1,722,244 research outputs found
An effective approach for designing circular pipes with the Colebrook-White formula
No full textTo designingpipe diameter, several flow resistance formulae are available in literature. Generally, these are relationships relating friction loss, relative roughness and Reynolds number. Here, an iterative procedure, based on the solution of a second order equation (standard approach), is presented. Then an effective and simple procedure (advanced approach) for designing circular pipes by means of the Colebrook-White transition formula is provided. Latter procedure is proven to be accurate in double precisionafter a limited number of iterative steps
Simulating fluid-structure interaction with SPH
No full textFluid-structure interaction (FSI) is the condition according to which a fluid comes in contact and interacts with
a structure. A simple application of the impulse-momentum theorem yields as a result the developing of pressure waves,
propagating inside both media. The present work presents a critical assessment of the Weakly Compressible Smoothed
Particle Hydrodynamics (WCSPH) method to simulate the first stages of a 2D fluid impacting onto a rigid vertical wall.
The effects on the results in terms of the sound speed c0, appearing inside the state equation, as well as of the velocity va
of the approaching fluid are here investigated
Modelling Complex Molecular and Biomaterial Systems: Simulation of Reactive Processes, Methodological Advances and Interplay with the Experiments
No full textNonlinear electrodynamics and CMB polarization
No full textThe polarization of Cosmic Microwave Background radiation (CMB) is reviewed in the context of nonlinear electrodynamics (NLED). We compute the polarization angle of photons propagating in a cosmological background with planar symmetry. The Lagrangian density describing the Pagels-Tomboulis (PT) nonlinear electrodynamics has the form L~X, where X = FF, and the parameter featuring the non-Maxwellian character of the PT nonlinear description of the electromagnetic interaction. After looking at the polarization components in the plane orthogonal to the (x)-direction of propagation of the CMB photons, the polarization angle is defined in terms of the eccentricity of the universe, a geometrical property whose evolution on cosmic time (from the last scattering surface to the present) is constrained by the strength of magnetic fields over extragalactic distances. Then, by making use of recent data on CMB polarization from WMAP and BOOMERanG, we infer the constraints on the parameter characterizing the nonlinearity of electrodynamics alla P
COBRAMM: A Tunable QM/MM Approach to Complex Molecular Architectures. Modelling the Excited and Ground State Properties of Sized Molecular Systems
No full textThis contribution describes a new implementation of a general hybrid approach with a modular structure (called COBRAMM: Computations in Bologna Relating Ab‐initio and Molecular Mechanics Methods) that is able to integrate some specialized softwares and acts as a flexible computational environment, thus increasing the flexibility/efficiency of both QM, and MM, and QM/MM calculations. Specifically, QM/MM ground and excited states geometry optimizations, frequency calculations, conical intersection searches and adiabatic/non‐adiabatic molecular dynamics can be performed on a large molecular system, that can be split up to three different layers corresponding to different levels of accuracy. Here we report, together with a description of the method and its implementation, some test examples on very different chemical problems, which span the wide and diversified area of chemistry (from ground to excited states topics) and show the flexibility, general applicability and accuracy of the presented hybrid approach in the modelling of complex molecular architectures
Iridium Complexes with Terdentate Ligands: Cyclometalated vs. Polypyridine Analogues; Hybrid vs. “Pure” DFT
No full textThe present investigation has a twofold aim. In the first place, the ground‐state spectroscopic features of three IrIII polypyridyl complexes were studied by computational means, in order to elucidate the nature of their UV‐visible spectra and electrochemical behaviour. Coincidentally, the limited but variegated set of molecules was employed to analyse how different exchange‐correlation functionals model the properties of interest in the presence of transition metal atoms. These iridium complexes involve various terdentate ligands: a N∧N∧N bonded ttpy derivative ( ttpy = 4′ ‐tolyl‐ 2:2′,6′:2′′ ‐terpyridine), a C∧N∧C bonded dppy derivative ( dppy = 2,6 ‐diphenyl‐pyridine), a C∧N∧N pbp derivative ( pbp = 6 ‐phenyl‐ 2:2′ ‐bipyridine). The axially‐symmetric‐homoleptic or heteroleptic‐arrangement gives the low‐lying excited states of these molecules a unique directional nature. Analysis of the electronic transitions calculated by first‐principle method based on time‐dependent density functional theory (TDDFT) gives insight on their steady‐state visible absorption bands. The comparison of generalised‐gradient approximation (GGA) functionals (BLYP and G96LYP) with hybrid GGA functionals (B3LYP and PBE0) values the consequences of the nonlocal Hartree–Fock exchange embedded in the latter
Ab initio Investigation to Model Stilbene Photo-Physical Properties by Combining CC2 Topological Investigation and CASPT2 Energy Corrections
