Accuracy and Transferability in Machine Learned Potentials for Carbon

In this thesis, we discuss the approach taken to construct an accurate machine learning (ML) model for atomistic simulations of carbon, constructed using the Gaussian approximation potential (GAP) methodology. We begin by discussing the process for constructing a potential for a single phase, graphene. We then extend this to produce a general-purpose potential, named GAP-20, which describes the properties of the bulk crystalline and amorphous phases, crystal surfaces, and defect structures with a high degree of accuracy. We combine structural databases for amorphous carbon and graphene, which we extend substantially by adding suitable configurations, for example, for defects in graphene and other nanostructures. The final potential is fitted to reference data computed using the optB88-vdW density functional theory (DFT) functional. Dispersion interactions, which are crucial to describe multilayer carbonaceous materials, are therefore implicitly included. We additionally account for long-range dispersion interactions using a semianalytical two-body term and show that an improved model can be obtained through an optimization of the manybody smooth overlap of atomic positions descriptor. We rigorously test the potential on lattice parameters, bond lengths, formation energies, and phonon dispersions of numerous carbon allotropes. We compare the formation energies of an extensive set of defect structures, surfaces, and surface reconstructions to DFT reference calculations. The present work demonstrates the ability to combine, in the same ML model, the previously attained flexibility required for amorphous carbon with the high accuracy necessary for crystalline graphene which we introduce in this thesis, thereby providing an interatomic potential that will be applicable to a wide range of applications concerning diverse forms of bulk and nanostructured carbon

Birds of a Feather Talk Together: User Influence on Language Adoption

Language is in constant flux be it from changes in meaning to the introduction of new terms. At the user level it changes by users accommodating their language in relation to whom they are in contact with. By mining diffusion's of new terms across social networks we detect the influence between users and communities. This is then used to compute the user activation threshold at which they adopt new terms dependent on their neighbours. We apply this method to four different networks from two popular on-line social networks (Reddit and Twitter). This research highlights novel results: by testing the network through random shuffles we show that the time at which a user adopts a term is dependent on the local structure, however, a large part of the influence comes from the global structure and that influence between users and communities is not significantly dependent on network structures

Exploring inefficiencies in commercial real estate pricing

Thesis (S.M. in Real Estate Development)--Massachusetts Institute of Technology, Dept. of Urban Studies and Planning, 2007.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Includes bibliographical references (p. 54-55).This paper is an investigation into the relationship between real estate capitalization rates and income growth. The paper includes a cross-sectional analysis of approximately 25 markets in the United States. The paper analyzes apartment and office markets separately, for two different periods of time. Multi-variable regression techniques are used to explore the relationships between cap rates in different markets and rent growth, appreciation, and employment growth, as well as Liquidity and Supply Constraint factors. For this analysis, periods of time were chosen ranging from 1996 to 2002 so that subsequent rental growth, appreciation, and employment growth data was available. With this information, future growth of these variables is back tested. The results of the regressions are then compared to the theoretical relationships that should exist between cap rates and future income growth and appreciation. The results show that market pricing does not accurately price future income growth in a consistent manner. This provides significant evidence that the real estate capital market is inefficient in its pricing of assets in different markets based on the future rental behavior in individual markets. The last aspect of the thesis is the development of an investment strategy that capitalizes on the inefficiencies discovered in the analysis. The strategy enables the investor to make investments that should demonstrate superior risk-adjusted returns based on the space market fundamentals in individual markets.by Patrick D. Rowe.S.M.in Real Estate Developmen

Long-term vascular access ports as a means of sedative administration in a rodent fMRI survival model

