Abstract: We study the transition state of pericyclic reactions at elevated temperature with unbiased ab initio molecular dynamics. We find that the transition state of the intramolecular rearrangements for barbaralane and bullvalene remains aromatic at high temperature despite the significant thermal atomic motions. Structural, magnetic, and electronic properties of the dynamical transition state show the concertedness and aromatic character. Free-energy Calculations also support the validity of the transition state theory for the present rearrangement reactions. The calculations demonstrate that cyclic delocalization represents a strong force to synchronize the thermal atomic motions even at high temperatures.
Abstract: We introduce a class of interatomic potential models that can be automatically generated from data consisting of the energies and forces experienced by atoms, as derived from quantum mechanical calculations. The models do not have a fixed functional form and hence are capable of modeling complex potential energy landscapes. They are systematically improvable with more data. We apply the method to bulk crystals, and test it by calculating properties at high temperatures. Using the interatomic potential to generate the long molecular dynamics trajectories required for such calculations saves orders of magnitude in computational cost.
Abstract: Computer simulation results are reported for a realistic polarizable potential model of water in the supercooled region. Three states, corresponding to the low density amorphous ice, high density amorphous ice, and very high density amorphous ice phases are chosen for the analyses. These states are located close to the liquid-liquid coexistence lines already shown to exist for the considered model. Thermodynamic and structural quantities are calculated, in order to characterize the properties of the three phases. The results point out the increasing relevance of the interstitial neighbors, which clearly appear in going from the low to the very high density amorphous phases. The interstitial neighbors are found to be, at the same time, also distant neighbors along the hydrogen bonded network of the molecules. The role of these interstitial neighbors has been discussed in connection with the interpretation of recent neutron scattering measurements. The structural properties of the systems are characterized by looking at the angular distribution of neighboring molecules, volume and face area distribution of the Voronoi polyhedra, and order parameters. The cumulative analysis of all the corresponding results confirms the assumption that a close similarity between the structural arrangement of molecules in the three explored amorphous phases and that of the ice polymorphs I-h, III, and VI exists. (c) 2008 American Institute of Physics.
Abstract: The transferability of of few simple rigid non-polarizable water models were tested by Gibbs Ensemble Monte Carlo simulations to predict their vapor-liquid phase equilibria, and by isothermal-isobaric (Parrinello-Rahman) Monte Carlo simulations of the 13 known crystalline phases of ice. The temperature dependence of the corresponding second virial coefficients was also determined and then used to test the internal consistency of the simulated vapor-phase densities. The model predictions appear satisfactory for liquid water for ambient conditions, but they fail to mimic accurately the properties of the ice polymorphs and the orthobaric vapor phase. The major shortcomings of the models were in the overestimation by a factor of two (similar to 4-6 kJ/mol) of the internal energy difference between the high-pressure ice phases and the hexagonal phase. This unacceptable discrepancy is caused by the parameterization to reproduce the density of liquid water at ambient conditions, that accounts for the significant polarization effects in the condensed phases in terms of augmented dipole moments, with the consequent detrimental effect on the estimations of the vapor-phase properties. (c) 2007 Published by Elsevier B.V.
Abstract: The authors propose a new classical model for the water molecule. The geometry of the molecule is built on the rigid TIP5P model and has the experimental gas phase dipole moment of water created by four equal point charges. The model preserves its rigidity but the size of the charges increases or decreases following the electric field created by the rest of the molecules. The polarization is expressed by an electric field dependent nonlinear polarization function. The increasing dipole of the molecule slightly increases the size of the water molecule expressed by the oxygen-centered sigma parameter of the Lennard-Jones interaction. After refining the adjustable parameters, the authors performed Monte Carlo simulations to check the ability of the new model in the ice, liquid, and gas phases. They determined the density and internal energy of several ice polymorphs, liquid water, and gaseous water and calculated the heat capacity, the isothermal compressibility, the isobar heat expansion coefficients, and the dielectric constant of ambient water. They also determined the pair-correlation functions of ambient water and calculated the energy of the water dimer. The accuracy of theirs results was satisfactory. (C) 2007 American Institute of Physics.
