Velocity-gauge real-time TDDFT within a numerical atomic orbital basis set

The interaction of laser fields with solid-state systems can be modeled efficiently within the velocity-gauge formalism of real-time time dependent density functional theory (RT-TDDFT). In this article, we discuss the implementation of the velocity-gauge RT-TDDFT equations for electron dynamics within a linear combination of atomic orbitals (LCAO) basis set framework. Numerical results obtained from our LCAO implementation, for the electronic response of periodic systems to both weak and intense laser fields, are compared to those obtained from established real-space grid and Full-Potential Linearized Augumented Planewave approaches. Potential applications of the LCAO based scheme in the context of extreme ultra-violet and soft X-ray spectroscopies involving core-electronic excitations are discussed.Comment: 32 pages, 7 figures Updated acknowledgements and outlook. No changes to result

Equation of motion coupled-cluster approach for intrinsic losses in x-ray spectra

We present an equation of motion coupled cluster approach for calculating and understanding intrinsic inelastic losses in core level x-ray absorption spectra (XAS). The method is based on a factorization of the transition amplitude in the time-domain, which leads to a convolution of an effective one-body spectrum and the core-hole spectral function. The spectral function characterizes these losses in terms of shake-up excitations and satellites, and is calculated using a cumulant representation of the core-hole Green's function that includes non-linear corrections. The one-body spectrum also includes orthogonality corrections that enhance the XAS at the edge

A Transferable H2O Interaction Potential Based on a Single Center Multipole Expansion: SCME

A transferable potential energy function for describing the interaction between water molecules is presented. The electrostatic interaction is described rigorously using a multipole expansion. Only one expansion center is used per molecule to avoid the introduction of monopoles. This single center approach turns out to converge and give close agreement with ab initio calculations when carried out up to and including the hexadecapole. Both dipole and quadrupole polarizability is included. All parameters in the electrostatic interaction as well as the dispersion interaction are taken from ab initio calculations or experimental measurements of a single water molecule. The repulsive part of the interaction is parametrized to fit ab initio calculations of small water clusters and experimental measurements of ice Ih. The parametrized potential function was then used to simulate liquid water and the results agree well with experiment, even better than simulations using some of the point charge potentials fitted to liquid water. The evaluation of the new interaction potential for condensed phases is fast because point charges are not present and the interaction can, to a good approximation, be truncated at a finite range.Comment: 30 pages, 15 figures, 11 table

Asymmetric constraints on limits to species ranges influence consumer-resource richness over an environmental gradient

ArticleABSTRACT Aim There is little consensus about the relative roles of biotic versus abiotic factors in setting limits to species distributions or in generating geographical patterns of species richness. However, despite the probable importance of host availability in governing the distribution and diversity of consumers, few studies have simultaneously tested the effects of resource distribution and diversity on consumer ranges and richness patterns. Location Sierra de Guadarrama, central Spain. Methods We examined the effects of biotic resources, consumer attributes and climate on the ranges and species richness patterns of 43 specialist butterflies at 40 sites over a 1800-m elevational gradient. Evidence for resource use was based on comprehensive field records of oviposition and larval feeding on host plants. Results We show that limitation by either biotic interactions with resources (the distributions and parts eaten of the larval host plants) or intrinsic dispersal ability was stronger at upper than lower elevational range limits for butterflies. Both resource and consumer richness followed a unimodal, humped pattern over the elevational gradient, but host plant richness peaked 300 m lower than butterfly richness. In addition, whereas changes in butterfly species richness were roughly symmetrical around peak richness over the gradient studied, the host plants showed markedly lower species richness at high elevations (> 1750 m). Butterfly species richness increased with host plant resource diversity and relative humidity, with a steeper response to host plant richness in cooler sites (at higher elevations). Main conclusions The results demonstrate the role of bottom-up control by resource availability in limiting consumer distributions and richness. Importantly, resource limitation had increasing relevance towards the coolest parts of environmental gradients and those poorest in resource species, with potential consequences for ecological responses to environmental change.Universidad Rey Juan Carlos/Comunidad de Madrid . Grant Number: URJC-CM-2006-CET-0592 Spanish Ministry of Economy and Competitiveness . Grant Numbers: CGL2005-06820/BOS , CGL2008-04950/BOS , CGL2011-30259 , CGL2013-48277-P , CGL2014-57784-

