DSpace Collection:
http://hdl.handle.net/2117/395
Sat, 28 Feb 2015 00:47:55 GMT2015-02-28T00:47:55Zwebmaster.bupc@upc.eduUniversitat Politècnica de Catalunya. Servei de Biblioteques i DocumentaciónoDynamic structure function of a cold Fermi gas at unitarity
http://hdl.handle.net/2117/26519
Title: Dynamic structure function of a cold Fermi gas at unitarity
Authors: Astrakharchik, Grigori; Boronat Medico, Jordi; Krotscheck, Eckhard; Lichtenegger, Thomas
Abstract: We present a theoretical study of the dynamic structure function of a resonantly interacting two-component Fermi gas at zero temperature. Our approach is based on dynamic many-body theory able to describe excitations in strongly correlated Fermi systems. The fixed-node diffusion Monte Carlo method is used to produce the ground-state correlation functions which are used as an input for the excitation theory. Our approach reproduces recent Bragg scattering data in both the density and the spin channel. In the BCS regime, the response is close to that of the ideal Fermi gas. On the BEC side, the Bose peak associated with the formation of dimers dominates the density channel of the dynamic response. When the fraction of dimers is large our theory departs from the experimental data, mainly in the spin channel.Thu, 26 Feb 2015 08:58:44 GMThttp://hdl.handle.net/2117/265192015-02-26T08:58:44ZAstrakharchik, Grigori; Boronat Medico, Jordi; Krotscheck, Eckhard; Lichtenegger, ThomasnoDynamic response, Electron gas, Monte Carlo methods, Diffusion Monte Carlo method, Dynamic structure, Excitation theory, Ground-state correlation, Many-body theory, Theoretical study, Two-component fermi gas, Zero temperatures, FermionsWe present a theoretical study of the dynamic structure function of a resonantly interacting two-component Fermi gas at zero temperature. Our approach is based on dynamic many-body theory able to describe excitations in strongly correlated Fermi systems. The fixed-node diffusion Monte Carlo method is used to produce the ground-state correlation functions which are used as an input for the excitation theory. Our approach reproduces recent Bragg scattering data in both the density and the spin channel. In the BCS regime, the response is close to that of the ideal Fermi gas. On the BEC side, the Bose peak associated with the formation of dimers dominates the density channel of the dynamic response. When the fraction of dimers is large our theory departs from the experimental data, mainly in the spin channel.Universality in molecular halo clusters
http://hdl.handle.net/2117/26455
Title: Universality in molecular halo clusters
Authors: Stipanovic, P.; Markic, L. Vranjes; Beslic, Ivana; Boronat Medico, Jordi
Abstract: The ground state of weakly bound dimers and trimers with a radius extending well into the classically forbidden region is explored, with the goal to test the predicted universality of quantum halo states. The focus of the study is molecules consisting of T down arrow, D down arrow, He-3, He-4, and alkali atoms, where the interaction between particles is much better known than in the case of nuclei, which are traditional examples of quantum halos. The study of realistic systems is supplemented by model calculations in order to analyze how low-energy properties depend on the interaction potential. The use of variational and diffusion Monte Carlo methods enabled a very precise calculation of both the size and binding energy of the trimers. In the quantum halo regime, and for large values of scaled binding energies, all clusters follow almost the same universal line. As the scaled binding energy decreases, Borromean states separate from tango trimers.Fri, 20 Feb 2015 16:17:53 GMThttp://hdl.handle.net/2117/264552015-02-20T16:17:53ZStipanovic, P.; Markic, L. Vranjes; Beslic, Ivana; Boronat Medico, JordinoInteraction cross-sections, Surface integral method, Quantum halos, Ground-state, Perturbation-theory, Systems, Potentials, Radii, HE, ScatteringThe ground state of weakly bound dimers and trimers with a radius extending well into the classically forbidden region is explored, with the goal to test the predicted universality of quantum halo states. The focus of the study is molecules consisting of T down arrow, D down arrow, He-3, He-4, and alkali atoms, where the interaction between particles is much better known than in the case of nuclei, which are traditional examples of quantum halos. The study of realistic systems is supplemented by model calculations in order to analyze how low-energy properties depend on the interaction potential. The use of variational and diffusion Monte Carlo methods enabled a very precise calculation of both the size and binding energy of the trimers. In the quantum halo regime, and for large values of scaled binding energies, all clusters follow almost the same universal line. As the scaled binding energy decreases, Borromean states separate from tango trimers.First-principles modeling of quantum nuclear effects and atomic interactions in solid He-4 at high pressure
