non-magnetic down to 2 K. The Sommerfeld
specific heat coefficient of mJ/mol K indicates heavy fermion
behavior. We report the results of generalized gradient approximation (GGA)+
calculations of PuPtIn and as yet unsynthesized isovalent
PuPtGa. The strength of the - hybridization of PuPtIn is
similar to the PuCoIn superconductor. The bare and -weighted
susceptibility within the constant-matrix-element approximation is calculated,
showing a maximum along the direction at . A similar and
slightly stronger maximum is also found in the structurally related
heavy-fermion materials PuCoGa and PuCoIn. The absence of
superconductivity in PuPtIn is examined based on the results of our
calculations.Comment: Version accepted for publicatio
Exercise and Omega-3 Polyunsaturated Fatty Acid Supplementation for the Treatment of Hepatic Steatosis in Hyperphagic OLETF Rats
Background and Aims. This study examined if exercise and omega-3 fatty acid (n3PUFA) supplementation is an effective treatment for hepatic steatosis in obese, hyperphagic Otsuka Long-Evans Tokushima Fatty (OLETF) rats. Methods. Male OLETF rats were divided into 4 groups (n=8/group): (1) remained sedentary (SED), (2) access to running wheels; (EX) (3) a diet supplemented with 3% of energy from fish oil (n3PUFA-SED); and (4) n3PUFA supplementation plus EX (n3PUFA+EX). The 8 week treatments began at 13 weeks, when hepatic steatosis is present in OLETF-SED rats. Results. EX alone lowered hepatic triglyceride (TAG) while, in contrast, n3PUFAs failed to lower hepatic TAG and blunted the ability of EX to decrease hepatic TAG levels in n3PUFAs+EX. Insulin sensitivity was improved in EX animals, to a lesser extent in n3PUFA+EX rats, and did not differ between n3PUFA-SED and SED rats. Only the EX group displayed higher complete hepatic fatty acid oxidation (FAO) to CO2 and carnitine palmitoyl transferase-1 activity. EX also lowered hepatic fatty acid synthase protein while both EX and n3PUFA+EX decreased stearoyl CoA desaturase-1 protein. Conclusions. Exercise lowers hepatic steatosis through increased complete hepatic FAO, insulin sensitivity, and reduced expression of de novo fatty acid synthesis proteins while n3PUFAs had no effect
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