The dynamics of thin vibrated granular layers

We describe a series of experiments and computer simulations on vibrated granular media in a geometry chosen to eliminate gravitationally induced settling. The system consists of a collection of identical spherical particles on a horizontal plate vibrating vertically, with or without a confining lid. Previously reported results are reviewed, including the observation of homogeneous, disordered liquid-like states, an instability to a `collapse' of motionless spheres on a perfect hexagonal lattice, and a fluctuating, hexagonally ordered state. In the presence of a confining lid we see a variety of solid phases at high densities and relatively high vibration amplitudes, several of which are reported for the first time in this article. The phase behavior of the system is closely related to that observed in confined hard-sphere colloidal suspensions in equilibrium, but with modifications due to the effects of the forcing and dissipation. We also review measurements of velocity distributions, which range from Maxwellian to strongly non-Maxwellian depending on the experimental parameter values. We describe measurements of spatial velocity correlations that show a clear dependence on the mechanism of energy injection. We also report new measurements of the velocity autocorrelation function in the granular layer and show that increased inelasticity leads to enhanced particle self-diffusion.Comment: 11 pages, 7 figure

Carbon based membranes as filtering materials for gaseous mixtures.

Treballs Finals de Grau de Química, Facultat de Química, Universitat de Barcelona, Any: 2021, Tutors: Fermín Huarte Larrañaga, Pablo Gamallo BelmonteCarbon-based membranes are a novel approach to gas separation. More precisely, new graphene-like structures are of utmost importance in this field of research. The scope of this work is to prove the effectiveness of grazyne membranes in the separation of different gaseous mixtures: carbon dioxide (CO2) with methane (CH4) and CO2 with oxygen (O2). To determine the efficiency of the membrane, a molecular dynamics simulation is carried via Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) undergoing an adaptive intermolecular reactive bond order (AIREBO) force field. Grazynes are a recently proposed family of 2D carbon allotropes consisting in graphene-like stripes bonded via acetylenic links, which allow for the design of pores of variable size, an important property for gas separation. For these simulations, the studied membrane was [1],[2]{2}-grazyne. The focus of the research was to determine their permeability and selectivity for both mixtures at different sets of pressures and constant temperature. To achieve this, a box was simulated in which a piston-like wall was set at different heights. Due to computational restraints, simulations at low pressure values (i.e. lower than 10 atm) were performed with c(2x2) supercells. The results were conclusive in determining the [1],[2]{2}-grazyne membrane as infinitely selective for CO2 over CH4 between 1 and 20 atm, meaning the membrane was impermeable for methane. For the CO2/O2 mixture, further simulations were performed with [1],[3]- and [1],[m]{1}-grazynes (m=1,2,3) as no selective separation could be carried out. No conclusive data could be obtained from such simulations, as the only selective separations occurred when only a single molecule was filtered

Computational Physics on Graphics Processing Units

The use of graphics processing units for scientific computations is an emerging strategy that can significantly speed up various different algorithms. In this review, we discuss advances made in the field of computational physics, focusing on classical molecular dynamics, and on quantum simulations for electronic structure calculations using the density functional theory, wave function techniques, and quantum field theory.Comment: Proceedings of the 11th International Conference, PARA 2012, Helsinki, Finland, June 10-13, 201

Electronic Excitations in Complex Molecular Environments: Many-Body Green's Functions Theory in VOTCA-XTP

Many-body Green's functions theory within the GW approximation and the Bethe-Salpeter Equation (BSE) is implemented in the open-source VOTCA-XTP software, aiming at the calculation of electronically excited states in complex molecular environments. Based on Gaussian-type atomic orbitals and making use of resolution of identify techniques, the code is designed specifically for non-periodic systems. Application to the small molecule reference set successfully validates the methodology and its implementation for a variety of excitation types covering an energy range from 2-8 eV in single molecules. Further, embedding each GW-BSE calculation into an atomistically resolved surrounding, typically obtained from Molecular Dynamics, accounts for effects originating from local fields and polarization. Using aqueous DNA as a prototypical system, different levels of electrostatic coupling between the regions in this GW-BSE/MM setup are demonstrated. Particular attention is paid to charge-transfer (CT) excitations in adenine base pairs. It is found that their energy is extremely sensitive to the specific environment and to polarization effects. The calculated redshift of the CT excitation energy compared to a nucelobase dimer treated in vacuum is of the order of 1 eV, which matches expectations from experimental data. Predicted lowest CT energies are below that of a single nucleobase excitation, indicating the possibility of an initial (fast) decay of such an UV excited state into a bi-nucleobase CT exciton. The results show that VOTCA-XTP's GW-BSE/MM is a powerful tool to study a wide range of types of electronic excitations in complex molecular environments

Accelerated Broadband Spectra and Attosecond Charge Migration Simulations using Real-Time Time-Dependent Density Functional Theory

In this dissertation, the calculations of light-matter interactions offer insight into the structure and dynamical response of electrons in molecular systems. Such information is useful for characterizing molecules, electronic structure, photochemistry, photomaterials, and a host of other applications. In the first part of this work, simulations of broadband absorption spectra are accelerated by the use of Pad´e approximanants of Fourier Transforms and dipole decomposition. Electronic absorption spectra from valence and core levels are obtained using time-dependent methods and compared to results from established perturbative techniques. In addition, core level absorption spectra are calculated for a nickel porphyrin and shown to be consistent with experimental XANES spectra. Not only is the speed up of these real-time simulations significant (at least 5x faster), but such methods offer the ability to directly calculate the dipole response from molecular orbital pairs involved in a given transition. In the second part of this dissertation, time-dependent density functional theory (TDDFT) calculations were performed to capture core-hole-initiated charge migration. By using welldefined initial states, well-known problems of resonant excitation (with TDDFT) caused by the adiabatic approximation are avoided. Using such initial states, core-hole-initiated valence charge migration is obtained in nitrosobenzene by TDDFT. These results are shown to be in good agreement with those of more accurate methods, provided the use of a hybrid functional to incorporate both electron correlation and exchange. In addition, the timedependent electron localization function (TD-ELF) can convert the density into a Lewis-dot picture of bonds and lone pairs of electrons. Building upon these charge migration studies, initial states in the valence orbitals were also constructed. In valence ionization, the electron is partially removed from multiple channels or orbitals; thus, multiple orbitals are involved in constructing the valence hole initial states. Due to the sudden ionization approximation made by construction of the vi initial states, a broadband-like response causes many dynamical modes to be excited. Here, the amplitude and phase of Fourier transforms are used to extract and classify the modes as charge migration or density-like excitation. These modes are then used to calculate metrics like migration distance and speed. Using this information in conjuction with the TD-ELF, it becomes possible to not only interpret charge migration, but to predict it by established chemical principles