http://hdl.handle.net/2117/26453
Title: First-principles modeling of quantum nuclear effects and atomic interactions in solid He-4 at high pressure
Authors: Cazorla Silva, Claudio; Boronat Medico, Jordi
Abstract: We present a first-principles computational study of solid He-4 at T = 0 K and pressures up to similar to 160 GPa. Our computational strategy consists in using van der Waals density functional theory (DFT-vdW) to describe the electronic degrees of freedom in this material, and the diffusion Monte Carlo (DMC) method to solve the Schrodinger equation describing the behavior of the quantum nuclei. For this, we construct an analytical interaction function based on the pairwise Aziz potential that closely matches the volume variation of the cohesive energy calculated with DFT-vdW in dense helium. Interestingly, we find that the kinetic energy of solid He-4 does not increase appreciably with compression for P >= 85 GPa. Also, we show that the Lindemann ratio in dense solid He-4 amounts to 0.10 almost independently of pressure. The reliability of customary quasiharmonic DFT (QH DFT) approaches in describing quantum nuclear effects in solids is also studied. We find that QH DFT simulations, although provide a reasonable equation of state in agreement with experiments, are not able to reproduce correctly these critical effects in compressed He-4. In particular, we disclose huge discrepancies of at least similar to 50% in the calculated He-4 kinetic energies using both the QH DFT and present DFT-DMC methods.Fri, 20 Feb 2015 15:30:21 GMThttp://hdl.handle.net/2117/264532015-02-20T15:30:21ZCazorla Silva, Claudio; Boronat Medico, JordinoMonte-Carlo algorithms, Augmented-wave method, X-Ray-diffraction, Equation-of-state, Ground-state, Molecular-hydrogen, Megabar pressures, Single-crystal, Dense helium, TemperatureWe present a first-principles computational study of solid He-4 at T = 0 K and pressures up to similar to 160 GPa. Our computational strategy consists in using van der Waals density functional theory (DFT-vdW) to describe the electronic degrees of freedom in this material, and the diffusion Monte Carlo (DMC) method to solve the Schrodinger equation describing the behavior of the quantum nuclei. For this, we construct an analytical interaction function based on the pairwise Aziz potential that closely matches the volume variation of the cohesive energy calculated with DFT-vdW in dense helium. Interestingly, we find that the kinetic energy of solid He-4 does not increase appreciably with compression for P >= 85 GPa. Also, we show that the Lindemann ratio in dense solid He-4 amounts to 0.10 almost independently of pressure. The reliability of customary quasiharmonic DFT (QH DFT) approaches in describing quantum nuclear effects in solids is also studied. We find that QH DFT simulations, although provide a reasonable equation of state in agreement with experiments, are not able to reproduce correctly these critical effects in compressed He-4. In particular, we disclose huge discrepancies of at least similar to 50% in the calculated He-4 kinetic energies using both the QH DFT and present DFT-DMC methods.Beam focalization in reflection from flat dielectric subwavelength gratings
http://hdl.handle.net/2117/26401
Title: Beam focalization in reflection from flat dielectric subwavelength gratings
Authors: Cheng, Yu Chieh; Zeng, H; Trullàs Simó, Joaquim; Cojocaru, Crina; Malinauskas, Mangirdas; Jukna, T; Wiersma, D.S.; Staliunas, Kestutis
Abstract: We experimentally demonstrate the recently predicted effect of near-field focusing for light beams from flat dielectric subwavelength gratings (SWGs). This SWGs were designed for visible light 532 nm and fabricated by direct laser writing in a negative photoresist, with the refractive index n = 1.5 and the period d = 314 nm. The laterally invariant gratings can focus light beams without any optical axis to achieve the transversal invariance. We show that focal distances can be obtained up to 13 mu m at normal reflection for TE polarization.Wed, 18 Feb 2015 07:43:19 GMThttp://hdl.handle.net/2117/264012015-02-18T07:43:19ZCheng, Yu Chieh; Zeng, H; Trullàs Simó, Joaquim; Cojocaru, Crina; Malinauskas, Mangirdas; Jukna, T; Wiersma, D.S.; Staliunas, KestutisnoNegative refraction, Photonic crystals, Transmission, Laser, LensWe experimentally demonstrate the recently predicted effect of near-field focusing for light beams from flat dielectric subwavelength gratings (SWGs). This SWGs were designed for visible light 532 nm and fabricated by direct laser writing in a negative photoresist, with the refractive index n = 1.5 and the period d = 314 nm. The laterally invariant gratings can focus light beams without any optical axis to achieve the transversal invariance. We show that focal distances can be obtained up to 13 mu m at normal reflection for TE polarization.Luttinger-liquid behavior of one-dimensional He-3
