Home About us Contact
|
Home About us Contact | ||
|
|
||
Gaussian Functions (gaussian + function)
Selected AbstractsTHE FITNESS EFFECT OF MUTATIONS ACROSS ENVIRONMENTS: A SURVEY IN LIGHT OF FITNESS LANDSCAPE MODELSEVOLUTION, Issue 12 2006Guillaume Martin Abstract The fitness effects of mutations on a given genotype are rarely constant across environments to which this genotype is more or less adapted, that is, between more or less stressful conditions. This can have important implications, especially on the evolution of ecological specialization. Stress is thought to increase the variance of mutations' fitness effects, their average, or the number of expressed mutations. Although empirical evidence is available for these three mechanisms, their relative magnitude is poorly understood. In this paper, we propose a simple approach to discriminate between these mechanisms, using a survey of empirical measures of mutation effects in contrasted environments. This survey, across various species and environments, shows that stress mainly increases the variance of mutations effects on fitness, with a much more limited impact on their average effect or on the number of expressed mutations. This pattern is consistent with a simple model in which fitness is a Gaussian function of phenotypes around an environmentally determined optimum. These results suggest that a simple, mathematically tractable landscape model may not be quantitatively as unrealistic as previously suggested. They also suggest that mutation parameter estimates may be strongly biased when measured in stressful environments. [source] Mixing of components in two-component aggregationAICHE JOURNAL, Issue 9 2006Themis Matsoukas Abstract The problem of binary component aggregation with kernels that are independent of composition is considered. The bivariate distribution as the product of two distributions is studied, one that refers to the size of the aggregates, and one that describes the distribution of the component of interest (solute), and obtain the governing equations for all three. The distribution of solute within aggregates of size v has a steady-state solution, that is independent of the size distribution: it is a Gaussian function whose mean and variance are both proportional to the aggregate size v. To quantify the degree of blending, the sum-square X2, of the deviation of the amount solute from its mean, is studied. Two cases are identified for which X2 is constant during aggregation: (a) "partially mixed" seeds regardless of kernel; and (b) sum-type kernels regardless of seed distribution. Simulations confirm the results for these two cases, and further indicate that in the general case, X2 is nearly constant. The degree of mixing is determined solely by the initial distribution of components, but does not depend on the kernel. Optimum initial conditions that minimize the time required to reach a desired level of homogeneity between components are identified. © 2006 American Institute of Chemical Engineers AIChE J, 2006 [source] Omni-directional gap of 1-D photonic crystals based on porous silicon with a Gaussian profile refractive indexPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 2 2007J. Arriaga Abstract Using the transfer matrix method we calculate the omni-directional band gap of a 1-D photonic crystal consisting of alternating layers of two dielectric materials A and B with refractive index nA and nB, respectively. The refractive index of layer A is constant and the refractive index of layer B varies according to the envelope of a Gaussian function. We find that under certain circumstances it is possible to obtain 100% reflectivity for both polarizations and any value of the incident angle of the electromagnetic waves. Although the structure considered does not posses a higher omni-directional band gap than the periodic sequence of low and high constant refractive indexes, it can be used to produce a new type of omni-directional mirrors without abrupt interfaces. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] A comparison of label-based review and ALE meta-analysis in the Stroop taskHUMAN BRAIN MAPPING, Issue 1 2005Angela R. Laird Abstract Meta-analysis is an important tool for interpreting results of functional neuroimaging studies and is highly influential in predicting and testing new outcomes. Although traditional label-based review can be used to search for agreement across multiple studies, a new function-location meta-analysis technique called activation likelihood estimation (ALE) offers great improvements over conventional methods. In ALE, reported foci are modeled as Gaussian functions and pooled to create a statistical whole-brain image. ALE meta-analysis and the label-based review were used to investigate the Stroop task in normal subjects, a paradigm known for its effect of producing conflict and response inhibition due to subjects' tendency to perform word reading as opposed to color naming. Both methods yielded similar activation patterns that were dominated by response in the anterior cingulate and the inferior frontal gyrus. ALE showed greater involvement of the anterior cingulate as compared to that in the label-based technique; however, this was likely due to the increased spatial level of distinction allowed with the ALE method. With ALE, further analysis of the anterior cingulate revealed evidence for somatotopic mapping within the rostral and caudal cingulate zones, an issue that has been the source