18 research outputs found
Understanding high pressure hydrogen with a hierarchical machine-learned potential
The hydrogen phase diagram has a number of unusual features which are
generally well reproduced by density functional calculations. Unfortunately,
these calculations fail to provide good physical insights into why those
features occur. In this paper, we parameterize a model potential for molecular
hydrogen which permits long and large simulations. The model shows excellent
reproduction of the phase diagram, including the broken-symmetry Phase II, an
efficiently-packed phase III and the maximum in the melt curve. It also gives
an excellent reproduction of the vibrational frequencies, including the maximum
in the vibrational frequency and negative thermal expansion. By
detailed study of lengthy molecular dynamics, we give intuitive explanations
for observed and calculated properties. All solid structures approximate to
hexagonal close packed, with symmetry broken by molecular orientation. At high
pressure, Phase I shows significant short-ranged correlations between molecular
orientations. The turnover in Raman frequency is due to increased coupling
between neighboring molecules, rather than weakening of the bond. The liquid is
denser than the close-packed solid because, at molecular separations below
2.3\AA, the favoured relative orientation switches from
quadrupole-energy-minimising to steric-repulsion-minimising. The latter allows
molecules to get closer together, without atoms getting closer but this cannot
be achieved within the constraints of a close-packed layer
Consensus statement on abusive head trauma in infants and young children
Abusive head trauma (AHT) is the leading cause of fatal head injuries in children younger than 2 years. A multidisciplinary team bases this diagnosis on history, physical examination, imaging and laboratory findings. Because the etiology of the injury is multifactorial (shaking, shaking and impact, impact, etc.) the current best and inclusive term is AHT. There is no controversy concerning the medical validity of the existence of AHT, with multiple components including subdural hematoma, intracranial and spinal changes, complex retinal hemorrhages, and rib and other fractures that are inconsistent with the provided mechanism of trauma. The workup must exclude medical diseases that can mimic AHT. However, the courtroom has become a forum for speculative theories that cannot be reconciled with generally accepted medical literature. There is no reliable medical evidence that the following processes are causative in the constellation of injuries of AHT: cerebral sinovenous thrombosis, hypoxic-ischemic injury, lumbar puncture or dysphagic choking/vomiting. There is no substantiation, at a time remote from birth, that an asymptomatic birth-related subdural hemorrhage can result in rebleeding and sudden collapse. Further, a diagnosis of AHT is a medical conclusion, not a legal determination of the intent of the perpetrator or a diagnosis of murder. We hope that this consensus document reduces confusion by recommending to judges and jurors the tools necessary to distinguish genuine evidence-based opinions of the relevant medical community from legal arguments or etiological speculations that are unwarranted by the clinical findings, medical evidence and evidence-based literature
Nuclear Spin Crossover in Dense Molecular Hydrogen
The laws of quantum mechanics are often tested against the behaviour of the
lightest element in the periodic table, hydrogen. One of the most striking
properties of molecular hydrogen is the coupling between molecular rotational
properties and nuclear spin orientations, giving rise to the spin isomers
ortho- and para-hydrogen. At high pressure, as intermolecular interactions
increase significantly, the free rotation of H2 molecules is increasingly
hindered, and consequently a modification of the coupling between molecular
rotational properties and the nuclear spin system can be anticipated. To date,
high-pressure experimental methods have not been able to observe nuclear spin
states at pressures approaching 100 GPa and consequently the effect of high
pressure on the nuclear spin statistics could not be directly measured. Here,
we present in-situ high-pressure nuclear magnetic resonance data on molecular
hydrogen in its hexagonal phase I up to 123 GPa at room temperature. While our
measurements confirm the presence of I=1 ortho-hydrogen at low pressures, above
70 GPa, where inter- and intramolecular distances become comparable, we observe
a crossover in the nuclear spin statistics from a spin-1 quadrupolar to a
spin-1/2 dipolar system, evidencing the loss of spin isomer distinction. These
observations represent a unique case of a nuclear spin crossover phenomenon in
quantum solids