Geometry optimization in the presence of external forces: a theoretical model for enforced structural changes in molecules

2010 ◽  
Vol 108 (14) ◽  
pp. 1845-1856 ◽  
Author(s):  
Krzysztof Wolinski ◽  
Jon Baker
1972 ◽  
Vol 2 (1) ◽  
pp. 13-22 ◽  
Author(s):  
H. Behm ◽  
H. Gutiérrez ◽  
M. Requena

The relationship of changing demographic characteristics to health status and medical care in Latin America is examined in this article. The rapid demographic growth, high birth and death rates, although with a downward trend, the excessive expansion of the large cities, and the dispersion of the rural population are correlated with the unsatisfactory levels of health and living of the masses of the population, lack of medical care whose quality varies according to social class, and the limitations of the health systems. In the light of this analysis the authors discuss some of the tasks and prospects of the health sector. They contend that the situation described is basically due to inefficient social, economic, and political structures and an unhealthy dependence on external forces. Only radical structural changes will enable millions of Latin Americans to really have access to medical care, health, and life itself, an access which is their inalienable right and which today is denied them.


2012 ◽  
Vol 134 (7) ◽  
Author(s):  
Jagir R. Hussan ◽  
Mark L. Trew ◽  
Peter J. Hunter

A theoretical model of the cross-linking topology of ventricular muscle tissue is developed. Using parameter estimation the terms of the theoretical model are estimated for normal and pathological conditions. The model represents the anisotropic structure of the tissue, reproduces published experimental data and characterizes the role of different tissue components in the observed macroscopic behavior. Changes in the material parameters are consistent with expected structural changes and the model is extended to reproduce force-Calcium relationships. Model results are invoked to argue that semisoft behavior and the material axis anisotropy arise from the constraints on the extracellular matrix cross-linking topology.


2019 ◽  
Author(s):  
◽  
Whitney Annette Goodwin ◽  

Prehispanic northeastern Honduran communities were situated at the border between southeastern Mesoamerica and lower Central America. Previous studies of pottery style suggest that local groups shifted their affiliation from north to south at the end of the Classic period (ca. AD 1000). This study examines the contexts in which pottery, as a medium for style, was used, and how the food people prepared, stored, or served in these vessels offers a perspective complementary to pottery style for understanding how identity was actively negotiated in this region. In this view, other parts of the foodways system – the foods chosen to be processed or cooked in pottery, the particular methods of preparation or serving – can also have their own form of style that has the potential to be as important in materializing identities as the designs on pottery vessels. Excavation at the Selin Farm site documented shell midden mounds containing large deposits of shell, pottery, and other materials disposed of as part of feasting events that took place throughout the Selin Period (AD 300-1000). These stratified deposits are the result of repeated consumption and disposal practices that represent groups of people that came together to form a community of consumption in the past. Data from excavation, lithic and faunal analyses, and typological, morphological, and residue analyses of pottery point to variation in the form, content, and motivations behind these events over space and time. By tracing the nature and scale of these feasting events over time and space at Selin Farm, this study provides data critical to situating the processes behind identity negotiation at the local level and tying the micropolitics of individual events to broader social and political changes in the region. The timing of changes in local pottery styles and foodways suggests they occurred partly as a result of interaction with groups to the north and south that both spoke to cultural understandings and similarities while also highlighting differences and reinforcing boundaries. However, variation in feasting practices across contexts at Selin Farm demonstrates, for the first time, internal heterogeneity within a northeastern community that helps explain processes of change without relying exclusively on external forces, while also not denying their influence in shaping local change. The study of identity negotiation at Selin Farm demonstrates that aggrandizers, expansionist chiefdoms, or outside influences were not responsible for cultural change in the small-scale societies of Central America. The people who lived and feasted at the site were not passive recipients of innovations from the north or the south. There were complex internal social and political strategies being employed by individuals and groups that led to the structural changes that took place in the region. Through interaction with each other and with outside groups they were continually guiding the formation, maintenance, and transformation of group identity through the manipulation of shared practices and everyday activities, punctuated by the feasting events described here.


