267 research outputs found
High-pressure Debye-Waller and Grueneisen parameters of Au and Cu
The lattice vibrations are determined in the quasi-harmonic approximation for
elemental Au and Cu to twice their normal density by first-principles
electronic band-structure calculations. It is found for these materials that
the important moments of the phonon density of states can be obtained to high
accuracy from short-ranged force constant models. We discuss the implications
for the Grueneisen parameters on the basis of calculated phonon moments and
their approximations by using bulk moduli and Debye-Waller factors.Comment: 4 pages, 2 figures to appear in the proceedings of the 13th APS
Topical Conference on Shock Compression of Condensed Matter (scheduled for
April 2004
Phase Transitions in High Purity Zr Under Dynamic Compression
We present results from ramp compression experiments on high-purity Zr that
show the , , as well as
reverse phase transitions. Simulations with a
multi-phase equation of state and phenomenological kinetic model match the
experimental wave profiles well. While the dynamic
transition occurs GPa above the equilibrium phase boundary, the
transition occurs within 0.9~GPa of equilibrium. We
estimate that the dynamic compression path intersects the equilibrium line at GPa, and K. The thermodynamic path in the
interior of the sample lies K above the isentrope at the point of
the transition. Approximately half of this
dissipative temperature rise is due to plastic work, and half is due to the
non-equilibrium transition. The inferred rate of
the transition is several orders of magnitude
higher than that measured in dynamic diamond anvil cell (DDAC) experiments in
an overlapping pressure range. We discuss a model for the influence of shear
stress on the nucleation rate. The small fractional volume change at the transition amplifies the effect
of shear stress, and we estimate that for this case shear stress is equivalent
to a pressure increase in the range of several GPa. Correcting our transition
rate to a hydrostatic rate brings it approximately into line with the DDAC
results, suggesting that shear stress plays a significant role in the
transformation rate
Thermal Stabilization of the HCP Phase in Titanium
We have used a tight-binding model that is fit to first-principles
electronic-structure calculations for titanium to calculate quasi-harmonic
phonons and the Gibbs free energy of the hexagonal close-packed (hcp) and omega
crystal structures. We show that the true zero-temperature ground-state is the
omega structure, although this has never been observed experimentally at normal
pressure, and that it is the entropy from the thermal population of phonon
states which stabilizes the hcp structure at room temperature. We present the
first completely theoretical prediction of the temperature- and
pressure-dependence of the hcp-omega phase transformation and show that it is
in good agreement with experiment. The quasi-harmonic approximation fails to
adequately treat the bcc phase because the zero-temperature phonons of this
structure are not all stable
Electron correlation in C_(4N+2) carbon rings: aromatic vs. dimerized structures
The electronic structure of C_(4N+2) carbon rings exhibits competing
many-body effects of Huckel aromaticity, second-order Jahn-Teller and Peierls
instability at large sizes. This leads to possible ground state structures with
aromatic, bond angle or bond length alternated geometry. Highly accurate
quantum Monte Carlo results indicate the existence of a crossover between C_10
and C_14 from bond angle to bond length alternation. The aromatic isomer is
always a transition state. The driving mechanism is the second-order
Jahn-Teller effect which keeps the gap open at all sizes.Comment: Submitted for publication: 4 pages, 3 figures. Corrected figure
Electron affinities of the first- and second- row atoms: benchmark ab initio and density functional calculations
A benchmark ab initio and density functional (DFT) study has been carried out
on the electron affinities of the first- and second-row atoms. The ab initio
study involves basis sets of and quality, extrapolations to
the 1-particle basis set limit, and a combination of the CCSD(T), CCSDT, and
full CI electron correlation methods. Scalar relativistic and spin-orbit
coupling effects were taken into account. On average, the best ab initio
results agree to better than 0.001 eV with the most recent experimental
results. Correcting for imperfections in the CCSD(T) method improves the mean
absolute error by an order of magnitude, while for accurate results on the
second-row atoms inclusion of relativistic corrections is essential. The latter
are significantly overestimated at the SCF level; for accurate spin-orbit
splitting constants of second-row atoms inclusion of (2s,2p) correlation is
essential. In the DFT calculations it is found that results for the 1st-row
atoms are very sensitive to the exchange functional, while those for second-row
atoms are rather more sensitive to the correlation functional. While the LYP
correlation functional works best for first-row atoms, its PW91 counterpart
appears to be preferable for second-row atoms. Among ``pure DFT'' (nonhybrid)
functionals, G96PW91 (Gill 1996 exchange combined with Perdew-Wang 1991
correlation) puts in the best overall performance. The best results overall are
obtained with the 1-parameter hybrid modified Perdew-Wang (mPW1) exchange
functionals of Adamo and Barone [J. Chem. Phys. {\bf 108}, 664 (1998)], with
mPW1LYP yielding the best results for first-row, and mPW1PW91 for second-row
atoms. Indications exist that a hybrid of the type mPW1LYP +
mPW1PW91 yields better results than either of the constituent functionals.Comment: Phys. Rev. A, in press (revised version, review of issues concerning
DFT and electron affinities added
Successful control of a hospital-wide outbreak of OXA-48 producing Enterobacteriaceae in the Netherlands, 2009 to 2011
On 31 May 2011, after notification of Klebsiella pneumoniae(KP)(OXA-48);(CTX-M-15) in two patients, nosocomial transmission was suspected in a Dutch hospital. Hospital-wide infection control measures and an outbreak investigation were initiated. A total of 72,147 patients were categorised into groups based on risk of OXA-48 colonisation or infection, and 7,527 were screened for Enterobacteriaceae(OXA-48) by polymerase chain reaction (PCR). Stored KP isolates (n=408) were retrospectively tested for OXA-48 and CTX-M-1 group extended-spectrum beta-lactamases (ESBL). 285 KP isolates from retrospective and prospective patient screening were genotyped by amplified fragment length polymorphism (AFLP). 41 isolates harbouring different Enterobacteriaceae species were analysed by plasmid multilocus sequence typing (pMLST). No nosocomial transmission of Enterobacteriaceae(OXA-48) was detected after 18 July 2011. Enterobacteriaceae(OXA-48) were found in 118 patients (KP (n=99), Escherichia coli (n=56), >= 1 Enterobacteriaceae(OXA-48) species (n=52)),of whom 21 had clinical infections. 39/41 (95%) of OXA-48 containing plasmids were identical in pMLST. Minimum inhibitory concentrations (MICs) of KPOXA-48 and E. coli(OXA-48) for imipenem and meropenem ranged from = 16 mg/L, and 153/157 (97%) had MIC >0.25mg/L for ertapenem. AFLP identified a cluster of 203 genetically linked isolates (62 KPOXA-48;(CTX-M15); 107 KPCTX-M-15; 34 KPOXA-48). The 'oldest' KPCTX-M-15 and KPOXA-48 clonal types originated from February 2009 and September 2010, respectively. The last presumed outbreak-related KPOXA-48 was detected in April 2012. Uncontrolled transmission of KP (CTX-M-15) evolved into a nosocomial outbreak of KPOXA-48; CTX-M15 with large phenotypical heterogeneity. Although the outbreak was successfully controlled, the contribution of individual containment measures and of the hospital relocating into a new building just before outbreak notification was impossible to quantify
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