53 research outputs found
Regularized Born-Oppenheimer molecular dynamics
While the treatment of conical intersections in molecular dynamics generally
requires nonadiabatic approaches, the Born-Oppenheimer adiabatic approximation
is still adopted as a valid alternative in certain circumstances. In the
context of Mead-Truhlar minimal coupling, this paper presents a new closure of
the nuclear Born-Oppenheimer equation, thereby leading to a molecular dynamics
scheme capturing geometric phase effects. Specifically, a semiclassical closure
of the nuclear Ehrenfest dynamics is obtained through a convenient prescription
for the nuclear Bohmian trajectories. The conical intersections are suitably
regularized in the resulting nuclear particle motion and the associated Lorentz
force involves a smoothened Berry curvature identifying a loop-dependent
geometric phase. In turn, this geometric phase rapidly reaches the usual
topological index as the loop expands away from the original singularity. This
feature reproduces the phenomenology appearing in recent exact nonadiabatic
studies, as shown explicitly in the Jahn-Teller problem for linear vibronic
coupling. Likewise, a newly proposed regularization of the diagonal correction
term is also shown to reproduce quite faithfully the energy surface presented
in recent nonadiabatic studies.Comment: Third version with minor changes. To appear in Phys. Rev.
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Rovibrational Dynamics of Nuclei and Molecules
We study quantized rotation-vibration dynamics with applications to nuclear and molecular models. Firstly we consider small vibrations of Skyrmions (topological solitons which model atomic nuclei), developing new approximations to their quantum energy spectra which incor- porate both rotation-vibration and isorotation-vibration corrections. We find that the forms of these corrections are highly restricted as a consequence of the large symmetry groups of Skyrmions, and we determine them using representation theory. We explore the implications for the Helium-4 nucleus and the Lithium-7/Beryllium-7 isodoublet, comparing our findings with experimental data.
We propose a model for the Carbon-12 nucleus based on point â”-particles restricted to isosceles triangular configurations, inspired by linear chain and equilateral triangular Skyrmions. The configuration space is not a manifold but has a graph-like structure, and we make use of Quantum Graph Theory to study the quantized dynamics. The resulting energy spectrum reproduces the experimental data rather well.
Nuclear physicists are interested in more than just the quantum energy spectrum: elec- tromagnetic transition rates, for instance, measure -decay between two nuclear states and can be measured in the laboratory. We develop a formalism to compute electromagnetic transition rates within rotation-vibration models and compare the results of our Carbon-12 model and a recent Oxygen-16 model to experimental data. We go on to propose some ways in which the Oxygen-16 model might be improved.
Finally, we turn from nuclear physics to molecular physics and study the protonated methane molecular ion, introducing a quantum graph model for the complex rotation-vibration dynamics. We find good agreement with other numerical work where available and compute states up to angular momentum J = 4 for the first time
Preserve Non-Stationary Long-Term Dynamics via Selected Incomplete Dual Bases
The author adopts previously developed methods of quantum propagation which
use trajectory-guided sets of Gaussian Coherent States for the use with SU(2)
Coherent States. Motivated by recent experiments, the author applies the
technique to the simulation of quantum dynamics in a chain of coupled qubits.
Because of the short time dynamics can be reproduced on a selected small basis
set of Coupled SU(2) Coherent States. To recover long-time dynamics observed in
the experiment propagation on a small localised basis is combined with
projection on an optimised static basis
Exactly solvable 1D model explains the low-energy vibrational level structure of protonated methane
A new one-dimensional model is proposed for the low-energy vibrational
quantum dynamics of CH5+ based on the motion of an effective particle confined
to a 60-vertex graph with a single edge length parameter.