Abstract: The structural changes occurring in supercooled liquid water upon moving from one coexisting liquid phase to the other have been investigated by computer simulation using a polarizable interaction potential model. The obtained results favorably compare with recent neutron scattering data of high and low density water. In order to assess the physical origin of the observed structural changes, computer simulation of several ice polymorphs has also been carried out. Our results show that there is a strict analogy between the structure of various disordered (supercooled) and ordered (ice) phases of water, suggesting that the occurrence of several different phases of supercooled water is rooted in the same physical origin that is responsible for ice polymorphism. (c) 2007 American Institute of Physics.
Abstract: Two water molecules connected by hydrogen bond in hexagonal ice can have four possible configurations. These configurations are distinguished by the relative orientation of the two molecules and termed for obvious reasons as c-cis, h-cis, c-trans, and h-trans. The occurrence of symmetry permitted dimer orientations is a characteristic feature of each ice polymorph. In the proton-ordered structures the occurrence of orientations is strictly determined, while in the proton-disordered structures it can vary within certain limits. We performed Monte Carlo simulations using the so-called TIP5P-EW, TIP4P-EW and TIP4P-2005 interaction models to study this isomerism for the polymorphs of ice. We found that the variation of energy with the frequency of different dimer orientations in the proton-disordered phases is large enough to influence the results of phase stability studies. Knowing the distributions of dimer orientations of the ice IX-ice III ordered-disordered polymorph pairs, we could estimate the internal energy of ice IX using dimer energies assigned to certain orientations in the disordered phase of ice III. In agreement with experimental evidences at low temperatures the TIP4P-EW and TIP4P-2005 potentials predicted lower energy for ice VIII than for ice VII. (c) 2007 Elsevier B.V. All rights reserved.
Abstract: We analyzed the ability of variants of the SPC/E and TIP4P types of water models to describe the temperature dependence of their second virial coefficients, liquid-vapor phase envelopes, and corresponding coexistence vapor pressure. We complete the characterization of the two most promising models by testing their adequacy to predict the structure of the 13 known crystalline phases of ice by (Parrinello-Rahman) isothermal-isobaric Monte Carlo simulations. While these models perform well for the description of properties to which their force fields were fitted (density, heat of vaporization, structure at the level of pair correlations), their transferability to the entire phase diagram is unsatisfactory, i.e., none could significantly mitigate the shortcomings of the original models. In fact, the most appropriate alternative appears to be the TIP4P-EW model, i.e., the recent reparametrization of the original TIP4P water model. Model parametrizations aimed at improving the description of ice behavior fail even in the description of the liquid phase. (c) 2006 American Institute of Physics.
Abstract: We perform extensive Gibbs Ensemble Monte Carlo simulations to study the capability of some recently re-parameterizations of the original TIP4P model intended to predict accurately the vapor-liquid coexistence envelope of water, its critical point, and its temperature dependence for the vapor pressure and second virial coefficient, and complement this analysis with the characterization of some specific crystalline faces of ice. We also disclose some trends between the resulting dipole moment of the models and the Lennard-Jones parameters, the location of the negative charge, as well as the estimated critical temperature. Finally, we discuss the inability of these models to predict accurately and simultaneously the melting temperature and the temperature of maximum density. (C) 2006 Elsevier B.V All rights reserved.
Abstract: As a reference for follow-up studies toward more accurate model parametrizations, we performed molecular-dynamics and Monte Carlo simulations for all known crystalline phases of ice, as described by the simple point-charge/extended and TIP4P water models. We started from the measured structures, densities, and temperatures, and carried out classical canonical simulations for all these arrangements. All simulated samples were cooled down close to 0 K to facilitate the comparison with theoretical estimates. We determined configurational internal energies as well as pressures, and monitored how accurately the measured configurations were preserved during the simulations. While these two models predicted very similar thermophysical and structural properties for water at ambient conditions, the predicted features for the corresponding ice polymorphs may differ significantly. (C) 2005 American Institute of Physics.
