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Prediction of a new potential high-pressure structure of FeSiO
We predict a new candidate high-temperature high-pressure structure of
FeSiO with space-group symmetry Cmmm by applying an evolutionary algorithm
within DFT+U that we call post-perovskite II (PPv-II). An exhaustive search
found no other competitive candidate structures with ABO composition. We
compared the X-ray diffraction (XRD) pattern of FeSiO PPv-II with
experimental results of the recently reported H-phase of (Fe,Mg)SiO. The
intensities and positions of two main X-ray diffraction peaks of PPv-II
FeSiO compare well with those of the H-phase. We also calculated the static
equation of state, the enthalpy and the bulk modulus of the PPv-II phase and
compared it with those of perovskite (Pv) and post-perovskite (PPv) phases of
FeSiO. According to the static DFT+U computations the PPv-II phase of
FeSiO is less stable than Pv and PPv phases under lower mantle pressure
conditions at 0 K and has a higher volume. PPv-II may be entropically
stabilized, and may be a stable phase in Earths lower mantle, coexisting
with -PbO (Columbite-structured) silica and perovskite, or with
magnesiowustite or ferropericlase, depending on bulk composition
Thermal effects on lattice strain in hcp Fe under pressure
We compute the c/a lattice strain versus temperature for nonmagnetic hcp iron
at high pressures using both first-principles linear response quasiharmonic
calculations based on the full potential linear-muffin-tin-orbital (LMTO)
method and the particle-in-cell (PIC) model for the vibrational partition
function using a tight-binding total-energy method. The tight-binding model
shows excellent agreement with the all-electron LMTO method. When hcp structure
is stable, the calculated geometric mean frequency and Helmholtz free energy of
hcp Fe from PIC and linear response lattice dynamics agree very well, as does
the axial ratio as a function of temperature and pressure. On-site
anharmonicity proves to be small up to the melting temperature, and PIC gives a
good estimate of its sign and magnitude. At low pressures, hcp Fe becomes
dynamically unstable at large c/a ratios, and the PIC model might fail where
the structure approaches lattice instability. The PIC approximation describes
well the vibrational behavior away from the instability, and thus is a
reasonable approach to compute high temperature properties of materials. Our
results show significant differences from earlier PIC studies, which gave much
larger axial ratio increases with increasing temperature, or reported large
differences between PIC and lattice dynamics results.Comment: 9 figure
Chemical accuracy from quantum Monte Carlo for the Benzene Dimer
We report an accurate study of interactions between Benzene molecules using
variational quantum Monte Carlo (VMC) and diffusion quantum Monte Carlo (DMC)
methods. We compare these results with density functional theory (DFT) using
different van der Waals (vdW) functionals. In our QMC calculations, we use
accurate correlated trial wave functions including three-body Jastrow factors,
and backflow transformations. We consider two benzene molecules in the parallel
displaced (PD) geometry, and find that by highly optimizing the wave function
and introducing more dynamical correlation into the wave function, we compute
the weak chemical binding energy between aromatic rings accurately. We find
optimal VMC and DMC binding energies of -2.3(4) and -2.7(3) kcal/mol,
respectively. The best estimate of the CCSD(T)/CBS limit is -2.65(2) kcal/mol
[E. Miliordos et al, J. Phys. Chem. A 118, 7568 (2014)]. Our results indicate
that QMC methods give chemical accuracy for weakly bound van der Waals
molecular interactions, comparable to results from the best quantum chemistry
methods.Comment: Accepted for publication in the Journal of Chemical Physics, Vol.
143, Issue 11, 201
Low-pressure phase diagram of crystalline benzene from quantum Monte Carlo
We study the low-pressure (0 to 10 GPa) phase diagram of crystalline benzene
using quantum Monte Carlo (QMC) and density functional theory (DFT) methods. We
consider the , , and structures as the best candidates
for phase I and phase II. We perform diffusion quantum Monte Carlo (DMC)
calculations to obtain accurate static phase diagrams as benchmarks for modern
van der Waals density functionals. We use density functional perturbation
theory to compute phonon contribution in the free-energy calculations. Our DFT
enthalpy-pressure phase diagram indicates that the and
structures are the most stable phases within the studied pressure range. The
DMC Gibbs free-energy calculations predict that the room temperature to
phase transition occurs at 2.1(1) GPa. This prediction is consistent
with available experimental results at room temperature. Our DMC calculations
show an estimate of 50.60.5 kJ/mol for crystalline benzene lattice energy
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The use of drug calendars for the diagnosis of cutaneous drug eruptions in the age of electronic medical records
A morbilliform drug eruption is the most common condition leading to a dermatology consultation for a patient in the hospital. Timing is an important diagnostic tool since the onset of a skin rash usually takes place within days-to-weeks of the start of the implicated drug. A comprehensive, thorough, and reliable drug history by the clinician is essential. Therefore, to assist in the task of determining the causative medication of a new skin rash in a hospitalized patient, the creation of a drug calendar is recommended. The development of an electronic version of the drug calendar offers several benefits over the manual version. As the use of electronic medical records continues to become the standard in medicine, the electronic drug calendar will serve as an invaluable tool for the diagnosis of drug hypersensitivity
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