Charge density investigations of the compounds [Mg{(pz*)3C}2] ( 1 ) (pz* = 3,5‐dimethylpyrazolyl) and (Ph2P=S)C14H9 ( 2 ) show two disturbing features in each case. First, the models derived from high‐resolution data sets at different temperatures differ significantly. Additionally, residual density appears close to or even at the atomic positions, especially for data sets measured at 100 K. This indicates significant errors that could be caused by thermal diffuse scattering (TDS). A reduction of the integration box size leads to a substantial improvement in the model quality and removes the differences in the models. At the same time it indicates TDS to be the reason for these errors. However, this method is very time consuming and an alternative is needed. In endeavouring to improve the method, it was noticed that the refinement of resolution‐dependent scale factors can be employed as a validation tool to detect such errors. In a nested interval approach, a correction factor {α = a[sin(θ)/λ]2 + b[sin(θ)/λ]3} is determined that minimizes these errors and improves the model quality. This model is now based on refinement of only a single scale factor. 相似文献
Crystallographic least squares are a fundamental tool for crystal structure analysis. In this paper their properties are derived from functions estimating the degree of similarity between two electron‐density maps. The new approach leads also to modifications of the standard least‐squares procedures, potentially able to improve their efficiency. The role of the scaling factor between observed and model amplitudes is analysed: the concept of unlocated model is discussed and its scattering contribution is combined with that arising from the located model. Also, the possible use of an ancillary parameter, to be associated with the classical weight related to the variance of the observed amplitudes, is studied. The crystallographic discrepancy factors, basic tools often combined with least‐squares procedures in phasing approaches, are analysed. The mathematical approach here described includes, as a special case, the so‐called vector refinement, used when accurate estimates of the target phases are available. 相似文献
Evolving networks with a constant number of edges may be
modelled using a rewiring process. These models are used to
describe many real-world processes including the evolution of
cultural artifacts such as family names, the evolution of gene
variations, and the popularity of strategies in simple
econophysics models such as the minority game. The model is
closely related to Urn models used for glasses, quantum gravity
and wealth distributions. The full mean field equation for the
degree distribution is found and its exact solution and generating
solution are given. 相似文献
The main goals of this paper are to: i) relate two iteration-complexity bounds derived for the Mizuno-Todd-Ye predictor-corrector
(MTY P-C) algorithm for linear programming (LP), and; ii) study the geometrical structure of the LP central path. The first
iteration-complexity bound for the MTY P-C algorithm considered in this paper is expressed in terms of the integral of a certain
curvature function over the traversed portion of the central path. The second iteration-complexity bound, derived recently
by the authors using the notion of crossover events introduced by Vavasis and Ye, is expressed in terms of a scale-invariant
condition number associated with m × n constraint matrix of the LP. In this paper, we establish a relationship between these bounds by showing that the first one
can be majorized by the second one. We also establish a geometric result about the central path which gives a rigorous justification
based on the curvature of the central path of a claim made by Vavasis and Ye, in view of the behavior of their layered least
squares path following LP method, that the central path consists of long but straight continuous parts while the remaining curved part is relatively “short”.
R. D. C. Monteiro was supported in part by NSF Grants CCR-0203113 and CCF-0430644 and ONR grant N00014-05-1-0183. T. Tsuchiya
was supported in part by Japan-US Joint Research Projects of Japan Society for the Promotion of Science “Algorithms for linear
programs over symmetric cones” and the Grants-in-Aid for Scientific Research (C) 15510144 of Japan Society for the Promotion
of Science. 相似文献
Scaling predictions for bottle‐brush polymers with a rigid backbone and flexible side chains under good solvent conditions are discussed and their validity is assessed by a comparison with Monte Carlo simulations of a simple lattice model. It is shown that typically only a rather weak stretching of the side chains is realized, and then the scaling predictions are not applicable. Also two‐component bottle brush polymers are considered, where two types (A,B) of side chains are grafted, assuming that monomers of different kind repel each other. In this case, variable solvent quality is allowed. Theories predict “Janus cylinder”‐type phase separation along the backbone in this case. The Monte Carlo simulations, using the pruned‐enriched Rosenbluth method (PERM) give evidence that the phase separation between an A‐rich part of the cylindrical molecule and a B‐rich part can only occur locally. The correlation length of this microphase separation can be controlled by the solvent quality. This lack of a phase transition is interpreted by an analogy with models for ferromagnets in one space dimension.
A series of mononuclear complexes based on lanthanide ions has been synthesized and X-ray characterized. The compounds [LnIIIL2(NO3)3(H2O)2] (Ln = La, Ce, Pr, Nd, Sm, Gd and Tm; L = 2,6-bis(2-formylphenoxymethyl)pyridine) are found to be isomorphous and isostructural. Ligand L systematically coordinates through one carbonyl functionality, and the resulting complexes are placed on a twofold axis in crystals belonging to C2/c space-group. Emission spectra for Ln = La, Pr, Nd revealed a correlation between the Ln–O coordination bond length and the photoluminescent properties of the complexes, in line with a Förster–Dexter mechanism for intramolecular energy transfer. Ligand L is therefore a suitable sensitizer for lanthanide ions. 相似文献
Abstract A quantitative method to analyse fresh water samples for ultratrace pollutants such as Cd, Cr, Fe, Mn, Ni, Pb, V and Zn has been developed. the impurities in the water sample were preconcentrated by the method of slow evaporation and collected on specpure graphite which was subsequently analysed by d.c. are excitation using NaF as the carrier. the mean relative standard deviation of the method employed is + 15%. the accuracy of the method reported has been found to be quite satisfactory as borne out by the results of the Intercomparison Run conducted by the International Atomic Energy Agency, Vienna for the determination of trace elements in water. the detection limits for the various elements are as follows: Ni, Pb, V : 1 ng/ml; Cd, Cr, Mn: 2 ng/ml; Fe: 15 ng/ml and Zn : 20 ng/ml. Some of these detection limits have been compared with those obtained in some of the recent techniques such as AAS, Laser Induced Fluorescence (LIF) and Laser Enhanced Ionization (LEI) or Opto-Galvanic Spectrometry(òGS). It has been found that there is a tenfold increase in sensitivity for Pb in the present method as compared to the above techniques except LEI while the detection limits obtained for other elements are comparable with those in other techniques. 相似文献
