http://hdl.handle.net/2117/395 Thu, 23 May 2013 15:58:26 GMT 2013-05-23T15:58:26Z webmaster.bupc@upc.edu Universitat Politècnica de Catalunya. Servei de Biblioteques i Documentació no http://hdl.handle.net/2117/19248 Title: Phase transitions with infinitely many absorbing states in complex networks Authors: Sander, Renan; Ferreira, Silvio C.; Pastor Satorras, Romualdo Abstract: We investigate the properties of the threshold contact process (TCP), a process showing an absorbing-state phase transition with infinitely many absorbing states, on random complex networks. The finite-size scaling exponents characterizing the transition are obtained in a heterogeneous mean-field (HMF) approximation and compared with extensive simulations, particularly in the case of heterogeneous scale-free networks. We observe that the TCP exhibits the same critical properties as the contact process, which undergoes an absorbing-state phase transition to a single absorbing state. The accordance among the critical exponents of different models and networks leads to conjecture that the critical behavior of the contact process in a HMF theory is a universal feature of absorbing-state phase transitions in complex networks, depending only on the locality of the interactions and independent of the number of absorbing states. The conditions for the applicability of the conjecture are discussed considering a parallel with the susceptible-infected-susceptible epidemic spreading model, which in fact belongs to a different universality class in complex networks. Wed, 15 May 2013 12:52:57 GMT http://hdl.handle.net/2117/19248 2013-05-15T12:52:57Z Sander, Renan; Ferreira, Silvio C.; Pastor Satorras, Romualdo no SCALE-FREE NETWORKS, SYSTEMS, LATTICE, MODEL We investigate the properties of the threshold contact process (TCP), a process showing an absorbing-state phase transition with infinitely many absorbing states, on random complex networks. The finite-size scaling exponents characterizing the transition are obtained in a heterogeneous mean-field (HMF) approximation and compared with extensive simulations, particularly in the case of heterogeneous scale-free networks. We observe that the TCP exhibits the same critical properties as the contact process, which undergoes an absorbing-state phase transition to a single absorbing state. The accordance among the critical exponents of different models and networks leads to conjecture that the critical behavior of the contact process in a HMF theory is a universal feature of absorbing-state phase transitions in complex networks, depending only on the locality of the interactions and independent of the number of absorbing states. The conditions for the applicability of the conjecture are discussed considering a parallel with the susceptible-infected-susceptible epidemic spreading model, which in fact belongs to a different universality class in complex networks. http://hdl.handle.net/2117/18430 Title: Evolution in a changing environment Authors: Baronchelli, Andrea; Chater, Nick; Christiansen, M.H.; Pastor Satorras, Romualdo Abstract: We propose a simple model for genetic adaptation to a changing environment, describing a fitness landscape characterized by two maxima. One is associated with “specialist” individuals that are adapted to the environment; this maximum moves over time as the environment changes. The other maximum is static, and represents “generalist” individuals not affected by environmental changes. The rest of the landscape is occupied by “maladapted” individuals. Our analysis considers the evolution of these three subpopulations. Our main result is that, in presence of a sufficiently stable environmental feature, as in the case of an unchanging aspect of a physical habitat, specialists can dominate the population. By contrast, rapidly changing environmental features, such as language or cultural habits, are a moving target for the genes; here, generalists dominate, because the best evolutionary strategy is to adopt neutral alleles not specialized for any specific environment. The model we propose is based on simple assumptions about evolutionary dynamics and describes all possible scenarios in a non-trivial phase diagram. The approach provides a general framework to address such fundamental issues as the Baldwin effect, the biological basis for language, or the ecological consequences of a rapid climate change. Tue, 19 Mar 2013 19:45:52 GMT http://hdl.handle.net/2117/18430 2013-03-19T19:45:52Z Baronchelli, Andrea; Chater, Nick; Christiansen, M.H.; Pastor Satorras, Romualdo no We propose a simple model for genetic adaptation to a changing environment, describing a fitness landscape characterized by two maxima. One is associated with “specialist” individuals that are adapted to the environment; this maximum moves over time as the environment changes. The other maximum is static, and represents “generalist” individuals not affected by environmental changes. The rest of the landscape is occupied by “maladapted” individuals. Our analysis considers the evolution of these three subpopulations. Our main result is that, in presence