Effects Of Nearest Neighbor Interactions Research Articles

Determination of Young's Modulus of Active Pharmaceutical Ingredients by Relaxation Dynamics at Elevated Pressures.

A new approach is theoretically proposed to study the glass transition of active pharmaceutical ingredients and a glass-forming anisotropic molecular liquid at high pressures. We describe amorphous materials as a fluid of hard spheres. Effects of nearest neighbor interactions and cooperative motions of particles on glassy dynamics are quantified through a local and collective elastic barrier calculated using the elastically collective nonlinear Langevin equation theory. Inserting two barriers into Kramer's theory gives the structural relaxation time. Then, we formulate a new mapping based on the thermal expansion process under pressure to intercorrelate particle density, temperature, and pressure. This analysis allows us to determine the pressure and temperature dependence of α relaxation. From this, we estimate the effective elastic modulus of amorphous materials and capture the effects of conformation on the relaxation process. Remarkably, our theoretical results agree well with experiments.

Charge order and antiferromagnetism in the extended Hubbard model

We study the extended Hubbard model on a two-dimensional half-filled square lattice using the dynamical cluster approximation. We present results on the phase boundaries between the paramagnetic metallic (normal) state and the insulating antiferromagnetic state, as well as between the antiferromagnetic and charge order states. We find hysteresis along the antiferromagnet/charge order and normal/charge order phase boundaries (at larger values of the on-site interaction), indicating first order phase transitions. We show that nearest neighbor interactions lower the critical temperature for the antiferromagnetic phase. We also present results for the effect of nearest neighbor interactions on the antiferromagnetic phase boundary and for the evolution of spectral functions and energetics across the phase transitions.

Time-reversal symmetry breaking phase in the Hubbard model: A variational cluster approach study

We study the stability of the time-reversal symmetry breaking staggered-flux phase of a single band Hubbard model, within the variational cluster approach. For intermediate and small values of the interaction $U$, we find metastable solutions for the staggered-flux phase, with a maximum current per bond at $U\ensuremath{\approx}3.2$. However, allowing for antiferromagnetic and superconducting long-range order it turns out that in the region at and close to half filling the antiferromagnetic phase is the most favorable energetically. The effect of nearest-neighbor interaction is also considered. Our results show that a negative nearest-neighbor interaction and finite doping favors the stability of the staggered-flux phase. We also present preliminary results for the three-band Hubbard model obtained with a restricted set of variational parameters. For this case, no spontaneous time-reversal symmetry breaking phase is found in our calculations.

COORDINATION IN THE DYNAMICS OF THE BRAIN AS INFERRED FROM EEG ANALYSIS

The present study observed the coordination between cortical areas during no task conditions as well as for the pathological condition of epilepsy, by application of the phase synchronization technique to the EEG signal in a multichannel recording. The index obtained from the phase entrainment investigation was properly scaled by a novel method to take into account the effect of nearest neighbor interactions. This scaled index was analyzed temporally to learn about the behavior of regional interactions in time. The results obtained not only corroborate earlier known results, but also give deeper insight into actual brain functioning.

Fabric development with nearest‐neighbor interaction and dynamic recrystallization

Polycrystals undergoing ductile deformation develop lattice‐preferred orientation (fabric) as a result of intracrystalline slip. A consequence of fabric development is that bulk physical properties become anisotropic. Fabric development and macroscopic deformation are studied by examining three effects: nearest‐neighbor interaction (NNI) among crystals, polygonization, and migration recrystallization. The effects of NNI are modeled by arranging the crystals on a three‐dimensional cubic grid and assigning six neighbors to each crystal. The “strength” of interaction can vary from no interaction (homogeneous stress) to “strong” interaction (significant stress redistribution). Increasing the NNI leads to a more homogeneous strain. Fabric varies in both strength and symmetry at a given bulk strain, depending on the strength of NNI. For a prescribed fabric the strain rate increases as the NNI increases. Recrystallization is modeled from energy balance considerations, and polygonization is formulated in terms of stress differences. Both processes require knowledge of dislocation density, which is calculated in the model as a function of crystal strain and grain size, both of which vary with time. Models of fabric development in ice that include NNI lead to more realistic fabric evolution than models with homogeneous stress. However, available data on fabric evolution are inadequate to determine quantitatively the strength of NNI acting in ice.

Direct laser fabrication of dense microlens arrays in semiconductor-doped glass

A direct laser writing technique in semiconductor-doped glasses is used to fabricate microlens arrays with fill factors exceeding the close-packing limit for circular microlenses. This is achieved by both structural and thermal interactions between adjacent lenses during fabrication, which result in the formation of microlenses shaped in accordance with the translational symmetry of the array. The effect of nearest-neighbor interaction on various time scales is studied and shown to be critical to the distortions observed.

Analysis of TPD Spectra on Semiconductor Surfaces by Monte Carlo Simulations

ABSTRACTThis work aims to provide a systematic approach for deriving desorption mechanisms on semiconductor surfaces from a coupling of temperature programmed desorption (TPD) experiments and Monte Carlo (MC) simulations. MC simulations are used to evaluate different desorption schemes to identify mechanisms consistent with experimental results. Effects of nearest neighbor interactions, island formation, and surface reconstructions are quantified through the simulations. Methyl desorption from Ga-rich GaAs is used as a case study to illustrate the MC procedure for interpretation of TPD spectra.

Influence of 5'-nearest neighbors on the insertion kinetics of the fluorescent nucleotide analog 2-aminopurine by Klenow fragment.