http://hdl.handle.net/2117/26200
Title: Luttinger-liquid behavior of one-dimensional He-3
Authors: Astrakharchik, Grigori; Boronat Medico, Jordi
Abstract: The ground-state properties of one-dimensional He-3 are studied using quantum Monte Carlo methods. The equation of state is calculated in a wide range of physically relevant densities and is well interpolated by a power-series fit. The Luttinger liquid theory is found to describe the long-range properties of the correlation functions. The density dependence of the Luttinger parameter is explicitly found, and interestingly it shows a nonmonotonic behavior. Depending on the density, the static structure factor can be a smooth function of the momentum or might contain a peak of a finite or infinite height. Although no phase transitions are present in the system, we identify a number of physically different regimes, including an ideal Fermi gas, aTue, 03 Feb 2015 18:41:49 GMThttp://hdl.handle.net/2117/262002015-02-03T18:41:49ZAstrakharchik, Grigori; Boronat Medico, JordinoGround-state, Electron-gas, Anderson localization, Single-barrier, Quantum fluids, Monte-Carlo, Bose-gas, Transport, Crystal, SystemsThe ground-state properties of one-dimensional He-3 are studied using quantum Monte Carlo methods. The equation of state is calculated in a wide range of physically relevant densities and is well interpolated by a power-series fit. The Luttinger liquid theory is found to describe the long-range properties of the correlation functions. The density dependence of the Luttinger parameter is explicitly found, and interestingly it shows a nonmonotonic behavior. Depending on the density, the static structure factor can be a smooth function of the momentum or might contain a peak of a finite or infinite height. Although no phase transitions are present in the system, we identify a number of physically different regimes, including an ideal Fermi gas, aQuantum phase transition with a simple variational ansatz
http://hdl.handle.net/2117/25919
Title: Quantum phase transition with a simple variational ansatz
Authors: Lutsyshyn, Yaroslav; Astrakharchik, Grigori; Cazorla, C; Boronat Medico, Jordi
Abstract: We study the zero-temperature quantum phase transition between liquid and hcp solid He-4. We use the variational method with a simple yet exchange-symmetric and fully explicit wave function. It is found that the optimized wave function undergoes spontaneous symmetry breaking and describes the quantum solidification of helium at 22 atm. The explicit form of the wave function allows us to consider various contributions to the phase transition. We find that the employed wave function is an excellent candidate for describing both a first-order quantum phase transition and the ground state of a Bose solid.Mon, 19 Jan 2015 15:23:44 GMThttp://hdl.handle.net/2117/259192015-01-19T15:23:44ZLutsyshyn, Yaroslav; Astrakharchik, Grigori; Cazorla, C; Boronat Medico, JordinoShadow wave-function, Ground-state, Solid He-4, Monte-Carlo, Bulk He-4, Liquid, Helium, Supersolidity, Potentials, SystemsWe study the zero-temperature quantum phase transition between liquid and hcp solid He-4. We use the variational method with a simple yet exchange-symmetric and fully explicit wave function. It is found that the optimized wave function undergoes spontaneous symmetry breaking and describes the quantum solidification of helium at 22 atm. The explicit form of the wave function allows us to consider various contributions to the phase transition. We find that the employed wave function is an excellent candidate for describing both a first-order quantum phase transition and the ground state of a Bose solid.Distinguishability, degeneracy, and correlations in three harmonically trapped bosons in one dimension
http://hdl.handle.net/2117/25917
Title: Distinguishability, degeneracy, and correlations in three harmonically trapped bosons in one dimension
Authors: Garcia March, Miguel Angel; Julia Diaz, Bruno; Astrakharchik, Grigori; Boronat Medico, Jordi; Polls, A.