2010 ◽  
Vol 132 (15) ◽  
pp. 154903 ◽  
Author(s):  
Gerald Johannes Schneider ◽  
Dietmar Göritz

2015 ◽  
Vol 26 (15) ◽  
pp. 2727-2732 ◽  
Author(s):  
Peter-G. Barr-Gillespie

The hair bundle—the sensory organelle of inner-ear hair cells of vertebrates—exemplifies the ability of a cell to assemble complex, elegant structures. Proper construction of the bundle is required for proper mechanotransduction in response to external forces and to transmit information about sound and movement. Bundles contain tightly controlled numbers of actin-filled stereocilia, which are arranged in defined rows of precise heights. Indeed, many deafness mutations that disable hair-cell cytoskeletal proteins also disrupt bundles. Bundle assembly is a tractable problem in molecular and cellular systems biology; the sequence of structural changes in stereocilia is known, and a modest number of proteins may be involved.


Author(s):  
Juan C. Botero ◽  
Massimiliano Gobbi ◽  
Giampiero Mastinu

In this paper a new theoretical model to estimate the transmitted force in a snow-chain safety device is presented. Starting with a detailed analysis of the significant external forces acting on the system, the mathematical model is developed using some basic concepts of the contact mechanics theory. A MATLAB® code was developed in order to perform numerical simulations and experimental tests were carried out to validate the model. The results obtained show that for certain conditions of the driving surface and the tire's tread the force transmitted along the chain can be several times the longitudinal traction force applied to the tire itself. The importance of the interaction between the blocks on the tire and the chain segments is discussed. Some conclusions and recommendations are made in order to improve the design process of this kind of devices.


Soft Matter ◽  
2018 ◽  
Vol 14 (30) ◽  
pp. 6264-6277 ◽  
Author(s):  
A. Brzyska ◽  
W. Płaziński ◽  
K. Woliński

In this work we use the Enforced Geometry Optimization (EGO) approach to simulate force-induced structural changes in the monomer, and di- up to pentameric oligomers of neutral and non-sulfated carrageenan based oligosaccharides.


2020 ◽  
Author(s):  
Tim Stauch

<div> <div> <div> <p>A novel mechanochemical method for the simulation of molecules and molecular crystals under hydrostatic pressure, the eXtended Hydrostatic Compression Force Field (X-HCFF) approach, is introduced. In contrast to comparable methods, the desired pressure can be adjusted non-iteratively and molecules of general shape retain chemically reasonable geometries even at high pressures. The implementation of the X-HCFF approach is straightforward and the computational cost is practically the same as for a regular geometry optimization. Pressure can be applied by using any desired electronic structure method for which a nuclear gradient is available. The results of X-HCFF for pressure-dependent intramolecular structural changes in the investigated molecules and molecular crystals as well as a simple pressure-induced dimerization reaction are chemically intuitive and fall within the range of other established computational methods. Experimental spectroscopic data of a molecular crystal under pressure are reproduced accurately. </p> </div> </div> </div>


Author(s):  
K. Essalah ◽  
M. A. K. Sanhoury ◽  
M. T. Ben Dhia ◽  
M. R. Khaddar

Beryllium complexes of the types [BeCl2L2] (L = (Me2N)3P(O) (1), (Me2N)2P(O)F (2), Me2NP(O)F2 (3) and P(O)F3 (4)) have been theoretically studied by means of DFT geometry optimization and NMR chemical shift calculations (B3LYP/6-31G(d)). A good correlation was found between calculated and experimental data for complex 2. On going from complex 1 to 4, the Be-L bond underwent considerable lengthening, while that of Be-Cl was shortened (Be-O: 1.646 in 1 vs. 1.740 A° in 4; Be-Cl: 2.043 in 1 vs. 1.953 A° in 4). In the same way, the Be-O-P bond angle was found to decrease from 135° for 1 to 124° for 4. The trends are in good agreement with the calculated metal-ligand binding energies of complexes 1-4. Interestingly, the structural changes are accompanied by increased 9Be chemical shifts towards higher frequencies as the Me2N groups in the ligand are substituted by fluorine atoms. The results were compared to corresponding complexes with tin (IV) chloride, [SnCl4L2]. The theoretical data showed that the use of the 6-31G* basis set could efficiently predict the 9Be NMR chemical shifts in the complexes [BeCl2L2].


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