of a sufficiently stable environmental feature, as in the case of an unchanging aspect of a physical habitat, specialists can dominate the population. By contrast, rapidly changing environmental features, such as language or cultural habits, are a moving target for the genes; here, generalists dominate, because the best evolutionary strategy is to adopt neutral alleles not specialized for any specific environment. The model we propose is based on simple assumptions about evolutionary dynamics and describes all possible scenarios in a non-trivial phase diagram. The approach provides a general framework to address such fundamental issues as the Baldwin effect, the biological basis for language, or the ecological consequences of a rapid climate change. http://hdl.handle.net/2117/16982 Title: Excess protons in mesoscopic water-acetone nanoclusters Authors: Semino, Rocío; Martí Rabassa, Jordi; Guàrdia Manuel, Elvira; Laria, Daniel Abstract: We carried out molecular dynamics simulation experiments to examine equilibrium and dynamical characteristics of the solvation of excess protons in mesoscopic, [m:n] binary polar clusters comprising m = 50 water molecules and n = 6, 25, and 100 acetone molecules. Contrasting from what is found in conventional macroscopic phases, the characteristics of the proton solvation are dictated, to a large extent, by the nature of the concentration fluctuations prevailing within the clusters. At low acetone contents, the overall cluster morphology corresponds to a segregated aqueous nucleus coated by an external aprotic phase. Under these circumstances, the proton remains localized at the surface of the water core, in a region locally deprived from acetone molecules. At higher acetone concentrations, we found clear evidence of the onset of the mixing process. The cluster structures present aqueous domains with irregular shape, fully embedded within the acetone phase. Still, the proton remains coordinated to the aqueous phase, with its closest solvation shell composed exclusively by three water molecules. As the relative concentration of acetone increases, the time scales characterizing proton transfer events between neighboring water molecules show considerable retardations, stretching into the nanosecond time domain already for n ∼ 25. In water-rich aggregates, and similarly to what is found in the bulk, proton transfers are controlled by acetone/water exchange processes taking place at the second solvation shell of the proton. As a distinctive feature of the transfer mechanism, translocation pathways also include diffusive motions of the proton from the surface down into inner regions of the underlying water domain. Tue, 20 Nov 2012 18:12:21 GMT http://hdl.handle.net/2117/16982 2012-11-20T18:12:21Z Semino, Rocío; Martí Rabassa, Jordi; Guàrdia Manuel, Elvira; Laria, Daniel no We carried out molecular dynamics simulation experiments to examine equilibrium and dynamical characteristics of the solvation of excess protons in mesoscopic, [m:n] binary polar clusters comprising m = 50 water molecules and n = 6, 25, and 100 acetone molecules. Contrasting from what is found in conventional macroscopic phases, the characteristics of the proton solvation are dictated, to a large extent, by the nature of the concentration fluctuations prevailing within the clusters. At low acetone contents, the overall cluster morphology corresponds to a segregated aqueous nucleus coated by an external aprotic phase. Under these circumstances, the proton remains localized at the surface of the water core, in a region locally deprived from acetone molecules. At higher acetone concentrations, we found clear evidence of the onset of the mixing process. The cluster structures present aqueous domains with irregular shape, fully embedded within the acetone phase. Still, the proton remains coordinated to the aqueous phase, with its closest solvation shell composed exclusively by three water molecules. As the relative concentration of acetone increases, the time scales characterizing proton transfer events between neighboring water molecules show considerable retardations, stretching into the nanosecond time domain already for n ∼ 25. In water-rich aggregates, and similarly to what is found in the bulk, proton transfers are controlled by acetone/water exchange processes taking place at the second solvation shell of the proton. As a distinctive feature of the transfer mechanism, translocation pathways also include diffusive motions of the proton from the surface down into inner regions of the underlying water domain. http://hdl.handle.net/2117/16829 Title: Software refactoring: solving the time-dependent Schrodinger equation via fast Fourier transforms and parallel programming Authors: Khwaldeh, Ali; Tahat, Amani; Martí Rabassa, Jordi Abstract: In this study a multiprocessor C++ message passing interface implementation of a new bit-reversal algorithm to numerically solve the time dependent Schrodinger equation using a spectral method based on Fourier transform was presented. The major issues of parallel computer performance were discussed in terms