Van der Waals loops and the melting transition in two dimensions

Evidence for the existence of van der Waals loops in pressure p versus volume v plots has for some time supported the belief that melting in two dimensions is a first order phase transition. We report rather accurate equilibrium p(v) curves for systems of hard disks obtained from long Monte Carlo simulations. These curves, obtained in the constant volume ensemble, using periodic boundary conditions, exhibit well defined van der Waals loops. We illustrate their existence for finite systems that are known to undergo a continuous transition in the thermodynamic limit. To this end, we obtain magnetization m versus applied field curves from Monte Carlo simulations of the 2D Ising model, in the constant m ensemble, at the critical point. Whether van der Waals loops for disk systems behave in the thermodynamic limit as they do for the 2D Ising model at the critical point cannot be ruled out. Thus, the often made claim that melting in 2D is a first order phase transition, based on the evidence that van der Waals loops exist, is not sound.Comment: 10 pages, 6 Postscript figures (submitted to Phys.Rev.E). For related work, see http://pipe.unizar.es/~jf

π-Stacked polyphenolic dimers: A case study using dispersion-corrected methods

The accuracy of dispersion-corrected calculations (DFT-D2, DFT-D3 and DFT-NL) is assessed here, with large basis sets (def2-QZVP) to avoid incompleteness effects, for the most stable structure of a real-world polyphenol dimer chosen as an appropriate model. Natural polyphenols form such complexes with π-stacking playing a key stabilizing role. Our benchmark calculations predict its existence favored by 22–24 kcal/mol with respect to the isolated monomers, mainly driven by both π–π and H-bonding interactions. The adequate comparison of lower-cost DFT-based methods allowed bracketing their expected accuracy. These results thus pave the way towards reliable studies of challenging aggregation processes of natural products.The work in Alicante is supported by the ‘Ministerio de Educación y Ciencia’ of Spain and the ‘European Regional Development Fund’ through project CTQ2011-27253. The work in Mons is supported by the Belgian National Fund for Scientific Research (FNRS). The work in Limoges is supported by the ‘Conseil Régional du Limousin’ and COST actions FA1003 ‘East–West Collaboration for Grapevine Diversity Exploration and Mobilization of Adaptive Traits for Breeding’ and CM0804 ‘Chemical Biology with Natural Products’. The work in Malaysia is supported by Akademi Sains Malaysia through the SAGA Grant C20 and by the Ministry of Higher Education through the Grant 600-RMI/ST/FRGS 5/3/Fst (4/2011). The authors gratefully acknowledge the support by the Operational Program Research and Development for Innovation–European Regional Development Fund (Project CZ.1.05/2.1.00/03.0058 of the Ministry of Education, Youth and Sports of the Czech Republic). I.B. gratefully thanks the ‘Association Djerbienne de France’ (ADF) for the financial support

Experimental and theoretical characterization of the Zn - Zn bond in [Zn2(η5-C5Me5)2]

The existence and characterization of a bond between the Zn atoms in the recently synthesized complex [Zn2(5-C5Me5)2], as well as between Zn and ligand C atoms is firmly based on neutron diffraction and low-temperature X-ray synchrotron diffraction experiments. The multipolar analysis of the experimental electron density and its topological analysis by means of the Atoms in Molecules (AIM) approach reveals details of the Zn - Zn bond, such as its open-shell intermediate character (the results are consistent with a typical metal-metal single bond), as well as many other topological properties of the compound. Experimental results are also compared with theoretical ab initio calculations of the DFT (density functional theory) and MP2 (Mller-Plesset perturbation theory) electron densities, giving a coherent view of the bonding in the complex. For instance, charges calculated from the AIM approach applied to the atomic basin of each Zn atom are, on average, +0.72 e from both the experimental and the theoretical electron density, showing a moderate charge transfer from the metal, confirmed by the calculated topological indexes.Ministerio de Educación y Ciencia MAT2006-0199

