3790 research outputs found
Sort by
Cluster perturbation theory. II. Excitation energies for a coupled cluster target state
Get PDFIn cluster perturbation (CP) theory, we consider a target excitation space relative to a Hartree-Fock state and partition the target excitation space into a parent excitation space and an auxiliary excitation space. The zeroth-order state is in CP theory a coupled cluster (CC) state in the parent excitation space, and the target state is a CC state in the target excitation space. In this paper, we derive CP series for excitation energies in orders of the CC parent-state similarity-transformed fluctuation potential where the zeroth-order term in the series is an excitation energy for the CC parent state response eigenvalue equation and where the series formally converge to an excitation energy for the CC target state response eigenvalue equation. We give explicit expressions for the lowest-order excitation energy corrections. We also report calculations for CP excitation energy series for various parent and target excitation spaces and examine how well the lower-order corrections can reproduce the total excitation energies. Considering the fast local convergence we have observed for the CP excitation energy series, it becomes computationally attractive to use low-order corrections in CP series to obtain excitation energies of CC target state quality. For the CPS(D-n) series, the first-order correction vanishes, the second-order correction becomes the CIS(D) model, and for the CPS(D-3) model, our calculations suggest that excitation energies of CCSD quality are obtained. The numerical results also suggest that a similar behavior can be seen for the low-order excitation energy corrections for CP series where the parent state contains more than a singles excitation space, e.g., for the CPSD(T) model. We therefore expect the low-order excitation energy corrections in CP series soon to become state-of-the-art models for determining excitation energies of CC target state quality. Published under license by AIP Publishing.Publishe
Development of a REgion‐Specific Ecosystem Feedback Fire (RESFire) Model in the Community Earth System Model
Get PDFFires play a critical role in modulating regional and global climate through disturbances on
meteorological, biogeochemical, and hydrological processes, while fires are strongly affected by climate,
terrestrial ecosystems, and human activities. The complex climate‐fire‐ecosystem interactions with
anthropogenic disturbance are not well understood. We developed a REgion‐Specific ecosystem feedback
Fire (RESFire) model in the Community Earth System Model (CESM) that provides modeling capability to
reproduce the observed burning patterns and trends and to understand fire related climatic processes.
Comparing with the default Community Land Model version 4.5 fire model in CESM, the RESFire model
includes heterogeneous natural and anthropogenic constraints on fire ignition and spread, improved fire
impact parameterization including online fire emissions and fire induced land cover changes, and modeling
bias corrections for online fire weather simulation. Evaluation results based on the International Land
Model Benchmarking package show significant improvements in fire simulation performance. The overall
modeling score of burned area simulation increases from 0.50 with Community Land Model version 4.5 to
0.62 (RESFire driven by the observation‐reanalysis data) and 0.60 (RESFire driven by the bias‐corrected
Community Atmosphere Model version 5 simulation). The attribution analysis of decadal burned area
trends suggests distinct contributions of natural and anthropogenic forcing in different regions, which are
consistent with previous observations. The model also includes a fire impact module for estimating
atmospheric responses to fire emissions as well as fire disturbances on ecosystems, land cover, and surface
radiation budget. These results demonstrate the latest progress of global fire model development that enables
fully interactive climate‐fire‐ecosystem studies using CESM.
Plain Language Summary We improved the fire simulation capability in the Earth system
model to better understand the complex interactions among climate, fire, and ecosystems with
anthropogenic disturbance.PublishedYe
Modeling of a New Electron Acceleration Mechanism Ahead of Streamers
Get PDFHead-on collisions between negative and positive streamers have been proposed as a mechanism behind X-ray emissions by laboratory spark discharges. Recent simulations using plasma fluid and particle in cell models of a single head-on collision of two streamers of opposite polarities in ground pressure air predicted an insignificant number of thermal runaway electrons >1 keV and hence weak undetectable X-ray emissions. Because the current available models of a single streamer collision failed to explain the observations, we first use a Monte Carlo model coupled with multiple static dielectric ellipsoids immersed in a subbreakdown ambient electric field as a description of multiple streamer environment and we investigate the ability of multiple streamer-streamer head-on collisions to accelerate runaway electrons >1 keV up to energies similar to 200-300 keV instead of just one single head-on collision. The results of simulations show that the streamer head-on collision mechanism fails to accelerate electrons; instead, they decelerate in the positive streamer channel. In a second part, we use a streamer plasma fluid model to simulate a new streamer-electron acceleration mechanism based on a collision of a large negative streamer with a small neutral plasma patch in different Laplacian electric fields vertical bar E-0 vertical bar = (35, 40, 45) kV/cm, respectively. We observe the formation of a secondary short propagating negative streamer with a strong peak electric field >250 up to 378 kV/cm over a time duration of similar to 0.16 ns at the moment of the collision. The mechanism produces up to 10(6) runaway electrons with an upper energy limit of 24 keV.PublishedYe
The Power of Interest: Connecting the Real World to the Finance Classroom
