Helsinki University of Technology

About: Helsinki University of Technology is a based out in . It is known for research contribution in the topics: Artificial neural network & Finite element method. The organization has 8962 authors who have published 20136 publications receiving 723787 citations. The organization is also known as: TKK & Teknillinen korkeakoulu.

Linear-scaling self-consistent implementation of the van der Waals density functional

Abstract: An efficient linear-scaling approach to the van der Waals density functional in electronic-structure calculations is demonstrated. The nonlocal correlation potential needed in self-consistent calculations is derived in a practical form. This enables also an efficient determination of the Hellmann-Feynman forces on atoms. The numerical implementation employs adaptive quadrature grids in real space resulting in a fast and an accurate evaluation of the functional and the potential. The approach is incorporated in the atomic orbital code SIESTA. The application of the method to the S22 set of noncovalently bonded molecules and comparison with the quantum chemistry data reveal an overall agreement but show that different exchange functionals should be used for different types of bonds. The van der Waals vdW interaction plays an important role in various processes in biochemistry, in surface science, and in physics of layered materials. Although the basic underlying physics of dispersion forces is understood, an accurate and efficient calculation of the vdW interaction is still challenging. The post-Hartree-Fock approaches, based on perturbation theory, are able to treat the vdW interaction. For instance, the coupled-cluster methods 1 especially coupled-cluster with single and double and perturbative triple excitations, CCSDT are very accurate and often used in benchmark calculations. Unfortunately, the high precision comes with enormous computational expenses, making the method fully applicable only for small complexes with less than a few dozen of atoms. The second-order Moller-Plesset perturbation theory MP2, although less expensive and less accurate, also contains the essential physics for describing the vdW interaction. Both of the two methods are not straightforward to apply to extended systems and the existing approaches to periodic systems, as those in Refs. 2 and 3, are fairly expensive and not in common use. Another class of methods for electronic-structure calculations is based on the density-functional theory DFT. The standard approximations to the exchange-correlation energy in DFT, such as the local-density approximation LDA and the generalized-gradient approximation GGA, are relatively cheap and easily applicable also to periodic systems. Unfortunately, these methods cannot describe the dispersion interaction properly. The problem has been addressed by Langreth, Lundqvist and co-workers, 4‐6 who constructed a nonlocal correlation functional aiming to preserve the essential mechanisms behind the vdW interaction. This approach has been successfully utilized for selected test systems and applications. 7‐10 However, most of these results are obtained either using an a posteriori energy correction scheme or a self-consistent calculation with a restricted capability of relaxing the structures. The reason for this is the computational complexity of the self-consistent approach of Ref. 11, which was employed in the earlier self-consistent calculations. In this Rapid Communication, we demonstrate our alternative highly efficient real-space approach. We derive the functional derivative of the vdW density functional vdWDF and obtain a representation, which enables a simpler evaluation of the nonlocal correlation potential and forces for self-consistent calculations with full atomic relaxation. Our implementation employs numerical quadratures on adaptive grids, which vary depending on the local properties of the electron density and are optimized to calculate the vdW-DF. We perform calculations on the widely used molecular benchmark set S22. 12 The results for its molecular complexes provide an extensive test for the reliability of the vdW-DF. Simple examples such as noble gas dimers are not suitable for these purposes as they do not provide a complete picture with respect to the variation in the nature of the bonding. Extended systems are either not discussed as a systematic discussion would require a versatile test set, which does not exist.