of efficiency; speed up, cost and fraction of the execution time that could be parallelized. The scalable performance to a very high number of processors was addressed as well as compared with ideal values of Amdahl’s Law when presenting the parallel performance of the new developed algorithms. The results showed that message passing interface was an optimal method of implementing a parallelized bit-reversal algorithm. Fri, 02 Nov 2012 14:27:57 GMT http://hdl.handle.net/2117/16829 2012-11-02T14:27:57Z Khwaldeh, Ali; Tahat, Amani; Martí Rabassa, Jordi no In this study a multiprocessor C++ message passing interface implementation of a new bit-reversal algorithm to numerically solve the time dependent Schrodinger equation using a spectral method based on Fourier transform was presented. The major issues of parallel computer performance were discussed in terms of efficiency; speed up, cost and fraction of the execution time that could be parallelized. The scalable performance to a very high number of processors was addressed as well as compared with ideal values of Amdahl’s Law when presenting the parallel performance of the new developed algorithms. The results showed that message passing interface was an optimal method of implementing a parallelized bit-reversal algorithm. http://hdl.handle.net/2117/16719 Title: Slow dynamics and rare-region effects in the contact process on weighted tree networks Authors: Odor, G.; Pastor Satorras, Romualdo Abstract: We show that generic, slow dynamics can occur in the contact process on complex networks with a tree-like structure and a superimposed weight pattern, in the absence of additional (nontopological) sources of quenched disorder. The slow dynamics is induced by rare-region effects occurring on correlated subspaces of vertices connected by large weight edges and manifests in the form of a smeared phase transition. We conjecture that more sophisticated network motifs could be able to induce Griffiths phases, as a consequence of purely topological disorder. Thu, 11 Oct 2012 13:21:51 GMT http://hdl.handle.net/2117/16719 2012-10-11T13:21:51Z Odor, G.; Pastor Satorras, Romualdo no Complex networks, Contact process, Network motif, Quenched disorder, Slow dynamics, Topological disorder, Tree-like structures, Weighted tree We show that generic, slow dynamics can occur in the contact process on complex networks with a tree-like structure and a superimposed weight pattern, in the absence of additional (nontopological) sources of quenched disorder. The slow dynamics is induced by rare-region effects occurring on correlated subspaces of vertices connected by large weight edges and manifests in the form of a smeared phase transition. We conjecture that more sophisticated network motifs could be able to induce Griffiths phases, as a consequence of purely topological disorder. http://hdl.handle.net/2117/16713 Title: Activity driven modeling of time varying networks Authors: Perra, N.; Gonçalves, B.; Pastor Satorras, Romualdo; Vespignani, Alessandro Abstract: Network modeling plays a critical role in identifying statistical regularities and structural principles common to many systems. The large majority of recent modeling approaches are connectivity driven. The structural patterns of the network are at the basis of the mechanisms ruling the network formation. Connectivity driven models necessarily provide a time-aggregated representation that may fail to describe the instantaneous and fluctuating dynamics of many networks. We address this challenge by defining the activity potential, a time invariant function characterizing the agents' interactions and constructing an activity driven model capable of encoding the instantaneous time description of the network dynamics. The model provides an explanation of structural features such as the presence of hubs, which simply originate from the heterogeneous activity of agents. Within this framework, highly dynamical networks can be described analytically, allowing a quantitative discussion of the biases induced by the time-aggregated representations in the analysis of dynamical processes. Wed, 10 Oct 2012 18:10:01 GMT http://hdl.handle.net/2117/16713 2012-10-10T18:10:01Z Perra, N.; Gonçalves, B.; Pastor Satorras, Romualdo; Vespignani, Alessandro no COMPLEX NETWORKS, MARKOV GRAPHS, DYNAMICS, INTERNET, SYSTEMS, SPREAD Network modeling plays a critical role in identifying statistical regularities and structural principles common to many systems. The large majority of recent modeling approaches are connectivity driven. The structural patterns of the network are at the basis of the mechanisms ruling the network formation. Connectivity driven models necessarily provide a time-aggregated representation that may fail to describe the instantaneous and fluctuating dynamics of many networks. We address this challenge by defining the activity potential, a time invariant function characterizing the agents' interactions and constructing an activity driven model capable of encoding the instantaneous time description of the network dynamics. The model provides an explanation of structural