3,096 research outputs found
The sign problem in full configuration interaction quantum Monte Carlo: Linear and sub-linear representation regimes for the exact wave function
We investigate the sign problem for full configuration interaction quantum
Monte Carlo (FCIQMC), a stochastic algorithm for finding the ground state
solution of the Schr\"odinger equation with substantially reduced computational
cost compared with exact diagonalisation. We find -space Hubbard models for
which the solution is yielded with storage that grows sub-linearly in the size
of the many-body Hilbert space, in spite of using a wave function that is
simply linear combination of states. The FCIQMC algorithm is able to find this
sub-linear scaling regime without bias and with only a choice of Hamiltonian
basis. By means of a demonstration we solve for the energy of a 70-site
half-filled system (with a space of determinants) in 250 core hours,
substantially quicker than the 10 core hours that would be
required by exact diagonalisation. This is the largest space that has been
sampled in an unbiased fashion. The challenge for the recently-developed FCIQMC
method is made clear: expand the sub-linear scaling regime whilst retaining
exact on average accuracy. This result rationalizes the success of the
initiator adaptation (i-FCIQMC) and offers clues to improve it. We argue that
our results changes the landscape for development of FCIQMC and related
methods.Comment: 6 pages, 4 figures. The mentioned supplementary material is included
as "Ancillary files". Comments welcom
Eggshell appearance does not signal maternal corticosterone exposure in Japanese quail:an experimental study with brown-spotted eggs
Reproduction is a critical period for birds as they have to cope with many stressful events. One consequence of an acute exposure to stress is the release of corticosterone, the avian stress hormone. Prolonged stress can have negative impacts on the immune system, resulting in, for example, increased oxidative stress. Through maternal effects, females are known to modulate their investment in eggs content according to their own physiological condition. Less is known about maternal investment in eggshells, especially in pigments. The two main eggshell pigments may possess opposite antioxidant properties: protoporphyrin (brown) is a pro-oxidant, whereas biliverdin (blue-green) is an antioxidant. In Japanese quail, we know that the deposition of both pigments is related to female body condition. Thus, a chronic stress response may be reflected in eggshell coloration. Using female Japanese quails that lay brown-spotted eggs, we explored whether physiological exposure to corticosterone induces a change in female basal stress and antioxidant factors, and eggshell pigment concentration, spectrophotometric reflectance, and maculation coverage. We supplemented adult females over a 2 week period with either peanut oil (control) or corticosterone (treatment). We collected pre- and post-supplementation eggs and analysed the effect of corticosterone treatment on female physiology and eggshell appearance parameters. Except for corticosterone-fed birds which laid eggs with brighter spots, supplementation had no significant effect on female physiology or eggshell pigment concentration, reflectance and maculation. The change in eggshell spot brightness was not detected by a photoreceptor noise-limited color opponent model of avian visual perception. Our data confirms that eggshell reflectance in spotted eggs varies over the laying sequence, and spot reflectance may be a key factor that is affected by females CORT exposure, even if the changes are not detected by an avian visual model
Repurposing of Meropenem and Nadifloxacin for Treatment of Burn Patients?
The escalating number of multidrug resistant pathogens has demanded the swift development of new and potent antibiotics (ref. 2). Metallo-[beta]-lactamases (MBLs) continue to evolve, rendering the latest generation of carbapenem antibiotics useless (ref. 8). SPM-1, a recently discovered MBL, was isolated from a juvenile leukemia patient residing in a hospital in San Palo, Brazil just prior to the patient succumbing to septicemia brought on by Pseudomonas aeruginosa expressing SPM-1 (ref. 8). Screening of the Johns Hopkins Compound library of 1,514 FDA or FAD approved drugs (ref. 1) identified a novel SPM-1 inhibitor that is synergistically compatible with meropenem. Using clinically achievable concentrations, meropenem coupled with nadifloxacin inhibits Pseudomonas aeruginosa expressing SPM-1. This shotgun approach to new drug discovery provided a prompt solution to the grave problem of antibiotic resistant pathogens that are thriving in hospitals today
A Self-Attention Ansatz for Ab-initio Quantum Chemistry
We present a novel neural network architecture using self-attention, the
Wavefunction Transformer (Psiformer), which can be used as an approximation (or
Ansatz) for solving the many-electron Schr\"odinger equation, the fundamental
equation for quantum chemistry and material science. This equation can be
solved from first principles, requiring no external training data. In recent
years, deep neural networks like the FermiNet and PauliNet have been used to
significantly improve the accuracy of these first-principle calculations, but
they lack an attention-like mechanism for gating interactions between
electrons. Here we show that the Psiformer can be used as a drop-in replacement
for these other neural networks, often dramatically improving the accuracy of
the calculations. On larger molecules especially, the ground state energy can
be improved by dozens of kcal/mol, a qualitative leap over previous methods.
This demonstrates that self-attention networks can learn complex quantum
mechanical correlations between electrons, and are a promising route to
reaching unprecedented accuracy in chemical calculations on larger systems
Open-source development experiences in scientific software: the HANDE quantum Monte Carlo project
The HANDE quantum Monte Carlo project offers accessible stochastic algorithms
for general use for scientists in the field of quantum chemistry. HANDE is an
ambitious and general high-performance code developed by a
geographically-dispersed team with a variety of backgrounds in computational
science. In the course of preparing a public, open-source release, we have
taken this opportunity to step back and look at what we have done and what we
hope to do in the future. We pay particular attention to development processes,
the approach taken to train students joining the project, and how a flat
hierarchical structure aids communicationComment: 6 pages. Submission to WSSSPE
Accurate exchange-correlation energies for the warm dense electron gas
Density matrix quantum Monte Carlo (DMQMC) is used to sample exact-on-average
-body density matrices for uniform electron gas systems of up to 10
matrix elements via a stochastic solution of the Bloch equation. The results of
these calculations resolve a current debate over the accuracy of the data used
to parametrize finite-temperature density functionals. Exchange-correlation
energies calculated using the real-space restricted path-integral formalism and
the -space configuration path-integral formalism disagree by up to
\% at certain reduced temperatures and densities . Our calculations confirm the accuracy of the configuration
path-integral Monte Carlo results available at high density and bridge the gap
to lower densities, providing trustworthy data in the regime typical of
planetary interiors and solids subject to laser irradiation. We demonstrate
that DMQMC can calculate free energies directly and present exact free energies
for and .Comment: Accepted version: added free energy data and restructured text. Now
includes supplementary materia
Sublethal exposure, insecticide resistance, and community stress
Insecticides are an invaluable pest management tool and anthropogenic stressors of widespread environmental occurrence that are subject to biased perceptions based on the targeted application, market value of use, and regulatory requirements. As a result, short-term and simplistic efforts focusing on lethal effects toward individual species and populations prevail. Holistic and comprehensive studies exploring rather common sublethal insecticide exposures are rare, particularly considering their potential role in structuring populations and communities in diverse environmental settings and potentially interfering in a range of ecological interactions. Studies on insecticide resistance, for example, do not go beyond population-based studies, disregarding temporal and spatial effects in the associated community, and rarely considering the whole of sublethal exposure. Some of these knowledge gaps are here recognized and explored
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