Model of a fluid at small and large length scales and the hydrophobic effect

We present a statistical field theory to describe large length scale effects induced by solutes in a cold and otherwise placid liquid. The theory divides space into a cubic grid of cells. The side length of each cell is of the order of the bulk correlation length of the bulk liquid. Large length scale states of the cells are specified with an Ising variable. Finer length scale effects are described with a Gaussian field, with mean and variance affected by both the large length scale field and by the constraints imposed by solutes. In the absence of solutes and corresponding constraints, integration over the Gaussian field yields an effective lattice gas Hamiltonian for the large length scale field. In the presence of solutes, the integration adds additional terms to this Hamiltonian. We identify these terms analytically. They can provoke large length scale effects, such as the formation of interfaces and depletion layers. We apply our theory to compute the reversible work to form a bubble in liquid water, as a function of the bubble radius. Comparison with molecular simulation results for the same function indicates that the theory is reasonably accurate. Importantly, simulating the large length scale field involves binary arithmetic only. It thus provides a computationally convenient scheme to incorporate explicit solvent dynamics and structure in simulation studies of large molecular assemblies

Relating Molecular Morphology to Charge Mobility in Semicrystalline Conjugated Polymers

The molecular-level origins of the effects of annealing temperature and molecular weight on hole mobility in P3HT are elucidated using coarse-grained molecular dynamics, quantum chemical calculations, and kinetic Monte Carlo charge-transport simulations on a variety of realistic thin-film morphologies. The zero-field hole mobility is shown to increase as the annealing temperature or average molecular weights of samples are increased, in accordance with experimental results. Crystal structure analysis shows that the annealing temperature dependence of the mobility can be attributed to the size and structural order of the crystallites in both the chain-backbone and π-stacking directions. However, the molecular weight dependence of the mobility cannot be rationalized in the same way. Longer chains are shown to belong to more crystallites in the morphology, suggesting that the crystals become better connected as the molecular weight of the sample increases. We show that engineering samples to have an increased fraction of these long “tie chains” within the morphology improves mobility. As such, we propose that crystal connectivity in the noncrystalline portions of the morphology is similarly important in determining carrier mobility as crystallite size and order for semicrystalline conjugated polymers

Risk factors for recurrent C lostridium difficile infection in hematopoietic stem cell transplant recipients

Background Recurrent C lostridium difficile infection ( CDI ) represents a significant burden on the healthcare system and is associated with poor outcomes in hematopoietic stem cell transplant ( HSCT ) patients. Data are limited evaluating recurrence rates and risk factors for recurrence in HSCT patients. Methods HSCT patients who developed CDI between January 2010 and December 2012 were divided into 2 groups: non‐recurrent CDI (nr CDI ) and recurrent CDI ( rCDI ). Risk factors for rCDI were compared between groups. Rate of recurrence in HSCT patients was compared to that in other hospitalized patients. Results CDI was diagnosed in 95 of 711 HSCT patients (22 rCDI and 73 nr CDI ). Recurrence rates were similar in HSCT patients compared with other hospitalized patients (23.2% vs. 22.9%, P  > 0.99). Patients in the rCDI group developed the index case of CDI significantly earlier than the nr CDI group (3.5 days vs. 7.0 days after transplant, P  = 0.05). On univariate analysis, patients with rCDI were more likely to have prior history of CDI and neutropenia at the time of the index CDI case. Neutropenia at the time of the index CDI case was the only independent predictor of rCDI (78.8 vs. 34.8%, P  = 0.006) on multivariate analysis. Conclusions The rate of rCDI was similar between HSCT and other hospitalized patients, and the majority of patients developed the index case of CDI within a week of transplantation. Neutropenia at the index CDI case may be associated with increased rates of rCDI .Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/109272/1/tid12267.pd

Electronic damping of molecular motion at metal surfaces

A method for the calculation of the damping rate due to electron-hole pair excitation for atomic and molecular motion at metal surfaces is presented. The theoretical basis is provided by Time Dependent Density Functional Theory (TDDFT) in the quasi-static limit and calculations are performed within a standard plane-wave, pseudopotential framework. The artificial periodicity introduced by using a super-cell geometry is removed to derive results for the motion of an isolated atom or molecule, rather than for the coherent motion of an ordered over-layer. The algorithm is implemented in parallel, distributed across both ${\bf k}$ and ${\bf g}$ space, and in a form compatible with the CASTEP code. Test results for the damping of the motion of hydrogen atoms above the Cu(111) surface are presented.Comment: 10 pages, 3 figure