Abstract: We study a system of two bosons of one species and a third atom of a second species in a one-dimensional parabolic trap at zero temperature. We assume contact repulsive inter- and intraspecies interactions. By means of an exact diagonalization method we calculate the ground and excited states for the whole range of interactions. We use discrete group theory to classify the eigenstates according to the symmetry of the interaction potential. We also propose and validate analytical Ansatze gaining physical insight over the numerically obtained wave functions. We show that, for both approaches, it is crucial to take into account that the distinguishability of the third atom implies the absence of any restriction over the wave function when interchanging this boson with any of the other two. We find that there are degeneracies in the spectra in some limiting regimes, that is, when the interspecies and/or the intraspecies interactions tend to infinity. This is in contrast with the three-identical boson system, where no degeneracy occurs in these limits. We show that, when tuning both types of interactions through a protocol that keeps them equal while they are increased towards infinity, the systems's ground state resembles that of three indistinguishable bosons. Contrarily, the systems's ground state is different from that of three-identical bosons when both types of interactions are increased towards infinity through protocols that do not restrict them to be equal. We study the coherence and correlations of the system as the interactions are tuned through different protocols, which permit us to build up different correlations in the system and lead to different spatial distributions of the three atoms.Mon, 19 Jan 2015 14:47:48 GMThttp://hdl.handle.net/2117/259172015-01-19T14:47:48ZGarcia March, Miguel Angel; Julia Diaz, Bruno; Astrakharchik, Grigori; Boronat Medico, Jordi; Polls, A.noTonks-Girardeau gas, Impenetrable bosons, Atoms, Fermions, SystemsWe study a system of two bosons of one species and a third atom of a second species in a one-dimensional parabolic trap at zero temperature. We assume contact repulsive inter- and intraspecies interactions. By means of an exact diagonalization method we calculate the ground and excited states for the whole range of interactions. We use discrete group theory to classify the eigenstates according to the symmetry of the interaction potential. We also propose and validate analytical Ansatze gaining physical insight over the numerically obtained wave functions. We show that, for both approaches, it is crucial to take into account that the distinguishability of the third atom implies the absence of any restriction over the wave function when interchanging this boson with any of the other two. We find that there are degeneracies in the spectra in some limiting regimes, that is, when the interspecies and/or the intraspecies interactions tend to infinity. This is in contrast with the three-identical boson system, where no degeneracy occurs in these limits. We show that, when tuning both types of interactions through a protocol that keeps them equal while they are increased towards infinity, the systems's ground state resembles that of three indistinguishable bosons. Contrarily, the systems's ground state is different from that of three-identical bosons when both types of interactions are increased towards infinity through protocols that do not restrict them to be equal. We study the coherence and correlations of the system as the interactions are tuned through different protocols, which permit us to build up different correlations in the system and lead to different spatial distributions of the three atoms.Phase diagram of dipolar bosons in two dimensions with tilted polarization
http://hdl.handle.net/2117/25911
Title: Phase diagram of dipolar bosons in two dimensions with tilted polarization
Authors: Macia Rey, Adrián; Boronat Medico, Jordi; Mazzanti Castrillejo, Fernando Pablo
Abstract: We analyze the ground state of a system of dipolar bosons moving in the XY plane and such that their dipolar moments are all aligned in a fixed direction in space. We focus on the general case where the polarization field forms a generic angle a with respect to the Z axis. We use the path-integral ground-state method to analyze the static properties of the system as both a and the density n vary over a wide range where the system is stable. We use the maximum of the static structure function as an order parameter to characterize the different phases and the transition lines among them. We find that, in addition to a superfluid gas and a solid phase, the system reaches a stripe phase at large tilting angles that is entirely induced by the anisotropic character of the interaction. We also show that the quantum phase transition from the gas to the stripe phase is of second order and report approximate values for the critical exponents.Mon, 19 Jan 2015 14:16:59 GMThttp://hdl.handle.net/2117/259112015-01-19T14:16:59ZMacia Rey, Adrián; Boronat Medico, Jordi; Mazzanti Castrillejo, Fernando PablonoGasesWe analyze the ground state of a system of dipolar bosons moving in the XY plane and such that their dipolar moments are all aligned in a fixed direction in space. We focus on the general case where the