Structuring and sampling complex conformation space: Weighted ensemble dynamics simulations

Based on multiple simulation trajectories, which started from dispersively selected initial conformations, the weighted ensemble dynamics method is designed to robustly and systematically explore the hierarchical structure of complex conformational space through the spectral analysis of the variance-covariance matrix of trajectory-mapped vectors. Non-degenerate ground state of the matrix directly predicts the ergodicity of simulation data. The ground state could be adopted as statistical weights of trajectories to correctly reconstruct the equilibrium properties, even though each trajectory only explores part of the conformational space. Otherwise, the degree of degeneracy simply gives the number of meta-stable states of the system under the time scale of individual trajectory. Manipulation on the eigenvectors leads to the classification of trajectories into non-transition ones within the states and transition ones between them. The transition states may also be predicted without a priori knowledge of the system. We demonstrate the application of the general method both to the system with a one-dimensional glassy potential and with the one of alanine dipeptide in explicit solvent.Comment: 13 pages, 7 figures. Phys Rev E 2009 (in press

Regional Price Differences in Urban China 1986-2001: Estimation and Implication

Despite the intensive efforts made by economists to examine regional income inequality in China, limited attention has been paid to disentangle the contribution of regional price differentials. This paper examines regional price differential in urban China over the period 1986 to 2001. Spatial Price Index (SPI) is normally calculated using the Basket Cost Method, which defines a national basket and measures price variation of this common basket across different regions. The weakness of this method is that it arbitrarily assumes consumers’ preferences and has a strong reliance on good regional level price data, which are often not available. This paper adopts the Engel’s curve approach to estimate a Spatial Price Index for different provinces. The SPI obtained from the Engel’s curve approach indicates larger regional price variations than those obtained from the Basket Cost method. Further, regional price variations in urban China increased significantly during the late 1980s to early 1990s, stabilized at a relatively high level during the mid to end 1990s. Adjusting for the regional price variations our finding suggests that regional income inequality increased the most between the late 1980s and early 1990s, and stabilized in the mid 1990s, which contradicts previous findings using unadjusted income.spatial price index, Engel’s curve, income inequality, China

Diffusion Models for Double-ended Queues with Renewal Arrival Processes

We study a double-ended queue where buyers and sellers arrive to conduct trades. When there is a pair of buyer and seller in the system, they immediately transact a trade and leave. Thus there cannot be non-zero number of buyers and sellers simultaneously in the system. We assume that sellers and buyers arrive at the system according to independent renewal processes, and they would leave the system after independent exponential patience times. We establish fluid and diffusion approximations for the queue length process under a suitable asymptotic regime. The fluid limit is the solution of an ordinary differential equation, and the diffusion limit is a time-inhomogeneous asymmetric Ornstein-Uhlenbeck process (O-U process). A heavy traffic analysis is also developed, and the diffusion limit in the stronger heavy traffic regime is a time-homogeneous asymmetric O-U process. The limiting distributions of both diffusion limits are obtained. We also show the interchange of the heavy traffic and steady state limits

Manipulating thermal conductivity through substrate coupling

We report a new approach to the thermal conductivity manipulation -- substrate coupling. Generally, the phonon scattering with substrates can decrease the thermal conductivity, as observed in recent experiments. However, we find that at certain regions, the coupling to substrates can increase the thermal conductivity due to a reduction of anharmonic phonon scattering induced by shift of the phonon band to the low wave vector. In this way, the thermal conductivity can be efficiently manipulated via coupling to different substrates, without changing or destroying the material structures. This idea is demonstrated by calculating the thermal conductivity of modified double-walled carbon nanotubes and also by the ice nanotubes coupled within carbon nanotubes.Comment: 5 figure

Does having a preschool teacher with a bachelor's degree matter for children's development outcomes?