Distribución lateral de velocidades en cauces naturales

[ES] En este trabajo se presentan los resultados de la aplicación de dos modelos para la predicción de la distribución lateral de velocidades en cinco casos de cauces naturales. Los modelos considerados son el Método del Canal Dividido (DCM) a través de su implementación en el programa HEC-RAS, y el Método de la Distribución Lateral (LDM) a través de la solución analítica propuesta por Shiono y Knight. Los casos analizados corresponden a cinco secciones ubicadas sobre diversos ríos: el río Severn en Montford (UK), y las estaciones de medición Eldorado y Puerto Libertad sobre el Alto río Paraná, el río de La Suela, en la provincia de Córdoba, y el río Colorado en Buta Ranquil (Neuquén), todas estas en la República Argentina. Se presentan los valores ajustados de los parámetros de los modelos, haciendo hincapié en la interpretación física de los mismos. Se compara el desempeño de los modelos aplicados, y se analizan sus ventajas y desventajas relativas. Se concluye que, a pesar de que el modelo SKM es matemáticamente más complejo, la ganancia obtenida para estimar la distribución lateral de velocidades en cauces naturales es lo suficientemente significativa como para recomendar su utilización.Weber, JF.; Menéndez, ÁN.; Tarrab, L. (2005). Distribución lateral de velocidades en cauces naturales. Ingeniería del agua. 12(3):1-14. https://doi.org/10.4995/ia.2005.2566OJS114123Abril, B. (2003), Benchmark comparisons of the analytical and finite element solutions of the SKM. Technical Report EPSRC Research Grant - GR/R54880/01. University of Birmingham, UK.Cunge, J., Holly, F., Verwey, A. (1980). Practical Aspects of Computational River Hydraulics. Pitman, Boston.Dasso, C. (1977). Investigación del escurrimiento subsuperficial en la Cuenca del Río la Suela, Informe Final de Beca, Centro de Investigaciones Hídricas de la Región Semiárida, Villa Carlos Paz, Córdoba, Argentina.Einstein, H. A., Banks, R. B. (1950). "Fluid resistance of composite roughness". Transactions, American Geophysical Union Vol. 31 No. 4, pp. 603-610.HEC (2001). HEC - RAS Hydraulic Reference Manual, U.S. Army Corps of Engineers, Davis, CA.Knight D. W., Shiono, K., Pirt, J. (1989). "Prediction of depth mean velocity and discharge in natural rivers with overbank flow". Proceedings of the International Conference on Hydraulic and Environmental Modellling of Coastal, Estuarine and River Waters, pp. 419-428. Bradford.Lotter, G. K. (1933). "Soobrazheniia k gidravlicheskomu raschetu rusel s razlichnoi sherokhovatostiiu stenok (Considerations on hydraulic design of channels with different roughness of walls)". Izvestiia Vsesoiuznogo NuachnoIssledovatel'skogo Instituta Gidrotekhniki (Transactions, All-Union Scientific Research Institute of Hydraulic Engineering) Vol. 9 pp. 238-241. Leningrad.Menéndez, A. (2003). "Selection of optima mathematical models for fluvial problems". Proceedings of 3rd IAHR Symposium on River, Coastal and Estuarine Morphodynamics. Barcelona, Spain.Ministerio de Obras y Servicios Públicos de la República Argentina (MOP). (1973) Estudio hidrológico y sedimentológico del