Determination of the Maier-Saupe strength parameter from dielectric relaxation experiments: a molecular dynamics simulation study

Molecular dynamics simulations have been performed to investigate the rotational motion in the nematic and isotropic phases of a model mesogenic system in which the interactions between the molecules are represented by the Gay-Berne potential. First-rank end-over-end rotational relaxation times, analogous to those measured using dielectric relaxation spectroscopy for real mesogens with a longitudinal electric dipole, have been determined as a function of temperature and density. The relaxation times at temperatures throughout the nematic region are found to be larger than the values extrapolated from the isotropic phase to the same temperature. The simulation results are compared with the extended Debye theory for dielectric relaxation in the nematic phase. This relates the reduction in the relaxation rate to the retardation factor which depends on the Maier-Saupe strength parameter, and in turn is defined uniquely by the second-rank orientational order parameter. The simulations indicate that the retardation factor at constant strength parameter is density dependent, a feature neglected in the relaxation theory. We compare the simulation results where possible with experiment.     

HOW BIG AND HOW CLOSE? HABITAT PATCH SIZE AND SPACING TO CONSERVE A THREATENED SPECIES

Abstract. We present results of a spatially explicit, individual‐based stochastic dispersal model (HexSim) to evaluate effects of size and spacing of patches of habitat of Northern Spotted Owls (NSO; Strix occidentalis caurina) in Pacific Northwest, USA, to help advise recovery planning efforts. We modeled 31 artificial landscape scenarios representing combinations of NSO habitat cluster size (range 4–49 NSO pairs per cluster) and edge‐to‐edge cluster spacing (range 7–101 km), and an all‐habitat landscape. We ran scenarios using empirical estimates of NSO dispersal dynamics and distances and stage class vital rates (representing current population declines) and under adult survival rates adjusted to achieve an initially stationary population. Results suggested that long‐term (100‐yr) habitat occupancy rates are significantly higher with habitat clusters supporting ≥25 NSO pairs and ≤15 km spacing, and with overall landscapes of ≥35–40% habitat. Although habitat provision is key to NSO recovery, no habitat configuration provided for long‐term population persistence when coupled with currently observed vital rates. Results also suggested a key role of floaters (unpaired, nonterritorial, dispersing owls) in recolonizing vacant habitat, and that the floater population segment becomes increasingly depleted with greater population declines. We suggest additional areas of modeling research on this and other threatened species.     

Limits of the validity of the mass ratio independence of the Stokes—Einstein relation: molecular dynamics calculations and comparison with the Enskog theory

Detailed molecular dynamics simulations have been carried out to investigate the mass ratio dependence of the tracer self-diffusion coefficient D ₂ in binary, atomic Lennard-Jones mixtures as a function of density n and length diameter ratio σ₂₂/σ₁₁. For a compact representation of our results an exponential approach is employed. The results are compared with the Stokes-Einstein relation, which predicts no mass ratio dependence, and the Enskog theory. The validity ranges regarding the mass ratio dependence for Enskog and Stokes-Einstein like behaviour are given as well as the mass ratio dependence in the crossover regions between these two cases. For length parameter ratios σ₂₂/₁₁ <1, the Stokes-Einstein prediction is not valid for mass ratios 1/16≤m₂/m₁ ≤50. For length parameter ratios σ₂₂/σ₁₁>2 and for mass ratios m₂/m₁ < 1, the Stokes-Einstein regime (regarding the mass ratio dependence) is reached for smaller densities than for the same system but m₂/m₁ >1.     

Deuteron NMR study of dynamics and order in the columnar phase of a fluorinated discotic liquid crystal

Measurements have been made of deuteron quadrupolar splitting and spin—lattice relaxation times T _1Q and T _1z in the columnar phase of a ring deuterated 1-fluoro-2,3,6,7,10,11-hexahex-yloxytriphenylene (F-HAT6) as a function of temperature and at two different Larmor frequencies. The disc-like molecules are stacked, with only short range positional order, into columns which are arranged on a 2-dimensional hexagonal lattice. To describe small step reorientations of these molecules, a rotational diffusion model which uses a space-fixed frame to diagonalize the molecular diffusion tensor is adopted. The principal diffusion constants in this so called ‘anisotropic viscosity model’ Dα and Dβ are for rotations of a molecule around and perpendicular to the columnar axis, respectively. A global target analysis of the spectral densities J ₁(ω₀) and J ₂(2ω₀) at seven temperatures and two Larmor frequencies in a single minimization procedure was carried out. It was found that Dα Dβ, which is consistent with the picture that the motion towards or away from the local director (i.e., the columnar axis) tends to disrupt the packing of molecules within the column. Furthermore, the activation energies Eαβ and Eβ_a for these motions were found to be 67.7 kJmol^?1 and 56.3kJmol^?1. That these values are higher in comparison with those for HAT6 is interesting, and believed to be a result of a strong lateral dipole in F-HAT6.     

Comparison of the Morse and Deng-Fan potentials for X-H bonds in small molecules

We use the Morse and the Deng-Fan potentials to treat the X-H stretching motion in small molecules. The Deng-Fan potential is qualitatively similar to the Morse potential but has the correct asymptotic behaviour as the internuclear distance approaches 0. We use the Deng-Fan potential to calculate transition frequencies and intensities of overtones of X-H stretching vibrations. Parameters for the Morse oscillator can readily be found from a Birge-Sponer fit of experimental transition frequencies. We describe a procedure to find the parameters for the Deng-Fan oscillator. We compare the potentials, wavefunctions, and matrix elements of the Morse and the Deng-Fan oscillator models. In the examples considered the Deng-Fan potential does not predict observed energy levels and intensities significantly better than the Morse potential despite its correct asymptotic behaviour.     

