Getting excited: Challenges in quantum-classical studies of excitons in polymeric systems

A combination of classical molecular dynamics (MM/MD) and quantum chemical calculations based on the density functional theory (DFT) was performed to describe conformational properties of diphenylethyne (DPE), methylated-DPE and poly para phenylene ethynylene (PPE). DFT calculations were employed to improve and develop force field parameters for MM/MD simulations. Many-body Green's functions theory within the GW approximation and the Bethe-Salpeter equation were utilized to describe excited states of the systems. Reliability of the excitation energies based on the MM/MD conformations was examined and compared to the excitation energies from DFT conformations. The results show an overall agreement between the optical excitations based on MM/MD conformations and DFT conformations. This allows for calculation of excitation energies based on MM/MD conformations

The effect of the stochasticity of photoionization on 3D streamer simulations

Positive streamer discharges require a source of free electrons ahead of them for their growth. In air, these electrons are typically provided by photoionization. Here we investigate how stochastic fluctuations due to the discreteness of ionizing photons affect positive streamers in air. We simulate positive streamers between two planar electrodes with a 3D plasma fluid model, using both a stochastic and a continuum method for photoionization. With stochastic photoionization, fluctuations are visible in the streamer's direction, maximal electric field, velocity, and electron density. The streamers do not branch, and we find good agreement between the averaged stochastic results and the results with continuum photoionization. The streamers stay roughly axisymmetric, and we show that results obtained with an axisymmetric model indeed agree well with the 3D results. However, we find that positive streamers are sensitive to the amount of photoionization. When the amount of photoionization is doubled, there is even better agreement between the stochastic and continuum results, but with half the amount of photoionization, stochastic fluctuations become more important and streamer branching starts to occur

Effect of oxidation on POPC lipid bilayers:Anionic carboxyl group plays a major role

Phospholipids with unsaturated acyl chains are major targets of reactive oxygen species leading to formation of oxidized lipids. Oxidized phospholipids have a pronounced role in cell membrane damage. We investigated the effect of oxidation on physiological properties of phospholipid bilayers using atomistic molecular dynamics simulations. We studied phospholipid bilayer systems of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and its two stable oxidized products, 1-palmitoyl-2-(9′-oxo-nonanoyl)-sn-glycero-3-phosphocholine (PoxnoPC) and 1-palmitoyl-2-azelaoyl-sn-glycero-3-phosphocholine (PazePC). Structural properties of the POPC lipid bilayer upon the addition of PoxnoPC or PazePC with concentration ranging from 10% to 30% were described. The key finding is that PazePC lipids bend their polar tails toward the bilayer-water interface whereas PoxnoPC lipids orient their tail toward the bilayer interior. The bilayer thickness decreases such that the thickness reduction in bilayers containing PazePC is stronger than in bilayers containing PoxnoPC. The average area per lipid decreases with a stronger effect in bilayers containing PoxnoPC. The addition of PoxnoPC makes both POPC acyl chains slightly more ordered whereas the addition of PazePC reduces the order in the two POPC acyl chains. These structural changes lead to an enhancement in the permeabilities of the bilayers containing these two oxidized products depending on the type, and the amount of oxidation. This enhancement can be achieved with a lower concentration of PazePC (10% or 15%), whereas a higher concentration of PoxnoPC (20%) is required to achieve an apparent enhancement in permeability. While the permeability of bilayers containing PazePC is higher than bilayers containing PoxnoPC in the 10-20% concentration range, by increasing the concentration of the oxidized products to higher than 20%, permeability of the bilayers containing PazePC is reduced such that it is slightly smaller than those containing PoxnoPC.</p

Electrically isolated propagating streamer heads formed by strong electron attachment

Streamer discharges occur in the early stages of electric breakdown of gases in lightning, as well as in plasma and high voltage technology. They are growing filaments characterized by a curved charge layer at their tip that enhances the electric field ahead of them. In this study, we analyze the effect of strong electron attachment on the propagation of positive streamers. Strong attachment occurs in insulating gases like sulphur hexafluoride (SF6) or in air at increased density. We use the classical fluid approximation with photo-ionization for streamers in ambient air, and we artificially increase the electron attachment rate where the field is below the breakdown value. This modification approximates air pressures above 1 bar at room temperature. We find that the streamer head can keep propagating even though the ionized channel loses its conductivity closely behind the head; hence, even if it is electrically isolated. We describe how, depending on the attachment rate, the streamer propagation in a constant electric field can be accelerating, uniformly translating, or stagnating

Simulations of positive streamers in air in different electric fields: Steady motion of solitary streamer heads and the stability field

We simulate and characterize positive streamers in ambient air in homogeneous background electric fields from 4.5 to 26 kV/cm in a 4 cm gap. They can accelerate or decelerate depending on the background electric field. Many experiments have shown that a streamer keeps propagating in a stable manner in the so-called stability field of 4.5 to 5 kV/cm. Our fluid streamer simulations in STP air show that: (1) In a homogeneous field larger than 4.675 kV/cm, a single streamer accelerates, and in a lower field, it decelerates and eventually stagnates with a small radius and very high field enhancement. (2) In a field of 4.675 kV/cm, the streamer head propagates with an approximately constant velocity of 6.7 x 104 m/s and an optical radius of 55 μm over distances of several centimeters as a stable coherent structure. These values for the radius and velocity agree well with measurements of so-called minimal streamers. (3) Behind the uniformly translating streamer head, the channel conductivity decreases due to electron attachment and recombination, and the electric field returns to its background value about 1 cm behind the head. The propagation behavior of the solitary streamer agrees with the original definition of the stability field, which is the homogeneous field in which a streamer can propagate with a constant speed and shape

Simulations of positive streamers in air in different electric fields: Steady motion of solitary streamer heads and the stability field

We simulate and characterize positive streamers in ambient air in homogeneous background electric fields from 4.5 to 26 kV/cm in a 4 cm gap. They can accelerate or decelerate depending on the background electric field. Many experiments have shown that a streamer keeps propagating in a stable manner in the so-called stability field of 4.5 to 5 kV/cm. Our fluid streamer simulations in STP air show that: (1) In a homogeneous field larger than 4.675 kV/cm, a single streamer accelerates, and in a lower field, it decelerates and eventually stagnates with a small radius and very high field enhancement. (2) In a field of 4.675 kV/cm, the streamer head propagates with an approximately constant velocity of 6.7 x 104 m/s and an optical radius of 55 μm over distances of several centimeters as a stable coherent structure. These values for the radius and velocity agree well with measurements of so-called minimal streamers. (3) Behind the uniformly translating streamer head, the channel conductivity decreases due to electron attachment, and the electric field returns to its background value about 1 cm behind the head. The propaga