As part of the complex but intriguing question of what defines a highly qualified early childhood teacher (Kagan, Kauerz, & Tarrant, 2008), there has been a heated policy debate over whether to make a bachelor's degree (B.A.) the minimum education requirement for preschools' lead teachers in publicly funded programs (Zigler, Gilliam, & Barnett, 2011). A mixed and non-causal research base of the effect of a B.A. on preschool-teacher performance and child development outcomes is a partial source of the controversy (Kelley & Camilli, 2007; Early et al., 2007). Particularly, no experimental or quasi-experimental studies have been conducted for this topic (Barnett, 2011b). To fulfill the need for better causal inference, this dissertation first uses a nationally representative sample of American children born in 2001 who attended a preschool in 2005, to estimate the effect of having a lead teacher with a B.A. in preschool on the children's development outcomes assessed at aged 4, based on data from the Early Childhood Longitudinal Survey, Birth Cohort (ECLS-B). The term preschool is an umbrella term for all types of center-based programs. Based on three rigorous quantitative methods, including ordinary least squares with rich controls (OLS with rich controls) and two quasi-experimental methods (propensity score matching (PSM) and instrumental variables (IV)), this study finds: (1) In the model of OLS with rich controls, four of the eight comprehensive child development outcome constructs at age 4 are affected by teachers' B.A. status. Children with B.A. teachers are shown to exhibit higher early reading and math skills and fewer parent-reported internalizing behavior problems than children with non-B.A. teachers. No effects are found for story-telling skills, color recognition, parent-reported externalizing behavior problems or approaches to learning skills. Yet the children in the treatment group are reported by parents to have lower social competence. In PSM, B.A. positively predicts math skills and negatively affects social competence. In the IV estimates, a B.A. effect is only found for reducing parent-reported externalizing behavior. Comparatively, the PSM and IV estimates tend to be less statistically significant than the OLS estimates. This difference may be attributed to either bias or heterogeneity, given that the PSM and IV estimate may have removed some endogeneity of the treatment in a better way than OLS but they cannot represent the whole sample---the PSM estimate is for those matched and the IV estimate is only local to compliers. Further, when comparing teachers who have just a B.A. (as opposed to a B.A. or higher) with teachers who have an associate's degree (A.A.), the B.A. is found to have fewer statistically significant effects in the model of OLS with rich controls. Significant effects are found for two outcomes: Having a teacher with a B.A. increases math skills and reduces internalizing behavior problems. (2) There has not been much evidence of differential effects by preschool type, and the B.A. effects are no larger for children from low socioeconomic status (SES) families. Neither does the specialized education in early childhood education (ECE), as measured by whether a teacher has a degree in ECE or a related field and the number of college courses in ECE, interplay with the B.A. effect. (3) The supplemental analysis that uses two steps regression to link B.A., teacher-child interactions and child outcomes also returns some interesting findings. The treatment B.A. is found to increase the frequency of several classroom activities and the quality of teacher-child interactions (i.e., being more sensitive, less harsh, less detached and less permissive); but the two steps of the analysis only provide slight evidence for the mediating role of teacher-child interactions. Overall, there is some positive evidence of B.A. effects on children's early reading, math, the reduction of parent-reported internalizing behavior problems, the reduction of internalizing behavior problems and positive teacher behavior for the center-attending children in the ECLS-B dataset. Still, the evidence is not very strong given the inconsistency of findings across models and the negative effect of B.A. on parent-reported social competence. Such findings identified by rigorous methods in this study speak directly to the B.A. debate by adding a new piece of empirical information for a new generation of children and teachers; it adds some positive evidence to the pro side. Still, for future research and practice that aim to elevate quality, a full picture of cost-effectiveness and cost-benefit of the B.A. threshold policy is recommended and other teacher quality components should be considered

Primordial Non-Gaussianity from LAMOST Surveys

The primordial non-Gaussianity (PNG) in matter density perturbation is a very powerful probe of the physics of the very early Universe. The local PNG can induce a distinct scale-dependent bias on the large scale structure distribution of galaxies and quasars, which could be used for constraining it. We study the detection limits on PNG from the surveys of the LAMOST telescope. The cases of the main galaxy survey, the luminous red galaxy (LRG) survey, and the quasar survey of different magnitude limits are considered. We find that the MAIN1 sample (i.e. the main galaxy survey with one magnitude deeper than the SDSS main galaxy survey, or r<18.8) could only provide very weak constraint on PNG. For the MAIN2 sample (r<19.8) and the LRG survey, the 2\sigma (95.5%) limit on the PNG parameter f_{NL} are |f_{NL}|<145 and |f_{NL}|<114 respectively, comparable to the current limit from cosmic microwave background (CMB) data. The quasar survey could provide much more stringent constraint, and we find that the 2\sigma limit for |f_{NL}| is between 50 and 103, depending on the magnitude limit of the survey. With Planck-like priors on cosmological parameters, the quasar survey with g<21.65 would improve the constraints to |f_{NL}|<43 (2\sigma). We also discuss the possibility of further tightening the constraint by using the relative bias method proposed by Seljak(2008).Comment: 8 pages, 2 figures, RAA accepte