Substituent effects on gas-phase acidities of formic acid and its silicon and sulphur derivatives R − M(= X)XH(M = C., Si; X = O,S; R = H,F, CI,OH, NH2 and CH3)

Ab initio molecular orbital methods at the CBS-Q level of theory have been used to study the effect of substituent (F, Cl, NH₂, OH and CH₃) on the gas-phase acidities of formic acid, HCOOH, its silicon and sulphur derivatives R-M(= X)XH(M = C., Si; X = 0, S; R = F, Cl, OH, NH₂ and CH₃). For formic acid and its thio and dithio derivatives the acidity changes upon substitution are irregular and depend on both the type of substituent, position and degree of replacement of oxygen atoms by sulphur atoms. For sila carboxylic acids and their thio and dithio derivatives the calculated acidities regularly increase in the order: R-SiOOH < R-Si(=S)OH ? R-Si(=O)SH < R-SiSSH(R = H, F, Cl, OH, NH₂ and CH₃). The chloro derivatives are the strongest among the acids studied. The highest gas phase acidity (1277.6 kJmol^?1) has been calculated for ClC(=S)OH.     

An integral equation and Monte Carlo study of homo- and hetero- nuclear square-well diatomic fluids

Square-well homo-nuclear and hetero-nuclear diatomic fluids are studied using the Ornstein-Zernike equation and the recently proposed RHNC-VM closure. Monte Carlo canonical simulations have been performed to complete recent literature simulation data. The integral equation thermodynamic and structural results are compared with these and literature simulation data at three elongations over a large range of densities and temperatures. The RHNC-VM theory agrees excellently with the simulation thermodynamic and structural results. Its accuracy revealed slight errors in simulation data in work by Lisal and Nezbeda [1999, Molec. Phys., 96, 335]. The data have been re-simulated.     

FISHING YIELD,CURVATURE AND SPATIAL BEHAVIOR: IMPLICATIONS FOR MODELING MARINE RESERVES

ABSTRACT. Given a paucity of empirical data, policymakers are forced to rely on modeling to assess potential impacts of creating marine reserves to manage fisheries. Many modeling studies of reserves conclude that fishing yield will increase (or decrease only modestly) after creating a reserve in a heavily exploited fishery. However, much of the marine reserves modeling ignores the spatial heterogeneity of fishing behavior. Contrary to empirical findings in fisheries science and economics, most models assume explicitly or implicitly that fishing effort is distributed uniformly over space. This paper demonstrates that by ignoring this heterogeneity, yield‐per‐recruit models systematically overstate the yield gains (or understate the losses) from creating a reserve in a heavily exploited fishery. Conversely, at very low levels of exploitation, models that ignore heterogeneous fishing effort overstate the fishing yield losses from creating a reserve. Starting with a standard yield‐per‐recruit model, the paper derives a yield surface that maps spatially differentiated fishing effort into total long‐run fishing yield. It is the curvature of this surface that accounts for why the spatial distribution of fishing effort so greatly affects predicted changes from forming a reserve. The results apply generally to any model in which the long‐run fishing yield has similar curvature to a two‐patch Beverton‐Holt model. A simulation of marine reserve formation in the California red sea urchin fishery with Beverton‐Holt recruitment, eleven patches, and common larval pool dispersal dynamics reinforces these results.     

Analytical second derivatives for excited electronic states using the single excitation configuration interaction method: theory and application to benzo[a]pyrene and chalcone

A compact formulation and a semi-direct implementation are described of analytical second derivatives of the single excitation configuration interaction (CIS) energy. CPU time, memory usage and disc storage all scale with the same power of system size as the ground state Hartree—Fock (HF) method, with a coefficient only 2 to 3 times larger. Thus usually analytical excited state CIS frequencies are feasible when HF frequencies are feasible. As a computational example, analytical CIS/3-21G frequencies for benzo[a]pyrene are calculated more efficiently than with finite differences. As a chemical application, analytical CIS/6-31G^* frequencies are used to investigate non-planarity in the lowest π → π excited state of transchalcone.     

Optimal overhaul intervals with imperfect inspection and repair

Author for correspondence.Email:m.j.newby{at}city.ac.uk This paper is motivated by the idea of a maintenance-free operatingperiod whose objectives are to improve mission reliability andcarry out as much maintenance as possible as a second-line activity.The system may be in one of three states (good, faulty, andfailed), and expressions are developed for the average costper unit time until failure. The system is periodically inspected,the inspection being imperfect in the sense that it can resultin both false-positive and false-negative results. Simple faultscan be fixed, but a repair is imperfect, in that there is anon-zero probability of a fault remaining after a repair. Aftera fixed number of inspections, the system is overhauled. Ifthe system fails during operation, it is replaced at increasedcost. The sojourn time in each state has non-constant failurerate, and discretization and supplementary variables are usedto give a Markovian structure which allows easy computationof the average costs. Minimizing the average cost gives theoptimal number of inspections before overhauling the system.