Within this model, the quantum states of CH5+ are obtained in analytic form and
are related to combinatorial properties of . The bipartite
structure of gives a simple explanation for curious symmetries
observed in numerically exact variational calculations on CH5+
The bohmion method in nonadiabatic quantum hydrodynamics
Starting with the exact factorization of the molecular wavefunction, this
paper presents the results from the numerical implementation in nonadiabatic
molecular dynamics of the recently proposed bohmion method. Within the context
of quantum hydrodynamics, we introduce a regularized nuclear Bohm potential
admitting solutions comprising a train of -functions which provide a
finite-dimensional sampling of the hydrodynamic flow paths. The bohmion method
inherits all the basic conservation laws from its underlying variational
structure and captures electronic decoherence. After reviewing the general
theory, the method is applied to the well-known Tully models, which are used
here as benchmark problems. In the present case of study, we show that the new
method accurately reproduces both electronic decoherence and nuclear population
dynamics
Total Angular Momentum Conservation in Ab Initio Born-Oppenheimer Molecular Dynamics
We prove both analytically and numerically that the total angular momentum of
a molecular system undergoing adiabatic Born-Oppenheimer dynamics is conserved
only when pseudo-magnetic Berry forces are taken into account. This finding
sheds light on the nature of Berry forces for molecular systems with spin-orbit
coupling and highlights how ab initio Born-Oppenheimer molecular dynamics
simulations can successfully capture the entanglement of spin and nuclear
degrees of freedom as modulated by electronic interactions
The life cycle of the Acropora coral-eating flatworm (AEFW), Prosthiostomum acroporae; the influence of temperature and management guidelines
© The Author(s), 2019. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Barton, J. A., Hutson, K. S., Bourne, D. G., Humphrey, C., Dybala, C., & Rawlinson, K. A. The life cycle of the Acropora coral-eating flatworm (AEFW), Prosthiostomum acroporae; the influence of temperature and management guidelines. Frontiers in Marine Science, 6, (2019): 524, doi: 10.3389/fmars.2019.00524.As coral aquaculture is increasing around the world for reef restoration and trade, mitigating the impact of coral predators, pathogens and parasites is necessary for optimal growth. The Acropora coral-eating flatworm (AEFW), Prosthiostomum acroporae (Platyhelminthes: Polycladida: Prosthiostomidae) feeds on wild and cultivated Acropora species and its inadvertent introduction into reef tanks can lead to the rapid death of coral colonies. To guide the treatment of infested corals we investigated the flatwormâs life cycle parameters at a range of temperatures that represent those found in reef tanks, coral aquaculture facilities and seasonal fluctuations in the wild. We utilized P. acroporae from a long-term in vivo culture on Acropora species to examine the effects of temperature (3°C increments from 21 to 30°C) on flatworm embryonation period, hatching success, hatchling longevity, and time to sexual maturity. Our findings show that warmer seawater shortened generation times; at 27°C it took, on average, 11 days for eggs to hatch, and 35 days for flatworms to reach sexual maturity, giving a minimum generation time of 38 days, whereas at 24°C the generation time was 64 days. Warmer seawater (24â30°C) also increased egg hatching success compared to cooler conditions (21°C). These results indicate that warmer temperatures lead to higher population densities of P. acroporae. Temperature significantly increased the growth rate of P. acroporae, with individuals reaching a larger size at sexual maturity in warmer temperatures, but it did not influence hatchling longevity. Hatchlings, which can swim as well as crawl, can survive between 0.25 and 9 days in the absence of Acropora, and could therefore disperse between coral colonies and inter-connected aquaria. We used our data to predict embryonation duration and time to sexual maturity at 21â30°C, and discuss how to optimize current treatments to disrupt the flatwormâs life cycle in captivity.This study was funded by a James Cook University Development Grant, âParasite cultivation techniques: in vitro and in vivo culture methods for ecological and applied aquatic parasitology researchâ awarded to KH. Additional funding to KR and CD was raised through crowdfunding on Experiment.com (https://doi.org/10.18258/1621) and a donation from the Atlanta Reef Club, Duluth, GA, United States
First assessment of geophysical sensitivities from spaceborne Galileo and BeiDou GNSS-Reflectometry data collected by the UK TechDemoSat-1 Mission
The UKâs TechDemoSat-1 (TDS-1), launched 2014, has demonstrated the use of global positioning system (GPS) signals for monitoring ocean winds and sea ice. Here it is shown, for the first time, that Galileo and BeiDou signals detected by TDS-1 show similar promise. TDS-1 made seven raw data collections, recovering returns from Galileo and BeiDou, between November 2015 and March 2019. The retrieved open ocean delay Doppler maps (DDMs) are similar to those from GPS. Over sea ice, the Galileo DDMs show a distinctive triple peak. Analysis, adapted from that for GPS DDMs, gives Galileoâs signal-to-noise ratio (SNR), which is found to be inversely sensitive to wind speed, as for GPS. A Galileo track transiting from open ocean to sea ice shows a strong instantaneous SNR response. These results demonstrate the potential of future spaceborne constellations of GNSS-R (global navigation satellite systemâreflectometry) instruments for exploiting signals from multiple systems: GPS, Galileo, and BeiDou
Scouting for Climate Variable with Small Satellites
HydroGNSS is a small satellite mission under the new ESA Scout programme tapping into NewSpace, within ESAâs FutureEO programme. The mission will use an innovative GNSS-Reflectometry instrument to collect parameters related to the Essential Climate Variables (ECVs): soil moisture, inundation, freeze/thaw, biomass, ocean wind speed and sea ice extent. GNSS-Reflectometry is a type of bistatic radar utilizing abundant GNSS signals as signals of opportunity, empowering small satellites to provide measurement quality associated with larger satellites.
The HydroGNSS instrument introduces novel measurements compared to its predecessors on UKSA TechDemoSat-1 and NASA CYGNSS missions. These include: the acquisition of Galileo(E1) reflections, and firsts such as dual- polarization, complex âcoherent channelâ (amplitude/phase) and second frequency (L5/E5a) acquisitions. These measurements enable HydroGNSS to innovate the L2 products, e.g. improving the ground resolution and soil moisture measurement, as dual-polarized reflections allow the discrimination of vegetation effects from soil moisture.
HydroGNSS will:
â Complement and potentially gap fill other missions sensing soil moisture e.g. ESAâs SMOS and NASAâs SMAP missions.
â Complement ESAâs Biomass mission addressing coverage restrictions over Europe, North and Central America.
â Expand GNSS-Reflectometry techniques.
â Lay the foundations for a future constellation capable of offering continuity in high spatial-temporal resolution observations of the Earthâs weather and climate
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