polarization field forms a generic angle a with respect to the Z axis. We use the path-integral ground-state method to analyze the static properties of the system as both a and the density n vary over a wide range where the system is stable. We use the maximum of the static structure function as an order parameter to characterize the different phases and the transition lines among them. We find that, in addition to a superfluid gas and a solid phase, the system reaches a stripe phase at large tilting angles that is entirely induced by the anisotropic character of the interaction. We also show that the quantum phase transition from the gas to the stripe phase is of second order and report approximate values for the critical exponents.Molecular dynamics simulation of single ions in aqueous solutions: effects of the flexibility of the water molecules
http://hdl.handle.net/2117/25529
Title: Molecular dynamics simulation of single ions in aqueous solutions: effects of the flexibility of the water molecules
Authors: Guàrdia Manuel, Elvira
Abstract: We performed molecular dynamics simulations of single Na’ and F ions in aqueous solutions. Two single point charge water models with and without internal degrees of freedom were considered. Structural (radial distribution functions, orientation angles), dynamical (translational, vibrational, and reorientational motions), and other microscopic properties (hydration numbers, residence times) of ions and water molecules of their hydration shell were calculated. Our results are compared both with experimental data and with other simulation results using different interaction models. The influence of the flexibility of water molecules on the different properties is carefully discussed.Wed, 14 Jan 2015 19:01:42 GMThttp://hdl.handle.net/2117/255292015-01-14T19:01:42ZGuàrdia Manuel, ElviranoWe performed molecular dynamics simulations of single Na’ and F ions in aqueous solutions. Two single point charge water models with and without internal degrees of freedom were considered. Structural (radial distribution functions, orientation angles), dynamical (translational, vibrational, and reorientational motions), and other microscopic properties (hydration numbers, residence times) of ions and water molecules of their hydration shell were calculated. Our results are compared both with experimental data and with other simulation results using different interaction models. The influence of the flexibility of water molecules on the different properties is carefully discussed.Ionic association of Na+-Cl-, Na+-Na+ and Cl--Cl- in methanol: mean force potentials and friction kernels
http://hdl.handle.net/2117/25524
Title: Ionic association of Na+-Cl-, Na+-Na+ and Cl--Cl- in methanol: mean force potentials and friction kernels
Authors: Sesé Castel, Gemma; Guàrdia Manuel, Elvira; Padro Cardenas, Joan Angel
Abstract: Kinetic investigation of 1-octene bromination in AOT-isooctane-water microemulsions (13 = w = [H2O]/[AOT] = 24 and 6 = z = [IO]/[AOT] = 57) shows that the reaction is first-order in alkene and first-order in bromine, as usually found in protic media. Although both reagents are mainly located in the isooctane phase (Ktr, transfer coefficients from isooctane to water, are 1.5 × 10-5 and 8.8 × 10-3 for alkene and bromine, respectively), bromination occurs in an aqueous microenvironment, as illustrated by the high sensitivity of the bromination rate to the water content of the microemulsion. A kinetic pseudophase model describes the rate constant dependence on microemulsion composition satisfactorily by assuming competition between reactions at the interface and in the aqueous phase. Reasonable values for the coefficients of reagent partition between the interface and the two microphases and for the local bromination rate constants are obtained from the kinetic equations derived from the model. In particular, spectroscopically observed AOT-bromine complexation is in agreement with the high bromine concentration at the interface (K2, bromine partition coefficient from isooctane to interface, = 6.8). The water-phase bromination rate constant, kw = 1 × 108 M-1 s-1, is in the same range as that measured in bulk water. The lower limit for the interfacial rate constant, ki, is 103 M-1 s-1, a value close to that observed in poorly aqueous methanol (MeOH/H2O = 95/5 v/v). These data are compared with those recently obtained in the same microemulsions for solvolysis, a reaction which, like bromination, is water-promoted but supposed to take place at the interface only. The results are discussed in terms of the chemical properties of the water molecules encased in the microemulsion droplets.Wed, 14 Jan 2015 17:29:40 GMThttp://hdl.handle.net/2117/255242015-01-14T17:29:40ZSesé Castel, Gemma; Guàrdia Manuel, Elvira; Padro Cardenas, Joan AngelnoKinetic investigation of 1-octene bromination in AOT-isooctane-water microemulsions (13 = w = [H2O]/[AOT] = 24 and 6 = z = [IO]/[AOT] = 57) shows that the reaction is first-order in alkene and first-order in bromine, as usually found in protic media. Although both reagents are mainly located in the isooctane phase (Ktr, transfer coefficients from isooctane to water, are 1.5 × 10-5 and 8.8 × 10-3 for alkene and bromine, respectively), bromination occurs in an aqueous