Mechanical and thermal response of enamel to IR radiation - a finite element mesoscopic model

We present finite element models of human dental enamel that account for water-pores known to exist in this material, and use them to assess the influence of these pores on the temperature and stress profiles during and after single Er:YAG (2.9 µm) and CO2 (10.6 µm) laser pulses of duration 0.35 µs. Our results indicate that the temperature maximum is reached at the water-pores at the end of the laser pulse; this maximum seems to be independent of pore size for the CO2 laser but appears to be strongly dependent of pore size for the Er:YAG laser. The pressure reached at the water pore seems to be directly related to the temperature at the pore and it is significantly higher that the stress levels reached throughout the modelled structure, which indicates that water pores should play a significant role in the ablation mechanisms, even before water vaporization takes place. These results suggest that researchers conducting enamel ablation by Er:YAG lasers - or other lasers with wavelengths for which the absorption coefficients of the mineral and the water differ significantly - may want to select their samples and analyse their results taking into account factors that may alter the degree of mineralization of a tooth, such as age or type of tooth.Fundação para a Ciência e a Tecnologia (FCT) – Programa Operacional “Ciência , Tecnologia, Inovação” – POCTI/ESP/37944/2001, SFRH/BD/4725/200.Comunidade Europeia (CE). Fundo Europeu de Desenvolvimento Regional (FEDER)

O VI and Multicomponent H I Absorption Associated with a Galaxy Group in the Direction of PG0953+415: Physical Conditions and Baryonic Content

We report the discovery of an O VI absorption system at z(abs) = 0.14232 in a high resolution FUV spectrum of PG0953+415 obtained with the Space Telescope Imaging Spectrograph (STIS). Both lines of the O VI 1032, 1038 doublet and multicomponent H I Lya absorption are detected, but the N V doublet and the strong lines of C II and Si III are not apparent. We examine the ionization mechanism of the O VI absorber and find that while theoretical considerations favor collisional ionization, it is difficult to observationally rule out photoionization. If the absorber is collisionally ionized, it may not be in equilibrium due to the rapid cooling of gas in the appropriate temperature range. Non-equilibrium collisionally ionized models are shown to be consistent with the observations. A WIYN survey of galaxy redshifts near the sight line has revealed a galaxy at a projected distance of 395 kpc separated by ~130 km/s from this absorber, and three additional galaxies are found within 130 km/s of this redshift with projected separations ranging from 1.0 Mpc to 3.0 Mpc. Combining the STIS observations of PG0953+415 with previous high S/N GHRS observations of H1821+643, we derive a large number of O VI absorbers per unit redshift, dN/dz ~20. We use this sample to obtain a first estimate of the cosmological mass density of the O VI systems at z ~ 0. If further observations confirm the large dN/dz derived for the O VI systems, then these absorbers trace a significant reservoir of baryonic matter at low redshift.Comment: Accepted for publication in Ap.J., vol. 542 (Oct. 10, 2000

Modelling the influence of pore size on the response of materials to infrared lasers – an application to human enamel

We present an analytical model for a ceramic material (hydroxyapatite) containing nanometre-scale water pores, and use it to estimate the pressure at the pore as a function of temperature at the end of a single 0.35 µs laser pulse by Er:YAG (2.94 µm) and CO2 (10.6 µm) lasers. Our results suggest that the pressure at the pore is directly related to pore temperature, and that very high pressures can be generated simply by the thermal expansion of liquid water. Since the temperature reached at the pores at the end of the laser pulse is a strong function of pore size for Er:YAG lasers, but is independent of pore size for CO2 lasers, our present results provide a possible explanation for the fact that the enamel threshold ablation fluences are more variable for Er:YAG lasers than for CO2 lasers, and suggest that experimentalists should analyse their results accounting for factors (like age or type of tooth) that may change the pore size distribution in their samples.Comunidade Europeia (CE) - Fundo Comunitário Europeu (FEDER).Fundação para a Ciência e a Tecnologia (FCT)