features such as the presence of hubs, which simply originate from the heterogeneous activity of agents. Within this framework, highly dynamical networks can be described analytically, allowing a quantitative discussion of the biases induced by the time-aggregated representations in the analysis of dynamical processes. http://hdl.handle.net/2117/16707 Title: Water absorbed by polyaniline emeraldine tends to organize, forming nanodrops Authors: Casanovas, Jordi; Canales Gabriel, Manel; Fabregat Jové, Georgina; Meneguzzi, Álvaro; Alemán Llansó, Carlos Abstract: Interactions, in terms of both binding energies and microscopic organization, of water molecules absorbed by hydrophilic polyaniline emeraldine base have been investigated using quantum mechanical calculations, molecular dynamics simulation, FTIR spectroscopy, and 1H NMR. From an enthalpic point of view, water molecules interact more favorably with imine nitrogen atoms than with amine ones, even though the latter are entropically favored with respect to the former because of their two binding sites. Quantum mechanical results show that interaction energies of water molecules reversibly absorbed but organized individually around a binding site range from 3.0 to 6.3 kcal/mol, which is in good agreement with activation energies of 3–5 kcal/mol previously determined by thermodynamic measurements. The irreversible absorption of water to produce C–OH groups in rings of diimine units has been examined considering a three steps process in which water molecules act as both acidic and nucleophilic reagent. Although calculations predict that the whole process is disfavored by 5–8 kcal/mol only, FTIR and 1H NMR detected the existence of reversibly absorbed water but not of C–OH groups. Both the binding energies and the structural information provided by molecular dynamics simulations have been used to interpret the existence of two types of physisorbed water molecules: (i) those that interact individually with polymer chains and (ii) those immersed in nanodrops that are contained within the polymeric matrix. The binding energies calculated for these two types of water molecules are fully consistent with the thermodynamic activation energies previously reported. Wed, 10 Oct 2012 13:38:30 GMT http://hdl.handle.net/2117/16707 2012-10-10T13:38:30Z Casanovas, Jordi; Canales Gabriel, Manel; Fabregat Jové, Georgina; Meneguzzi, Álvaro; Alemán Llansó, Carlos no Interactions, in terms of both binding energies and microscopic organization, of water molecules absorbed by hydrophilic polyaniline emeraldine base have been investigated using quantum mechanical calculations, molecular dynamics simulation, FTIR spectroscopy, and 1H NMR. From an enthalpic point of view, water molecules interact more favorably with imine nitrogen atoms than with amine ones, even though the latter are entropically favored with respect to the former because of their two binding sites. Quantum mechanical results show that interaction energies of water molecules reversibly absorbed but organized individually around a binding site range from 3.0 to 6.3 kcal/mol, which is in good agreement with activation energies of 3–5 kcal/mol previously determined by thermodynamic measurements. The irreversible absorption of water to produce C–OH groups in rings of diimine units has been examined considering a three steps process in which water molecules act as both acidic and nucleophilic reagent. Although calculations predict that the whole process is disfavored by 5–8 kcal/mol only, FTIR and 1H NMR detected the existence of reversibly absorbed water but not of C–OH groups. Both the binding energies and the structural information provided by molecular dynamics simulations have been used to interpret the existence of two types of physisorbed water molecules: (i) those that interact individually with polymer chains and (ii) those immersed in nanodrops that are contained within the polymeric matrix. The binding energies calculated for these two types of water molecules are fully consistent with the thermodynamic activation energies previously reported. http://hdl.handle.net/2117/16697 Title: Zero-temperature phase diagram of the second layer of 4He adsorbed on graphene Authors: Gordillo Bargueño, Maria Carmen; Boronat Medico, Jordi Abstract: The phase diagram at zero temperature of 4He adsorbed on a helium incommensurate triangular solid on top of a single graphene sheet has been obtained using the diffusion Monte Carlo method. We have found that, in accordance with previous experimental and simulation results for graphite, the ground state of 4He on this setup is a liquid that, upon compression, transforms into a triangular solid. To define the stability limits of both liquid and solid phases, we considered not only the adsorption energies of the atoms located on the second layer but the average energy of the atoms in both layers. Our results show that the lower density limit for a stable liquid in the second layer is 0.163±0.005 Å−2 and that the lower limit for the existence of an incommensurate solid on the second layer is 0.186±0.003 Å−2. Both