microenvironment, as illustrated by the high sensitivity of the bromination rate to the water content of the microemulsion. A kinetic pseudophase model describes the rate constant dependence on microemulsion composition satisfactorily by assuming competition between reactions at the interface and in the aqueous phase. Reasonable values for the coefficients of reagent partition between the interface and the two microphases and for the local bromination rate constants are obtained from the kinetic equations derived from the model. In particular, spectroscopically observed AOT-bromine complexation is in agreement with the high bromine concentration at the interface (K2, bromine partition coefficient from isooctane to interface, = 6.8). The water-phase bromination rate constant, kw = 1 × 108 M-1 s-1, is in the same range as that measured in bulk water. The lower limit for the interfacial rate constant, ki, is 103 M-1 s-1, a value close to that observed in poorly aqueous methanol (MeOH/H2O = 95/5 v/v). These data are compared with those recently obtained in the same microemulsions for solvolysis, a reaction which, like bromination, is water-promoted but supposed to take place at the interface only. The results are discussed in terms of the chemical properties of the water molecules encased in the microemulsion droplets.Structure and dynamics of liquid ethanol
http://hdl.handle.net/2117/25516
Title: Structure and dynamics of liquid ethanol
Authors: Saiz, L; Padro Cardenas, Joan Angel; Guàrdia Manuel, Elvira
Abstract: Molecular dynamics simulations of liquid ethanol at four thermodynamic states ranging from T = 173 K to T = 348 K were carried out using the transferable OPLS potential model of Jorgensen (J. Phys. Chem. 1986, 90, 1276). Both static and dynamic properties are analyzed. The resulting properties show an overall agreement with available experimental data. Special attention is paid to the hydrogen bonds and to their influence on the molecular behavior. Results for liquid ethanol are compared with those for methanol in earlier computer simulation studies.Wed, 14 Jan 2015 15:38:22 GMThttp://hdl.handle.net/2117/255162015-01-14T15:38:22ZSaiz, L; Padro Cardenas, Joan Angel; Guàrdia Manuel, ElviranoMolecular dynamics simulations of liquid ethanol at four thermodynamic states ranging from T = 173 K to T = 348 K were carried out using the transferable OPLS potential model of Jorgensen (J. Phys. Chem. 1986, 90, 1276). Both static and dynamic properties are analyzed. The resulting properties show an overall agreement with available experimental data. Special attention is paid to the hydrogen bonds and to their influence on the molecular behavior. Results for liquid ethanol are compared with those for methanol in earlier computer simulation studies.Dielectric properties of liquid ethanol: a computer simulation study
http://hdl.handle.net/2117/25515
Title: Dielectric properties of liquid ethanol: a computer simulation study
Authors: Saiz, L; Guàrdia Manuel, Elvira; Padro Cardenas, Joan Angel
Abstract: Static and dynamic dielectric properties of liquidethanol have been studied as a function of the wave-vector number by computer simulation.Molecular dynamics simulations at room temperature have been performed using the optimized potentials for liquid simulations (OPLS) potential model proposed by Jorgensen [J. Phys. Chem. 90, 1276 (1986)]. The time dependent correlation functions of the longitudinal and transverse components of the dipole density as well as the individual and total dipole moment autocorrelation functions have been calculated. The infrared spectra and the dielectric relaxation of the liquid have been also analyzed. Results have been compared with the available experimental data. Special attention has been dedicated to investigate the molecular origin of the different analyzed properties.Wed, 14 Jan 2015 15:04:33 GMThttp://hdl.handle.net/2117/255152015-01-14T15:04:33ZSaiz, L; Guàrdia Manuel, Elvira; Padro Cardenas, Joan AngelnoLiquid dielectrics, Computer simulation
Dielectric properties
Ethanol
Correlation functionsStatic and dynamic dielectric properties of liquidethanol have been studied as a function of the wave-vector number by computer simulation.Molecular dynamics simulations at room temperature have been performed using the optimized potentials for liquid simulations (OPLS) potential model proposed by Jorgensen [J. Phys. Chem. 90, 1276 (1986)]. The time dependent correlation functions of the longitudinal and transverse components of the dipole density as well as the individual and total dipole moment autocorrelation functions have been calculated. The infrared spectra and the dielectric relaxation of the liquid have been also analyzed. Results have been compared with the available experimental data. Special attention has been dedicated to investigate the molecular origin of the different analyzed properties.Protons in supercritical water: a multistate empirical valence bond study
http://hdl.handle.net/2117/25513
Title: Protons in supercritical water: a multistate empirical valence bond study
Authors: Laria, D; Martí Rabassa, Jordi; Guàrdia Manuel, Elvira