Results of WEBT, VLBA and RXTE monitoring of 3C 279 during 2006-2007 ⋆

Abstract: Context. The quasar 3C 279 is among the most extreme blazars in terms of luminosity and variability of flux at all wavebands. Its vari ations in flux and polarization are quite complex and therefore require intensive monitoring observations at multiple wavebands to characterise and interpret the observed changes. Aims. In this paper, we present radio-to-optical data taken by the WEBT, supplemented by our VLBA and RXTE observations, of 3C 279. Our goal is to use this extensive database to draw inferences regarding the physics of the relativistic jet. Methods. We assemble multifrequency light curves with data from 30 ground-based observatories and the space-based instruments SWIFT (UVOT) and RXTE, along with linear polarization vs. time in the optical R band. In addition, we present a sequence of 22 images (with polarization vectors) at 43 GHz at resolution 0.15 milliarcse c, obtained with the VLBA. We analyse the light curves and polarization, as well as the spectral energy distributions at different epochs, corresponding to different brightness states. Results. We find that the IR-optical-UV continuum spectrum of the vari able component corresponds to a power law with a constant slope of−1.6, while in the 2.4‐10 keV X-ray band it varies in slope from−1.1 to−1.6. The steepest X-ray spectrum occurs at a flux minimum. Durin g a decline in flux from maximum in late 2006, the optical and 43 GHz core po larization vectors rotate by∼ 300 ◦ . Conclusions. The continuum spectrum agrees with steady injection of relativistic electrons with a power-law energy distribution of slope−3.2 that is steepened to−4.2 at high energies by radiative losses. The X-ray emission at flux minimum comes most likely from a new component that starts in an upstream section of the jet where inverse Compton scattering of seed photons from outside the jet is important. The rotation of the polarization vector implies that the jet contains a helical magnetic field that extends ∼ 20 pc past the 43 GHz core.

Molecular oxygen in the ρ Ophiuchi cloud

Abstract: Context: Molecular oxygen, O2, has been expected historically to be an abundant component of the chemical species in molecular clouds and, as such, an important coolant of the dense interstellar medium. However, a number of attempts from both ground and from space have failed to detect O2 emission.Aims: The work described here uses heterodyne spectroscopy from space to search for molecular oxygen in the interstellar medium. Methods: The Odin satellite carries a 1.1 m sub-millimeter dish and a dedicated 119 GHz receiver for the ground state line of O2. Starting in 2002, the star forming molecular cloud core ρ Oph A was observed with Odin for 34 days during several observing runs.Results: We detect a spectral line at v_LSR =+3.5 km s-1 with Δ v_FWHM=1.5 km s-1, parameters which are also common to other species associated with ρ Oph A. This feature is identified as the O2 (NJ = 11 - 1_0) transition at 118 750.343 MHz.Conclusions: The abundance of molecular oxygen, relative to H{2} , is 5 × 10-8 averaged over the Odin beam. This abundance is consistently lower than previously reported upper limits.Based on observations with Odin, a Swedish-led satellite project funded jointly by the Swedish National Space Board (SNSB), the Canadian Space Agency (CSA), the National Technology Agency of Finland (Tekes) and Centre National d'Etude Spatiale (CNES). The Swedish Space Corporation has been the industrial prime contractor and also is operating the satellite. Appendix A is only available in electronic form at http://www.aanda.org

Operational Architectonics of the Human Brain Biopotential Field: Towards Solving the Mind-Brain Problem

Abstract: Operational Architectonics of the Human Brain Biopotential Field: Towards Solving the Mind-Brain Problem The understanding of the interrelationship between brain and mind remains far from clear. It is well established that the brain's capacity to integrate information from numerous sources forms the basis for cognitive abilities. However, the core unresolved question is how information about the "objective" physical entities of the external world can be integrated, and how unified and coherent mental states (or Gestalts) can be established in the internal entities of distributed neuronal systems. The present paper offers a unified methodological and conceptual basis for a possible mechanism of how the transient synchronization of brain

Transglycosylation products of cellulase system of Trichoderma reesei

Abstract: From cellulose and cellobiose the formation of sophorose, laminaribiose, and gentiobiose was catalyzed byTrichoderma reesei culture filtrate containing exo- and endoglucanase and β-glucosidase activity and from cellobiose by a broken cell suspension fromT.reesei with β-glucosidase activity. The results indicate that β-glucosidase is the component responsible for transglycosylation reaction catalyzed byT.reesei cellulase enzyme complex.