values are in overall agreement with the results of torsional oscillator experiments and heat capacity measurements on graphite. The 4/7 and 7/12 registered solids are found to be metastable with respect to triangular incommensurate arrangements of the same density. Tue, 09 Oct 2012 17:55:08 GMT http://hdl.handle.net/2117/16697 2012-10-09T17:55:08Z Gordillo Bargueño, Maria Carmen; Boronat Medico, Jordi no The phase diagram at zero temperature of 4He adsorbed on a helium incommensurate triangular solid on top of a single graphene sheet has been obtained using the diffusion Monte Carlo method. We have found that, in accordance with previous experimental and simulation results for graphite, the ground state of 4He on this setup is a liquid that, upon compression, transforms into a triangular solid. To define the stability limits of both liquid and solid phases, we considered not only the adsorption energies of the atoms located on the second layer but the average energy of the atoms in both layers. Our results show that the lower density limit for a stable liquid in the second layer is 0.163±0.005 Å−2 and that the lower limit for the existence of an incommensurate solid on the second layer is 0.186±0.003 Å−2. Both values are in overall agreement with the results of torsional oscillator experiments and heat capacity measurements on graphite. The 4/7 and 7/12 registered solids are found to be metastable with respect to triangular incommensurate arrangements of the same density. http://hdl.handle.net/2117/16693 Title: Competing activation mechanisms in epidemics on networks Authors: Castellano, Claudio; Pastor Satorras, Romualdo Abstract: In contrast to previous common wisdom that epidemic activity in heterogeneous networks is dominated by the hubs with the largest number of connections, recent research has pointed out the role that the innermost, dense core of the network plays in sustaining epidemic processes. Here we show that the mechanism responsible of spreading depends on the nature of the process. Epidemics with a transient state are boosted by the innermost core. Contrarily, epidemics allowing a steady state present a dual scenario, where either the hub independently sustains activity and propagates it to the rest of the system, or, alternatively, the innermost network core collectively turns into the active state, maintaining it globally. In uncorrelated networks the former mechanism dominates if the degree distribution decays with an exponent larger than 5/2, and the latter otherwise. Topological correlations, rife in real networks, may perturb this picture, mixing the role of both mechanisms. Tue, 09 Oct 2012 13:42:34 GMT http://hdl.handle.net/2117/16693 2012-10-09T13:42:34Z Castellano, Claudio; Pastor Satorras, Romualdo no COMPLEX NETWORKS, RANDOM GRAPHS, SPREAD In contrast to previous common wisdom that epidemic activity in heterogeneous networks is dominated by the hubs with the largest number of connections, recent research has pointed out the role that the innermost, dense core of the network plays in sustaining epidemic processes. Here we show that the mechanism responsible of spreading depends on the nature of the process. Epidemics with a transient state are boosted by the innermost core. Contrarily, epidemics allowing a steady state present a dual scenario, where either the hub independently sustains activity and propagates it to the rest of the system, or, alternatively, the innermost network core collectively turns into the active state, maintaining it globally. In uncorrelated networks the former mechanism dominates if the degree distribution decays with an exponent larger than 5/2, and the latter otherwise. Topological correlations, rife in real networks, may perturb this picture, mixing the role of both mechanisms. http://hdl.handle.net/2117/16681 Title: Maintenance and reengineering of software: creating a Visual C++ graphical user interface to perform specific tasks related to soil structure interaction in poroelastic soil Authors: Khwaldeh, Ali; Tahat, Amani; Martí Rabassa, Jordi; Tahat, Mohammad Abstract: In this study, an interactive Graphical User Interface (GUI) application was introduced and tested. It worked as a utility program to facilitate and maximize the re-use of existing FORTRAN software. The GUI had been designed to generate tabulated seismic data (e.g., coefficient of reflection and propagation wave velocities: P-wave and SV-wave), in order to perform investigating the wave propagation in fluid saturated poroelastic medium, through using a simple two-dimensional soil-structure interaction model. Additionally, the theoretical background was also involved. The output data was of interest for understanding the effects of water saturation on the response of the ground and building structures to some degree earthquake excitation. Furthermore, the GUI application provided several utilities (e.g., viewing and plotting capabilities, file manipulation, editing as well as cloning the input parameters). Operating systems for which program had been designed were Windows XP, Windows Vista, Windows 7 and any (32) bits