río Alto Paraná: tramo Iguazú-Posadas. Comisión Nacional de la Cuenca del Plata (CONCAP), Dirección Nacional de Construcciones Portuarias y Vías Navegables (DNCPyVN) y Empresa Nacional de Agua y Energía Eléctrica (AyEE).Shiono, K., Knight, D. W. (1988). "Two-dimensional analytical solution for a compound channel". In Y. Iwasa, N. Tamai & A. Wada (eds.), Proceedings of 3rd International Symposium on Refined Flow Modeling and Turbulence Measurements pp. 503-510. Tokyo, Japan.Shiono, K., Knight, D. W. (1991). "Turbulent open-channel flows with variable depth across the channel". Journal of Fluid Mechanics Vol. 222 pp. 617-646. Great Britain.Tarrab, L. (2004). Estimación de la distribución lateral de velocidades en cauces naturales: ríos argentinos.Trabajo final de Ingeniería Civil - Fac. de Ciencias Exactas, Físicas y Naturales, Universidad Nacional de Córdoba, Argentina.Tarrab, L., Weber, J. F. (2004). Predicción del Coeficiente de Mezcla Transversal en Cauces Naturales. Mecánica Computacional, XXIII, 1343-1355. Asociación Argentina de Mecánica Computacional, San Carlos de Bariloche. ISSN: 1666-6070.Tarrab, L., Weber, J. F. (2004). El Método de la Distribución Lateral de Velocidades Aplicado a la Estimación de la Relación Altura Caudal en tres ríos Argentinos. XXI Congreso Latinoamericano de Hidráulica, São Pedro, Estado De São Paulo, Brasil, Octubre, 2004.Wark, J.B., Samuels, P.G., Ervine, D.A. (1990). "A practical method of estimating velocity and discharge in a compound channel". In White, W. R. (ed.), River flood hydraulics pp. 163-172. John Wiley & Sons, Chichester, UK.Weber, J. F. (2003). Modelación integrada uni y bidimensional de flujo en planicies de inundación. Tesis de Maestría en Ciencias de la Ingeniería - Mención en Recursos Hídricos - Fac. de Ciencias Exactas, Físicas y Naturales, Universidad Nacional de Córdoba, Argentina.Weber, J. F., Menéndez, A. N. (2003a). "Modelo LATERAL para simular la distribución lateral de velocidades en cauces naturales". Mecánica Computacional Vol. 22. Bahía Blanca, Argentina.Weber, J. F., Menéndez, A. N. (2003b). Desempeño de modelos de distribución lateral de velocidades en canales de sección compuesta. Primer Simposio Regional sobre Hidráulica de Ríos, RIOS2003, Buenos Aires, noviembre de 2003, Argentina.Weber, J. F., Tarrab, L. (2004). Predicción de la relación altura-caudal a través de un modelo analítico hidrodinámico: caso Alto Río Paraná. Cuadernos del Curiham,9.2, 101-108. ISSN 1514-2906. UNR Editora, Rosario, Argentina.Weber, J. F., Tarrab, L. (2003a). "Modelación de la distribución lateral de velocidades en cauces naturales - caso Alto Río Paraná". Mecánica Computacional Vol. 22. Bahía Blanca, Argentina.Weber, J. F., Tarrab, L. (2003b). Modelo analítico de la distribución lateral de velocidades en cauces naturales - caso Río Colorado. VIII Reunión sobre Recientes Avances en Física de Fluidos y sus Aplicaciones, FLUIDOS 2003, Tandil, noviembre 2003, Argentina.Weber, J. F., Tarrab, L. (2003c). Estudio de la hidrodinámica en cauces de montaña: caso Río La Suela. Primer Simposio Regional sobre Hidráulica de Ríos, RIOS2003, Buenos Aires, noviembre de 2003