Get PDFThis study examines how the incorporation of a project utilizing real- world financial data from the Bloomberg Professional Service affects student engagement in an introductory finance course. This mixed methods study demonstrates that the project provides insight into the role played by receptivity, relevance, discipline, and integration in bridging the gap between teaching and learning and between theory and practice.PublishedYe
Two-Dimensional gcPIC Simulation of Rising-Tone Chorus Waves in a Dipole Magnetic Field
No full textRising-tone chorus waves have already been successfully produced in a mirror magnetic field with the use of one- and two-dimensional particle-in-cell (PIC) simulations. However, in reality, the background magnetic field in the inner Earth's magnetosphere is a dipole magnetic field, unlike symmetric mirror fields. In this paper, with the two-dimensional (2-D) general curvilinear PIC (gcPIC) code, we investigate the generation of rising-tone chorus waves in the dipole magnetic field configuration. The plasma consists of three components: immobile ions, cold background, and hot electrons. In order to save computational resource, the topology of the magnetic field is roughly equal to that at L = 0.6 R-E, although the plasma parameters corresponding to those at L = 6 R-E (R-E is the Earth's radius) are used. Whistler mode waves are first excited around the magnetic equator by the hot electrons with a temperature anisotropy. The excited whistler mode waves propagate almost parallel and antiparallel to the background magnetic field in their source region, which is limited at vertical bar lambda vertical bar <= 3 degrees (where lambda is the magnetic latitude). When the waves leave from the source region and propagate toward high latitudes, both their amplitude and wave normal angle become larger. However, the group velocity of the waves is directed toward high latitudes almost along the magnetic field. During such a process, the waves have a frequency chirping, as shown by a rising tone in the frequency-time spectrogram. To our best knowledge, it is for the first time that rising-tone chorus are generated in a dipole magnetic field with a PIC simulation.Publishe
Data for: Adaptive seasonal shift towards investment in fewer, larger offspring: Evidence from field and laboratory studies
No full text1. Seasonal changes in reproduction have been described for many taxa. As reproductive seasons progress, females often shift from greater energetic investment in many small offspring towards investing less total energy into fewer, better provisioned (i.e. larger) offspring. The underlying causes of this pattern have not been assessed in many systems.
2. Two primary hypotheses have been proposed to explain these patterns. The first is an adaptive hypothesis from life history theory: early offspring have a survival advantage over those produced later. Accordingly, selection favors females that invest in offspring quantity early in the season and offspring quality later. The second hypothesis suggests these patterns are not intrinsic but result from passive responses to seasonal changes in the environment experienced by reproducing females (i.e. maternal environment).
3. To disentangle the causes underlying this pattern, which has been reported in brown anole lizards (Anolis sagrei), we performed complementary field and lab studies. The lab study carefully controlled maternal environments and quantified reproductive patterns throughout the reproductive season for each female. The field study measured similar metrics from free ranging lizards across an entire reproductive season.
4. In the lab, females increased relative effort per offspring as the reproductive season progressed; smaller eggs were laid earlier, largest eggs were laid later. Moreover, we observed significant among-individual variation in seasonal changes in reproduction, which is necessary for traits to evolve via natural selection. Because these patterns consistently emerge under controlled lab conditions, they likely represent an intrinsic and potentially adaptive adjustment of reproductive effort as predicted by life history theory.
5. The field study revealed similar trends, further suggesting that intrinsic patterns observed in the lab are strong enough to persist despite the environmental variability that characterizes natural habitats. The observed patterns are indicative of an adaptive seasonal shift in parental investment in response to a deteriorating offspring environment: allocating greater resources to late-produced offspring likely enhances maternal fitness.Ye
Physiological consequences of an altered flow regime on Alabama bass (Micropterus henshalli)
Get PDFThere are numerous studies on the effects of dams on aquatic biota, yet relatively little is known about whether hydropeaking activities cause physiological change in fish. Using Alabama bass (Micropterus henshalli) as a model, we evaluated whether hydropeaking in a regulated river altered glucocorticoid stress responsiveness relative to fish from an unregulated tributary. Blood samples were collected at the time of capture (baseline) and then collected again after a 1-hr period of confinement (response). Leukocyte profiles (blood smears) were created and plasma was extracted to assess plasma cortisol levels and neutrophils and lymphocyte (N:L) ratios, between sites and times to evaluate differences between sites and the two sampling periods. Baseline cortisol levels were higher in fish collected from the regulated river compared to those from unregulated site, but response levels of cortisol were similar between sites. Baseline and response level N:L ratios did not differ between sites. High baseline levels of cortisol suggested that fish exposed to regulated flows expressed an altered stress response and were likely in an allostatic state, i.e., attempting to acclimate. Further research is needed to understand how altered stress responses due to hydropeaking flows may be affecting fish.Ye
Beginning the Bicentenary: Why Geneva?
No full textArticle from the George Eliot Review, digitized and hosted by the George Eliot Review Online.Publishe