Cavity formation and the drying transition in the Lennard-Jones fluid

By simulation and theory, we study the probability of observing N molecular centers within molecular sized volumes for a Lennard-Jones fluid near liquid-vapor coexistence. For large volumes and small N, the probability distribution differs markedly from Gaussian. The free energy per unit surface area to form empty volumes (i.e., cavities) is a rapidly varying function of the radius for small cavities. It becomes constant for large volumes. The source of these behaviors is the occurrence of drying (i.e., solvent depletion) at the cavity surface. The crossover to drying occurs on microscopic length scales, with significant density depletion found for cavities with radii of the order of two or more Lennard-Jones diameters. Reasonable agreement is found between the simulation results and the theory developed by Lum, Chandler, and Weeks [J. Phys. Chem. B 103, 4570 (1999)].David M. Huang and David Chandle

Calculating transition dipole moments of phosphorescent emitters for efficient organic light-emitting diodes

The out-coupling of light from an organic light-emitting diode, and thus its efficiency, strongly depends on the orientation of the transition dipole moment (TDM) of the emitting molecules with respect to the substrate surface. Despite the importance of this quantity, theoretical investigations of the direction of the TDM of phosphorescent emitters based on iridium(iii) complexes remain limited. One challenge is to find an appropriate level of theory able to accurately predict the direction of the TDM. Here, we report relativistic time-dependent density functional theory (TDDFT) calculations of the TDM, emission energies and lifetimes for both the ground-state (S0) and triplet (T1) excited-state geometries of fac-tris(2-phenylpyridyl)iridium(iii) (Ir(ppy)3), using the two-component zero-order regular approximation (ZORA) or including spin-orbit coupling (SOC) perturbatively using the simpler one-component (scalar) formulation. We show that the one- and two-component approaches give similar emission energies and overall radiative lifetimes for each individual geometry. Use of the S0 geometry leads to two of the excited triplet substates being degenerate, with the degeneracy lifted for the T1 geometry, with the latter matching experiment. Two-component calculations using the T1 geometry give results for the direction of the TDM more consistent with experiment than calculations using the S0 geometry. Finally, we show that adding a dielectric medium does not affect the direction of TDM significantly, but leads to better agreement with the experimentally measured radiative lifetime.Mohammad Babazadeh, Paul L. Burn and David M. Huan

Instabilities in a Two-Component, Species Conserving Condensate

We consider a system of two species of bosons of equal mass, with interactions $U^{a}(|x|)$ and $U^{x}(|x|)$ for bosons of the same and different species respectively. We present a rigorous proof -- valid when the Hamiltonian does not include a species switching term -- showing that, when $U^{x}(|x|)>U^{a}(|x|)$, the ground state is fully "polarized" (consists of atoms of one kind only). In the unpolarized phase the low energy excitation spectrum corresponds to two linearly dispersing modes that are even a nd odd under species exchange. The polarization instability is signaled by the vani shing of the velocity of the odd modes.Comment: To appear in Phys. Rev.

Plans for laser spectroscopy of trapped cold hydrogen-like HCI

Laser spectroscopy studies are being prepared to measure the 1s ground state hyperfine splitting in trapped cold highly charged ions. The purpose of such experiments is to test quantum electrodynamics in the strong electric field regime. These experiments form part of the HITRAP project at GSI. A brief review of the planned experiments is presented.Comment: 4 pages, 4 figures, accepted for publication (NIMB

Plans for laser spectroscopy of trapped cold hydrogen-like HCI

Laser spectroscopy studies are being prepared to measure the 1s ground state hyperfine splitting in trapped cold highly charged ions. The purpose of such experiments is to test quantum electrodynamics in the strong electric field regime. These experiments form part of the HITRAP project at GSI. A brief review of the planned experiments is presented.Comment: 4 pages, 4 figures, accepted for publication (NIMB