ThX - a next-generation probe for the early detection of amyloid aggregates.

Neurodegenerative diseases such as Alzheimer's and Parkinson's are associated with protein misfolding and aggregation. Recent studies suggest that the small, rare and heterogeneous oligomeric species, formed early on in the aggregation process, may be a source of cytotoxicity. Thioflavin T (ThT) is currently the gold-standard fluorescent probe for the study of amyloid proteins and aggregation processes. However, the poor photophysical and binding properties of ThT impairs the study of oligomers. To overcome this challenge, we have designed Thioflavin X, (ThX), a next-generation fluorescent probe which displays superior properties; including a 5-fold increase in brightness and 7-fold increase in binding affinity to amyloidogenic proteins. As an extrinsic dye, this can be used to study unique structural amyloid features both in bulk and on a single-aggregate level. Furthermore, ThX can be used as a super-resolution imaging probe in single-molecule localisation microscopy. Finally, the improved optical properties (extinction coefficient, quantum yield and brightness) of ThX can be used to monitor structural differences in oligomeric species, not observed via traditional ThT imaging

Efficiency analysis of spur gears with a shifting profile

A model for the assessment of the energy efficiency of spur gears is presented in this study, which considers a shifting profile under different operating conditions (40–600 Nm and 1500–6000 rpm). Three factors affect the power losses resulting from friction forces in a lubricated spur gear pair, namely, the friction coefficient, sliding velocity and load sharing ratio. Friction forces were implemented using a Coulomb’s model with a constant friction coefficient which is the well-known Niemann formulation. Three different scenarios were developed to assess the effect of the shifting profile on the efficiency under different operating conditions. The first kept the exterior radii constant, the second maintained the theoretical contact ratio whilst in the third the exterior radii is defined by the shifting coefficient. The numerical results were compared with a traditional approach to assess the results.The authors would like to acknowledge Project DPI2013-44860 funded by the Spanish Ministry of Science and Technology and the COST ACTION TU 1105 for supporting this research

Measurement of the Lifetime Difference Between B_s Mass Eigenstates

We present measurements of the lifetimes and polarization amplitudes for B_s --> J/psi phi and B_d --> J/psi K*0 decays. Lifetimes of the heavy (H) and light (L) mass eigenstates in the B_s system are separately measured for the first time by determining the relative contributions of amplitudes with definite CP as a function of the decay time. Using 203 +/- 15 B_s decays, we obtain tau_L = (1.05 +{0.16}/-{0.13} +/- 0.02) ps and tau_H = (2.07 +{0.58}/-{0.46} +/- 0.03) ps. Expressed in terms of the difference DeltaGamma_s and average Gamma_s, of the decay rates of the two eigenstates, the results are DeltaGamma_s/Gamma_s = (65 +{25}/-{33} +/- 1)%, and DeltaGamma_s = (0.47 +{0.19}/-{0.24} +/- 0.01) inverse ps.Comment: 8 pages, 3 figures, 2 tables; as published in Physical Review Letters on 16 March 2005; revisions are for length and typesetting only, no changes in results or conclusion

The state of the Martian climate

60°N was +2.0°C, relative to the 1981–2010 average value (Fig. 5.1). This marks a new high for the record. The average annual surface air temperature (SAT) anomaly for 2016 for land stations north of starting in 1900, and is a significant increase over the previous highest value of +1.2°C, which was observed in 2007, 2011, and 2015. Average global annual temperatures also showed record values in 2015 and 2016. Currently, the Arctic is warming at more than twice the rate of lower latitudes

Cytogenetic analysis of Astylus antis (Perty, 1830) (Coleoptera, Melyridae): Karyotype, heterochromatin and location of ribosomal genes

Cytogenetic analysis of Astylus antis using mitotic and meiotic cells was performed to characterize the haploid and diploid numbers, sex determination system, chromosome morphology, constitutive heterochromatin distribution pattern and chromosomes carrying nucleolus organizer regions (NORs). Analysis of spermatogonial metaphase cells revealed the diploid number 2n = 18, with mostly metacentric chromosomes. Metaphase I cells exhibited 2n = 8II+Xyp and a parachute configuration of the sex chromosomes. Spermatogonial metaphase cells submitted to C-banding showed the presence of small dots of constitutive heterochromatin in the centromeric regions of nearly all the autosomes and on the short arm of the X chromosome (Xp), as well as an additional band on one of the arms of pair 1. Mitotic cells submitted to double staining with base-specific fluorochromes (DAPI-CMA3 ) revealed no regions rich in A+T or G+C sequences. Analysis of spermatogonial mitotic cells after sequential Giemsa/AgNO 3 staining did not reveal any specific mark on the chromosomes. Meiotic metaphase I cells stained with silver nitrate revealed a strong impregnation associated to the sex chromosomes, and in situ hybridization with an 18S rDNA probe showed ribosomal cistrons in an autosomal bivalent