No full textStilbene photoexcitation and consequent decay to the ground state has been investigated by mapping the Minimum Energy Path (MEP) from S1 spectroscopic state triggering an almost barrierless reaction pathway to an S1/S0 degenerate region. The particular influence of the σ‐π excitation on the S1 wave function, dominated by a π→π* character, reveals how the non‐dynamical correlation energy was important to correctly describe the excited state behaviour and the topological aspect of its potential energy surface. Several strategies of calculations, by using CASSCF//CASPT2 methods, were performed trying to improve the photochemical description nowadays known. Both symmetry and non symmetry preserving computations were performed; systematically was concluded that, because of the limit of CASSCF description enables only to introduce the correlation effect such as the ones due to σ‐π excitations, CASSCF and CASPT2 topologies are probably often not in agreement. Thus CC2 methodology was adopted o optimize the S1 geometries and obtain reasonable structures for the minima. Two S1/S0 accessible conical intersections featured by pyramidalized carbons were located on the first excited state explaining the ultrafast radiationless decay to the ground state and the photoproducts observed within the timescale of ps
Azobenzene cis-trans Photoisomerization Mechanism: Characterization of the Decay Ways from the Lowest pipi* Absorbing Singlet State
No full textIn this paper, we analyze the photoisomerization processes of azobenzene after its excitation in the bright S(ππ*). By state of the art/ ab initio / Complete Active Space calculations followed by perturbative corrections (CASPT2//CASSCF) we have identified the critical structures, the Minimum Energy Paths originating on the bright S(ππ*) and on other relevant excited states including the state S1(nπ*). The seams of conical intersections that are important in guiding the photoreaction are determined. We aim at establishing the mechanism of decay and of photoisomerization for the S(ππ*) state and at explaining the difference between the quantum yields found for the two lowest energy S1(nπ*) and S(ππ*) excited states. We found that an excited state based on the πN = NπN = N→π*N = Nπ*N = N configuration is a photoreaction intermediate that plays a very important role in the decay the bright S(ππ*). This doubly excited state, by driving the photoisomerization along the torsion path and by inducing a fast internal conversion to the S1(nπ*) occurring in a variety of geometries, explains all the most important features of the S(ππ*) azobenzene photoisomerization
An ab initio Study of Decay Mechanism of Adenine: the Facile Path of the Amino NH Bond Cleavage
No full textA comprehensive study of the radiationless decay processes of the lowest excited singlet states in the isolated 9H‐Adenine has been performed at the CASPT2//CASSCF level. The minimum energy paths of the La, Lb and nπ* singlet states along different skeletal distortions have been computed and the Conical Intersections (CIs) involving these states have been determined. The fast deactivation path of La along a skeletal deformation, which leads to a S0/La CI, as previously discussed, is confirmed. Moreover, low‐lying CIs between S0 and πσ* singlet states have been characterized, where σ* is the antibonding orbital localized on a N‐H bond of the amino (πσNH2*) or of the azine group (πσN9H*). We have found that the repulsive πσNH2* state associated with an amino N‐H bond can be populated through a barrierless way. Therefore, the decay path shows a bifurcation leading to two possible ways of radiationless deactivation: on one hand a non‐photochemical decay through the S0/La or S0/nπ* CIs and on the other hand a photochemical process via the possible access to the S0/πσNH2* CI that produces N‐H cleavage. In this way, we can explain the H atom loss found upon UV excitation. We have considered also the decay of higher energy bright states. We have found that these states can decay also by converting to the repulsive πσN9H* state associated with the azine NH bond. This new channel suggests an increase of H‐atom photoproduction yield by excitating Adenine with lower wavelength radiations. The study of the decay processes of an Adenine molecule in the double strand d(A)10⋅d(T)10 in water solvent is currently underway: Adenine is treated by the Quantum Mechanical (QM) approach and the remaining molecules are described at the Molecular Mechanics (MM) level. We use the COBRAMM program that is a tunable QM/MM approach to complex molecular architectures developed by our research group
Rhodopsin and GFP Chromophores: QM/MM Absorption Spectra in Solvent and Protein
No full textThis work presents a QM/MM investigation of the spectral properties of the 11‐cis‐retinal and the GFP chromophore in polar solvents and protein. The results of the computations are in surprising agreement with the experimental values indicating the accuracy of our computational approach. In addition it has been demonstrated the key role of the solvent to create a “virtual conter‐ion”. This effect is due to the reorientation (i.e. polarization) of the polar solvent close to the chromophore: the polarized permanent dipoles of the solvent act similarly to the counter ion, stabilizing the ground state respect to the charge transfer excited state
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