windows. The compilers were: Microsoft VC++ VS 6.0, Digital Visual FORTRAN 6.0 and Borland C++ Builder 6.0. The produced source code could be considered as successful case study for software engineers GUI developers, who were interested in creating (C++ /GUI), for FORTRAN by using the experience of such open source codes. Mon, 08 Oct 2012 18:25:30 GMT http://hdl.handle.net/2117/16681 2012-10-08T18:25:30Z Khwaldeh, Ali; Tahat, Amani; Martí Rabassa, Jordi; Tahat, Mohammad no In this study, an interactive Graphical User Interface (GUI) application was introduced and tested. It worked as a utility program to facilitate and maximize the re-use of existing FORTRAN software. The GUI had been designed to generate tabulated seismic data (e.g., coefficient of reflection and propagation wave velocities: P-wave and SV-wave), in order to perform investigating the wave propagation in fluid saturated poroelastic medium, through using a simple two-dimensional soil-structure interaction model. Additionally, the theoretical background was also involved. The output data was of interest for understanding the effects of water saturation on the response of the ground and building structures to some degree earthquake excitation. Furthermore, the GUI application provided several utilities (e.g., viewing and plotting capabilities, file manipulation, editing as well as cloning the input parameters). Operating systems for which program had been designed were Windows XP, Windows Vista, Windows 7 and any (32) bits windows. The compilers were: Microsoft VC++ VS 6.0, Digital Visual FORTRAN 6.0 and Borland C++ Builder 6.0. The produced source code could be considered as successful case study for software engineers GUI developers, who were interested in creating (C++ /GUI), for FORTRAN by using the experience of such open source codes. http://hdl.handle.net/2117/16428 Title: Tracking energy transfer from excited to accepting modes : application to water bend vibrational relaxation Authors: Rey Oriol, Rosendo; Hynes, James T. Abstract: We extend, via a reformulation in terms of Poisson brackets, the method developed previously (Rey et al., J. Phys. Chem. A, 2009, 113, 8949) allowing analysis of the pathways of an excited molecule's ultrafast vibrational relaxation in terms of intramolecular and intermolecular contributions. In particular we show how to ascertain, through the computation of power and work, which portion of an initial excess molecular energy (e.g. vibrational) is transferred to various degrees of freedom (e.g. rotational, translational) of the excited molecule itself and its neighbors. The particular case of bend excess energy relaxation in pure water is treated in detail, completing the picture reported in the work cited above. It is shown explicitly, within a classical description, that almost all of the initial water bend excitation energy is transferred—either indirectly, via Fermi resonance centrifugal coupling to the bend-excited water's rotation, or directly via intermolecular coupling— to local water librations, only involving molecules in the first two hydration shells of the vibrationally excited water molecule. Finally, it is pointed out that the Poisson bracket formulation can also be applied to elucidate the microscopic character of solvation and rotational dynamics, and should prove useful in developing a quantum treatment for energy flow in condensed phases. Tue, 04 Sep 2012 17:44:05 GMT http://hdl.handle.net/2117/16428 2012-09-04T17:44:05Z Rey Oriol, Rosendo; Hynes, James T. no TIME-DEPENDENT FLUORESCENCE, HYDROGEN-BOND EXCHANGE, SOLVATION DYNAMICS, LIQUID WATER, MOLECULAR-DYNAMICS, DIPOLAR MOLECULE, CONDENSED-PHASE, JUMP MECHANISM, POLAR-SOLVENT, H2O We extend, via a reformulation in terms of Poisson brackets, the method developed previously (Rey et al., J. Phys. Chem. A, 2009, 113, 8949) allowing analysis of the pathways of an excited molecule's ultrafast vibrational relaxation in terms of intramolecular and intermolecular contributions. In particular we show how to ascertain, through the computation of power and work, which portion of an initial excess molecular energy (e.g. vibrational) is transferred to various degrees of freedom (e.g. rotational, translational) of the excited molecule itself and its neighbors. The particular case of bend excess energy relaxation in pure water is treated in detail, completing the picture reported in the work cited above. It is shown explicitly, within a classical description, that almost all of the initial water bend excitation energy is transferred—either indirectly, via Fermi resonance centrifugal coupling to the bend-excited water's rotation, or directly via intermolecular coupling— to local water librations, only involving molecules in the first two hydration shells of the vibrationally excited water molecule. Finally, it is pointed out that the Poisson bracket formulation can also be applied to elucidate the microscopic character of solvation and rotational dynamics, and should prove useful in developing a quantum treatment for energy flow in condensed phases. http://hdl.handle.net/2117/16422 Title: Ferromagnetic transition of a two-component Fermi gas of hard spheres Authors: Arias de Saavedra, F.; Mazzanti Castrillejo, Fernando Pablo; Boronat Medico, Jordi; Polls, A. Abstract: We use microscopic many-body theory to analyze the problem of itinerant ferromagnetism in a repulsive atomic Fermi gas of hard spheres. Using simple arguments we show that the available theoretical predictions for the onset of the ferromagnetic transition predict a transition point at a density (kF a ∼ 1) that is too large to be compatible with the universal low-density expansion of the energy. We present variational calculations for the hard-sphere Fermi gas, in the framework of Fermi hypernetted chain theory, that shift the transition to higher densities (kF a ∼ 1.8). Backflow correlations, which are mainly active in the unpolarized system, are essential for this shift. Tue, 04 Sep 2012 15:03:23 GMT http://hdl.handle.net/2117/16422 2012-09-04T15:03:23Z Arias de Saavedra, F.; Mazzanti Castrillejo, Fernando Pablo; Boronat Medico, Jordi; Polls, A. no GROUND-STATE, LIQUID-HE-3, HELIUM, ENERGY We use microscopic many-body theory to analyze the problem of itinerant ferromagnetism in a repulsive atomic Fermi gas of hard spheres. Using simple arguments we show that the available theoretical predictions for the onset of the ferromagnetic transition predict a transition point at a density (kF a ∼ 1) that is too large to be compatible with the universal low-density expansion of the energy. We present variational calculations for the hard-sphere Fermi gas, in the framework of Fermi hypernetted chain theory, that shift the transition to higher densities (kF a ∼ 1.8). Backflow correlations, which are mainly active in the unpolarized system, are essential for this shift. http://hdl.handle.net/2117/16421 Title: Aqueous halide potentials from force matching of Car-Parrinello data Authors: Spangberg, Daniel; Guàrdia Manuel, Elvira; Masia, Marco Abstract: Recently many various research groups have devoted a huge effort to develop a realistic classical force field for ions in water. The parametrization techniques used could be gathered into two classes: (i) fit of the ab initio potential energy surface for clusters at gas phase, and (ii) fit of experimental properties. For both classes of force fields, a high level of accuracy has been achieved, which has led to important improvements in the modeling of ion–water systems. In this paper a new, complementary, approach is proposed to overcome the limitations and to get a deeper insight into the atomistic description of ion–water interactions. We use the recently developed force matching method to parametrize classical halide–water force fields for three different water models. Here we discuss both methodological issues and the level of agreement between the results obtained using this method to Car–Parrinello simulation results. Tue, 04 Sep 2012 13:44:45 GMT http://hdl.handle.net/2117/16421 2012-09-04T13:44:45Z Spangberg, Daniel; Guàrdia Manuel, Elvira; Masia, Marco no Aqueous solution, Car parrinello molecular dynamics, Force field, Forcematching, Halide, Water Recently many various research groups have devoted a huge effort to develop a realistic classical force field for ions in water. The parametrization techniques used could be gathered into two classes: (i) fit of the ab initio potential energy surface for clusters at gas phase, and (ii) fit of experimental properties. For both classes of force fields, a high level of accuracy has been achieved, which has led to important improvements in the modeling of ion–water systems. In this paper a new, complementary, approach is proposed to overcome the limitations and to get a deeper insight into the atomistic description of ion–water interactions. We use the recently developed force matching method to parametrize classical halide–water force fields for three different water models. Here we discuss both methodological issues and the level of agreement between the results obtained using this method to Car–Parrinello simulation results. http://hdl.handle.net/2117/16407 Title: Zero-temperature phase diagram of Yukawa bosons Authors: Osychenko, O.N.; Astrakharchik, Grigori; Mazzanti Castrillejo, Fernando Pablo; Boronat Medico, Jordi Abstract: We study the zero-temperature phase diagram of bosons interacting via screened Coulomb (Yukawa) potential by means of the diffusion Monte Carlo method. The Yukawa potential is used as a model interaction in the neutron matter, dusty plasmas, and charged colloids. As shown by Petrov et al. [Phys. Rev. Lett. 99, 130407 (2007)], interactions between weakly bound molecules of heavy and light fermionic atoms are described by an effective Yukawa potential with a strength related to the heavy-light mass ratio M/m, which might lead to crystallization in a two-dimensional geometry if the mass ratio of heavy-light fermions exceeds a certain critical value. In the present work we do a thorough study of the quantum three-dimensional Yukawa system. For strong interactions (equivalently, large mass ratios) the system experiences several phase transitions as the