Abstract: Molecular dynamics simulations have been performed to analyze microscopic details related to aqueous solvation of excess protons along the supercritical T = 673 K isotherm, spanning a density interval from a typical liquid down to vapor environments. The simulation methodology relies on a multistate empirical valence bond Hamiltonian model that includes a proton translocation mechanism. Our results predict a gradual stabilization of the solvated Eigen cation [H3O·(H2O)3]+ at lower densities, in detriment of the symmetric Zundel dimer [H·(H2O)2]+. At all densities, the average solvation structure in the close vicinity of the hydronium is characterized by three hydrogen bond acceptor water molecules and presents minor changes in the solute water distances. Characteristic times for the proton translocation jumps have been computed using population relaxation time correlation functions. Compared to room temperature results, the rates at high densities are 4 times faster and become progressively slower in steamlike environments. Diffusion coefficients for the excess proton have also been computed. In agreement with conductometric data, our results show that contributions from the Grotthus mechanism to the overall proton transport diminish at lower densities and predict that in steamlike environments, the proton diffusion is almost 1 order of magnitude slower than that for pure water. Spectroscopic information for the solvated proton is accordant to the gradual prevalence of proton localization in Eigen-like structures at lower densities.Wed, 14 Jan 2015 14:43:26 GMThttp://hdl.handle.net/2117/255132015-01-14T14:43:26ZLaria, D; Martí Rabassa, Jordi; Guàrdia Manuel, ElviranoMolecular dynamics simulations have been performed to analyze microscopic details related to aqueous solvation of excess protons along the supercritical T = 673 K isotherm, spanning a density interval from a typical liquid down to vapor environments. The simulation methodology relies on a multistate empirical valence bond Hamiltonian model that includes a proton translocation mechanism. Our results predict a gradual stabilization of the solvated Eigen cation [H3O·(H2O)3]+ at lower densities, in detriment of the symmetric Zundel dimer [H·(H2O)2]+. At all densities, the average solvation structure in the close vicinity of the hydronium is characterized by three hydrogen bond acceptor water molecules and presents minor changes in the solute water distances. Characteristic times for the proton translocation jumps have been computed using population relaxation time correlation functions. Compared to room temperature results, the rates at high densities are 4 times faster and become progressively slower in steamlike environments. Diffusion coefficients for the excess proton have also been computed. In agreement with conductometric data, our results show that contributions from the Grotthus mechanism to the overall proton transport diminish at lower densities and predict that in steamlike environments, the proton diffusion is almost 1 order of magnitude slower than that for pure water. Spectroscopic information for the solvated proton is accordant to the gradual prevalence of proton localization in Eigen-like structures at lower densities.Exploring the picosecond time domain of the solvation dynamics of coumarin 153 within beta-cyclodextrins
http://hdl.handle.net/2117/25511
Title: Exploring the picosecond time domain of the solvation dynamics of coumarin 153 within beta-cyclodextrins
Authors: RODRIGUEZ, J; Martí Rabassa, Jordi; Guàrdia Manuel, Elvira; Laria, D
Abstract: We report molecular dynamics simulation results of equilibrium and dynamical characteristics pertaining to the solvation of the dye coumarin 153 (C153) trapped within hydrophobic cavities of di- and trimethylated ß-cyclodextrins (CD) in aqueous solutions. We found that stable configurations of the encapsulated probe are characterized by a slanted docking, in which the plane of the C153 lies mostly parallel to one of the glucose units of the CD. “In and out” dynamical modes of the encapsulated probe present very small amplitudes. The rotational dynamics of the trapped coumarin can be cast in terms of a simple model that includes diffusive motions within a local restrictive environment coupled to the overall rotational motion of the CD. We have examined the early stages of the solvation response of the environment following a vertical excitation of the probe. Regardless of the degree of CD methylation, the water dynamical response seems to be completed within 2-3 ps and does not differ substantially from that observed for nonencapsulated probes. The CD response is characterized by a single, subpicosecond relaxation that involves intramolecular motions. We also explored dynamical modes that could account for the recently reported persistence of Stokes shifts in the nanosecond time domain. In all cases, the only sources of ultraslow dynamics that we detected were those associated with gauche-trans interconversions in primary hydroxyl chains of the CD, which do not seem to be directly connected to the electronic excitation of the probe.Wed, 14 Jan 2015 14:12:24 GMThttp://hdl.handle.net/2117/255112015-01-14T14:12:24ZRODRIGUEZ, J; Martí Rabassa, Jordi; Guàrdia Manuel, Elvira; Laria, DnoWe report molecular dynamics simulation results of equilibrium and dynamical characteristics pertaining to the solvation of the dye coumarin 153 (C153) trapped within hydrophobic