of some conflict in previous reviews of the anterior cingulate cortex. Hum Brain Mapp 25:6,21, 2005. © 2005 Wiley-Liss, Inc. [source] Polarized basis sets for accurate calculations of static and dynamic electric properties of moleculesJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 3 2010Angelika Baranowska Abstract We report on the development and testing of large polarized basis sets (LPolX, where X is the element symbol) for accurate calculations of linear and nonlinear electric properties of molecules. The method used to generate LPolX sets is based on our studies of the analytic dependence of Gaussian functions on external time-independent and time-dependent electric fields. At variance with the earlier investigations of small, highly compact (ZPolX) basis sets for moderately accurate calculations of electric properties of large molecules, the present goal is to obtain basis sets that are nearly saturated with respect to the selected class of electric properties and can be used for accurate studies of interaction-induced properties. This saturation makes the LPolX sets also useful in calculations of optical properties for chiral molecules. In this article, the LPolX sets are generated for X = H, C, N, O, and F, and examined in calculations of linear and nonlinear electric properties of four standard test systems: HF, N2, CO, and HCN. The study of the performance of LPolX basis sets has been carried out at different levels of approximation ranging from the SCF HF method to highly correlated CCSD(T) approach. The results obtained in this study compare favorably with accurate reference data and show a high level of saturation of LPolX basis sets with respect to the polarization effect due to external electric fields. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010 [source] Half-numerical evaluation of pseudopotential integralsJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 9 2006Roberto Flores-Moreno Abstract A half-numeric algorithm for the evaluation of effective core potential integrals over Cartesian Gaussian functions is described. Local and semilocal integrals are separated into two-dimensional angular and one-dimensional radial integrals. The angular integrals are evaluated analytically using a general approach that has no limitation for the l -quantum number. The radial integrals are calculated by an adaptive one-dimensional numerical quadrature. For the semilocal radial part a pretabulation scheme is used. This pretabulation simplifies the handling of radial integrals, makes their calculation much faster, and allows their easy reuse for different integrals within a given shell combination. The implementation of this new algorithm is described and its performance is analyzed. © 2006 Wiley Periodicals, Inc. J Comput Chem 27: 1009,1019, 2006 [source] Computational methods for studies of multiexciton complexes,PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 15 2006T. Vänskä Abstract Powerful computational methods are presented for studies of energy levels, photon-recombination rates, and phonon-relaxation rates of neutral and charged multiexciton complexes at correlated levels of theory. The electron,hole system is described by a two-band effective-mass Hamiltonian. The one-particle functions are expanded in a basis set consisting of anisotropic Gaussian functions. The many-body Hamiltonian constructed in the space of the antisymmetric products of one-particle functions is diagonalized using general coupled-cluster and configuration-interaction methods. The expansion coefficients of the coupled-cluster and configuration-interaction wave functions are obtained by solving the corresponding equations using direct iterative algorithms. We demonstrate the potential of the computational approaches by calculating total energies of multiexciton complexes at coupled-cluster and configuration-interaction levels. Computational methods for studies of radiative recombination and phonon-relaxation rates have also been developed and results are reported for radiative recombination rates and recombination energies of the exciton, biexciton, and of the positive and the negative trions confined in a InGaAs/GaAs quantum-dot sample. Phonon-relaxation rates have been calculated for a few low-lying ,g states of the exciton complex of the same quantum-dot sample. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] UV Spectra and Excitation Delocalization in DNA: Influence of the Spectral WidthCHEMPHYSCHEM, Issue 7 2005Emanuela Emanuele Abstract The singlet excited states of the model DNA duplex (dA)10.(dT)10 are studied. Calculations are performed in the exciton theory framework. Molecular dynamics calculations provide the duplex geometry. The dipolar coupling is determined using atomic transition charges. The monomer transition energies are simulated by Gaussian functions resembling the absorption bands of nucleosides in aqueous solutions. Most of the excited states are found to be delocalized over at least two bases and result from the mixing of different monomer states. Their properties are only weakly affected by conformational changes of the double helix. On average, the highest oscillator strength is carried by the upper eigenstates. The duplex absorption spectra are shifted a few nanometers to higher energies with respect to the spectra of noninteracting monomers. The states with larger spatial extent are located close to the maximum of the absorption spectrum. [source] | ||||||||||||||||