The effects of nearest neighbor interactions between a nucleotide base at the primer 3'-terminus and an incoming deoxyribonucleoside triphosphate on DNA polymerase catalyzed insertion were examined. Kinetics of inserting the fluorescent nucleotide analog 2-aminopurine deoxyribonucleotide (dAPMP) and dAMP opposite a template T by 3'-->5' exonuclease-deficient mutants of Klenow fragment (KF-) were measured on primer/templates of identical sequence except for the base pair at the 3'-primer terminus. In addition to its fluorescence properties, 2-aminopurine (AP) is an attractive probe because it is misinserted opposite T by polymerases at much higher frequencies than natural nucleotides. Misinsertion frequencies for AP are on the same order of magnitude as variations in misinsertion frequencies due to changes in local DNA sequence, which makes the statistical significance of these variations easier to document. We have established that changes in the fluorescence of AP can be used to follow the insertion of dAPMP on both steady-state and pre-steady-state time scales. Rates of insertion of dAPMP measured by fluorescence and by a polyacrylamide gel assay were similar and are sensitive to the identity of the base at the 3'-primer twice as fast as insertion following a primer terminus T. The difference in rates arises primarily from differences in kcat values, which were fastest next to G and slowest next to T, while apparent Km values were similar next to each of the 4 different nearest neighbors. The gel assay was used to measure AP misinsertion efficiencies by two methods: (1) by having dAPTP and dATP directly compete for insertion opposite T in the same reaction and (2) by measuring Vmax/Km values for each substrate in separate reactions. The results from the direct competition and separate kinetics measurements are similar. The misinsertion efficiency of dAPMP relative to dAMP opposite a template T was significantly higher next to a 3'-primer terminus G (f(ins) = 0.31 +/- 0.06) than next to T (f(ins) = 0.15 +/- 0.03) for the KF- single mutant (D42A). The corresponding misinsertion efficiencies next to a 3'-primer terminus G and T were 0.20 +/- 0.02 and 0.16, respectively, for the KF- double mutant (D355A, E357A). Relative rates of insertion of dAPMP and dAMP correlate with melting temperatures calculated for nearest neighbor doublets which reflect the relative base-stacking energies. In addition to changes in insertion kinetics, polymerase-DNA dissociation rates varied with the identity of the 3'-primer terminus, differing by as much as 7-20-fold depending on the polymerase and the primer/template.

Monte carlo simulations of carbon monoxide oxidation on Pt catalysts

This communication presents results of Monte Carlo (MC) simulations of the CO oxidation reaction on a flat Pt surface. Several algorithms were considered to account for the effects of nearest neighbor interactions on the probabilities of adsorption, desorption and surface reaction. The results from the various algorithms are compared with those reported in the literature as well as with the classical Langmuir-Hinshelwood (LH) model. The results indicate that the MC simulations are sensitive to the algorithms accounting for nearest neighbor interactions. The MC calculations predict the formation of clusters of CO which confines the reaction to the cluster perimeters, effectively decreasing the reactive surface area. Conditions that increase the probability of cluster formation lead to discrepancies between MC and LH model predictions and vice versa. Agreement between MC and LH calculations are obtained only on randomizing the surface after each MC iteration or when high CO desorption probabilities minimize cluster formation.

Monte Carlo simulations of carbon monoxide oxidation on Pt catalysts

This communication presents results of Monte Carlo (MC) simulations of the CO oxidation reaction on a flat Pt surface. Several algorithms were considered to account for the effects of nearest neighbor interactions on the probabilities of adsorption, desorption and surface reaction. The results from the various algorithms are compared with those reported in the literature as well as with the classical Langmuir-Hinshelwood (LH) model. The results indicate that the MC simulations are sensitive to the algorithms accounting for nearest neighbor interactions. The MC calculations predict the formation of clusters of CO which confines the reaction to the cluster perimeters, effectively decreasing the reactive surface area. Conditions that increase the probability of cluster formation lead to discrepancies between MC and LH model predictions and vice versa. Agreement between MC and LH calculations are obtained only on randomizing the surface after each MC iteration or when high CO desorption probabilities minimize cluster formation.

Effect of nearest-neighbor interactions on surface titrations

By considering the discreteness of surface groups and using either the random-mixing or quasichemical approximations for mixing ionized and un-ionized groups, analytical expressions are presented for the apparent dissociation constant in terms of nearest-neighbor interaction energies. These expressions are exact at zero and one degree of dissociation. A smeared surface charge model or a model calculating Δp K using only the electrostatic potential at a given charge due to the electrostatic potential of all the other charges is insufficient to explain the difference observed in the p K at a surface and an analogous nonaggregating molecule in bulk solution for titrations of dodecylamineoxide micelles and phosphatidic acid vesicles in the literature. In particular for a neutralized surface or when the electrostatic potentials cancel, there is still a discrepancy in the p K's even if image forces are considered. For dodecylamineoxide, assuming only a charge-charge interaction in the nearest-neighbor model is still inconsistent with the data, while an additional attractive interaction such as hydrogen bonding is consistent.

The effects of nearest neighbor interactions on the evaporation rate of cloud particles

A modified images method is presented and used to determine the vapor density field in arrays of up to nine “quasi-stationary” evaporating particles. The evaporation rate of each particle in an array is calculated as a function of the particle separation. Correction factors to Maxwell's evaporation rate expression to account for nearest neighbor interactions are proposed. The “R 2-law” is assessed and found to be only approximately correct when applied to interacting particles.