cavities of di- and trimethylated ß-cyclodextrins (CD) in aqueous solutions. We found that stable configurations of the encapsulated probe are characterized by a slanted docking, in which the plane of the C153 lies mostly parallel to one of the glucose units of the CD. “In and out” dynamical modes of the encapsulated probe present very small amplitudes. The rotational dynamics of the trapped coumarin can be cast in terms of a simple model that includes diffusive motions within a local restrictive environment coupled to the overall rotational motion of the CD. We have examined the early stages of the solvation response of the environment following a vertical excitation of the probe. Regardless of the degree of CD methylation, the water dynamical response seems to be completed within 2-3 ps and does not differ substantially from that observed for nonencapsulated probes. The CD response is characterized by a single, subpicosecond relaxation that involves intramolecular motions. We also explored dynamical modes that could account for the recently reported persistence of Stokes shifts in the nanosecond time domain. In all cases, the only sources of ultraslow dynamics that we detected were those associated with gauche-trans interconversions in primary hydroxyl chains of the CD, which do not seem to be directly connected to the electronic excitation of the probe.A continuous mixture of two different dimers in liquid water
http://hdl.handle.net/2117/25450
Title: A continuous mixture of two different dimers in liquid water
Authors: Pardo Soto, Luis Carlos; Henao Aristizábal, Andrés; Busch, Sebastian; Guàrdia Manuel, Elvira; Tamarit Mur, José Luis
Abstract: It is hitherto thought that liquid water is composed of tetrahedrally coordinated molecules with an asymmetric interaction of the central molecule with neighboring molecules. Kuhne et al., Nat. Commun., 2013, 4, 1450 suggested that this asymmetry, energetic rather than geometric, is the cornerstone to reconcile the homogeneous and inhomogeneous viewpoints of liquid water. In order to investigate the geometric origin of that asymmetry, we have scrutinized Molecular Dynamics (MD) simulations of water through a careful analysis of the five-dimensional probability distribution function of Euler angles in which the relative positions and orientations of water molecules are obtained. We demonstrate that, beyond the ubiquitous tetrahedral structure with well-defined molecular dimers, there is a series of possible molecular orientations that define the structure. These orientations are generated by rotating the neighboring molecule around the O-H axis that is involved in the hydrogen bond scheme. Two of the possible orientations have a higher probability, giving rise to two kinds of dimers: one close to the lowest energy of a water dimer in vacuum with an almost perpendicular alignment of the dipole moment, and another one with a parallel orientation of the dipole moment which is less tightly bound. These two different dimers have an effect on the orientation of further water dipole moments up to a distance of approximate to 6 angstrom. Liquid water can therefore be described as a continuous mixture of two kinds of dimers where the hydrogen bonds have the same geometry but the interaction energies are different due to a different mutual orientation of the dipoles of the participating water molecules.Wed, 14 Jan 2015 12:34:05 GMThttp://hdl.handle.net/2117/254502015-01-14T12:34:05ZPardo Soto, Luis Carlos; Henao Aristizábal, Andrés; Busch, Sebastian; Guàrdia Manuel, Elvira; Tamarit Mur, José LuisnoIt is hitherto thought that liquid water is composed of tetrahedrally coordinated molecules with an asymmetric interaction of the central molecule with neighboring molecules. Kuhne et al., Nat. Commun., 2013, 4, 1450 suggested that this asymmetry, energetic rather than geometric, is the cornerstone to reconcile the homogeneous and inhomogeneous viewpoints of liquid water. In order to investigate the geometric origin of that asymmetry, we have scrutinized Molecular Dynamics (MD) simulations of water through a careful analysis of the five-dimensional probability distribution function of Euler angles in which the relative positions and orientations of water molecules are obtained. We demonstrate that, beyond the ubiquitous tetrahedral structure with well-defined molecular dimers, there is a series of possible molecular orientations that define the structure. These orientations are generated by rotating the neighboring molecule around the O-H axis that is involved in the hydrogen bond scheme. Two of the possible orientations have a higher probability, giving rise to two kinds of dimers: one close to the lowest energy of a water dimer in vacuum with an almost perpendicular alignment of the dipole moment, and another one with a parallel orientation of the dipole moment which is less tightly bound. These two different dimers have an effect on the orientation of further water dipole moments up to a distance of approximate to 6 angstrom. Liquid water can therefore be described as a continuous mixture of two kinds of dimers where the hydrogen bonds have the same geometry but the interaction energies are different due to a different mutual orientation of the dipoles of the participating water molecules.