The purpose of this study is to develop a rodent functional magnetic resonance imaging (fMRI) survival model with the use of heparin-coated vascular access devices. Such a model would ease the administration of sedative agents, reduce the number of animals required in survival experiments and eliminate animal-to-animal variability seen in previous designs. Seven male Sprague-Dawley rats underwent surgical placement of an MRI-compatible vascular access port, followed by implantable electrode placement on the right median nerve. Functional MRI during nerve stimulation and resting-state functional connectivity MRI (fcMRI) were performed at times 0, 2, 4, 8 and 12 weeks postoperatively using a 9.4 T scanner. Anesthesia was maintained using intravenous dexmedetomidine and reversed using atipamezole. There were no fatalities or infectious complications during this study. All vascular access ports remained patent. Blood oxygen level dependent (BOLD) activation by electrical stimulation of the median nerve using implanted electrodes was seen within the forelimb sensory region (S1FL) for all animals at all time points. The number of activated voxels decreased at time points 4 and 8 weeks, returning to a normal level at 12 weeks, which is attributed to scar tissue formation and resolution around the embedded electrode. The applications of this experiment extend far beyond the scope of peripheral nerve experimentation. These vascular access ports can be applied to any survival MRI study requiring repeated medication administration, intravenous contrast, or blood sampling

Birds of a feather talk together: user influence on language adoption

Language is in constant flux be it from changes in meaning to the introduction of new terms. At the user level it changes by users accommodating their language in relation to whom they are in contact with. By mining diffusion’s of new terms across social networks we detect the influence between users and communities. This is then used to compute the user activation threshold at which they adopt new terms dependent on their neighbours. We apply this method to four different networks from two popular on-line social networks (Reddit and Twitter). This research highlights novel results: by testing the network through random shuffles we show that the time at which a user adopts a term is dependent on the local structure, however, a large part of the influence comes from the global structure and that influence between users and communities is not significantly dependent on network structures

Sex Differences in Endurance Exercise Capacity and Skeletal Muscle Lipid Metabolism in Mice

Previous studies suggest that sex differences in lipid metabolism exist with females demonstrating a higher utilization of lipids during exercise, which is mediated partly by increased utilization of muscle triglycerides. However, whether these changes in lipid metabolism contribute directly to endurance exercise performance is unclear. Therefore, the objective of this study was to investigate the contribution of exercise substrate metabolism to sex differences in endurance exercise capacity (EEC) in mice. Male and female C57BL/6-NCrl mice were subjected to an EEC test until exhaustion on a motorized treadmill. The treadmill was set at a 10% incline, and the speed gradually increased from 10.2 m/min to 22.2 m/min at fixed intervals for up to 2.5 h. Tissues and blood were harvested in mice immediately following the EEC. A cohort of sedentary, non-exercised male and female mice were used as controls. Females outperformed males by ~25% on the EEC. Serum levels of both fatty acids and ketone bodies were ~50% higher in females at the end of the EEC. In sedentary female mice, skeletal muscle triglyceride content was significantly greater compared to sedentary males. Gene expression analysis demonstrated that genes involved in skeletal muscle fatty acid oxidation were significantly higher in females with no changes in genes associated with glucose uptake or ketone body oxidation. The findings suggest that female mice have a higher endurance exercise capacity and a greater ability to mobilize and utilize fatty acids for energy

The natural setting of Caution Bay: climate, landforms, biota, and environmental zones

In this chapter, we review the present and past environment of Caution Bay set in a broader geographical context, including both terrestrial and marine habitats. Our primary objective is to sketch the general canvas upon which the past 6,000 or so years of local human presence, as represented by the Caution Bay archaeological record, played out. A secondary objective is to document the range of contemporary landforms and explore the spatial distribution and ecological dynamics of the various plant and animal communities that still occupy the present landscape, or did so at the time when Europeans first arrived in the 1870s. Knowledge of the contemporary landscape and its resources represents the starting point for inferring continuities and changes in ways of life for the region's past inhabitants as these are tracked back from the present to the mid-Holocene, and ultimately for understanding the choices people made as they balanced various primary extractive and commercial activities to maintain cultural practices, adopt and develop new ones, survive and prosper. Relationships between people and locales at Caution Bay were, and continue to be, dynamic, with people playing a major role in shaping both the physical and biological landscape, just as the landscape and its resources have influenced the course of human history in this area