density is increased, passing from gas to solid and to gas phase again.Weakly interacting Yukawa particles do not crystallize at any density. We find the minimal interaction strength at which the crystallization happens. In terms of the two-component fermionic system, this strength corresponds to a heavy-light mass ratio of M/m ∼ 180, so that it is impossible to realize the gas-crystal transition in a conventional bulk system. For the Yukawa model of fermionic mixtures we also analyze the possibility of building molecular systems with very large effective mass ratios by confining the heavy component to a sufficiently deep optical lattice. We show how the effective mass of the heavy component can be made arbitrarily large by increasing the lattice depth, thus leading to a tunable effective mass ratio that can be used to realize a molecular superlattice. Thu, 30 Aug 2012 09:48:20 GMT http://hdl.handle.net/2117/16407 2012-08-30T09:48:20Z Osychenko, O.N.; Astrakharchik, Grigori; Mazzanti Castrillejo, Fernando Pablo; Boronat Medico, Jordi no We study the zero-temperature phase diagram of bosons interacting via screened Coulomb (Yukawa) potential by means of the diffusion Monte Carlo method. The Yukawa potential is used as a model interaction in the neutron matter, dusty plasmas, and charged colloids. As shown by Petrov et al. [Phys. Rev. Lett. 99, 130407 (2007)], interactions between weakly bound molecules of heavy and light fermionic atoms are described by an effective Yukawa potential with a strength related to the heavy-light mass ratio M/m, which might lead to crystallization in a two-dimensional geometry if the mass ratio of heavy-light fermions exceeds a certain critical value. In the present work we do a thorough study of the quantum three-dimensional Yukawa system. For strong interactions (equivalently, large mass ratios) the system experiences several phase transitions as the density is increased, passing from gas to solid and to gas phase again.Weakly interacting Yukawa particles do not crystallize at any density. We find the minimal interaction strength at which the crystallization happens. In terms of the two-component fermionic system, this strength corresponds to a heavy-light mass ratio of M/m ∼ 180, so that it is impossible to realize the gas-crystal transition in a conventional bulk system. For the Yukawa model of fermionic mixtures we also analyze the possibility of building molecular systems with very large effective mass ratios by confining the heavy component to a sufficiently deep optical lattice. We show how the effective mass of the heavy component can be made arbitrarily large by increasing the lattice depth, thus leading to a tunable effective mass ratio that can be used to realize a molecular superlattice. http://hdl.handle.net/2117/16406 Title: Superfluidity of metastable glassy bulk para-hydrogen at low temperature Authors: Osychenko, O.N.; Rota, Riccardo; Boronat Medico, Jordi Abstract: Molecular para-hydrogen has been proposed theoretically as a possible candidate for superfluidity, but the eventual superfluid transition is hindered by its crystallization. In this work, we study a metastable non crystalline phase of bulk p-H2 by means of the Path Integral Monte Carlo method in order to investigate at which temperature this system can support superfluidity. By choosing accurately the initial configuration and using a non commensurate simulation box, we have been able to frustrate the formation of the crystal in the simulated system and to calculate the temperature dependence of the one-body density matrix and of the superfluid fraction. We observe a transition to a superfluid phase at temperatures around 1 K. The limit of zero temperature is also studied using the diffusion Monte Carlo method. Results for the energy, condensate fraction, and structure of the metastable liquid phase at T=0 are reported and compared with the ones obtained for the stable solid phase. Thu, 30 Aug 2012 09:31:27 GMT http://hdl.handle.net/2117/16406 2012-08-30T09:31:27Z Osychenko, O.N.; Rota, Riccardo; Boronat Medico, Jordi no Superfluidity, Superconductivity Molecular para-hydrogen has been proposed theoretically as a possible candidate for superfluidity, but the eventual superfluid transition is hindered by its crystallization. In this work, we study a metastable non crystalline phase of bulk p-H2 by means of the Path Integral Monte Carlo method in order to investigate at which temperature this system can support superfluidity. By choosing accurately the initial configuration and using a non commensurate simulation box, we have been able to frustrate the formation of the crystal in the simulated system and to calculate the temperature dependence of the one-body density matrix and of the superfluid fraction. We observe a transition to a superfluid phase at temperatures around 1 K. The limit of zero temperature is also studied using the diffusion Monte Carlo method. Results for the energy, condensate fraction, and structure of the metastable liquid phase at T=0 are reported and compared with the ones obtained for the stable solid phase.