Molecular dynamics simulations of the protonated G4 PAMAM dendrimer in an ionic liquid system.

Molecular dynamics simulations have been carried out for the ionic liquid system constituted by totally protonated PAMAM-EDA cations and Tf2N(-) anions. The conformational characteristics of the PMAM dendrimer (particularly the density profile around the dendrimer center) are compared with those obtained for the same dendrimer in water. We also investigate other features, such as the location of anions relative to the dendrimer molecules, and the interpenetration of the dendrimer cations in the ionic liquid system. It is shown that a substantial amount of anions are present in the outer part of the dendrimer, screening repulsive contacts between different cation molecules and favoring ionic conductivity. Dendrimer cations in the ionic liquid exhibit a significant degree of overlap.

A study of the influence of isotopic substitution on the melting point and temperature of maximum density of water by means of path integral simulations of rigid models.

The melting point of ice I(h), as well as the temperature of maximum density (TMD) in the liquid phase, has been computed using the path integral Monte Carlo method. Two new models are introduced, TIP4PQ_D2O and TIP4PQ_T2O, which are specifically designed to study D(2)O and T(2)O respectively. We have also used these models to study the "competing quantum effects" proposal of Habershon, Markland and Manolopoulos; the TIP4PQ/2005, TIP4PQ/2005 (D(2)O) and TIP4PQ/2005 (T(2)O) models are able to study the isotopic substitution of hydrogen for deuterium or tritium whilst constraining the geometry, while the TIP4PQ_D2O and TIP4PQ_T2O models, where the O-H bond lengths are progressively shortened, permit the study of the influence of geometry (and thus dipole moment) on the isotopic effects. For TIP4PQ_D2O-TIP4PQ/2005 we found a melting point shift of 4.9 K (experimentally the value is 3.68 K) and a TMD shift of 6 K (experimentally 7.2 K). For TIP4PQ_T2O-TIP4PQ/2005 we found a melting point shift of 5.2 K (experimentally the value is 4.49 K) and a TMD shift of 7 K (experimentally 9.4 K).

The phase diagram of water from quantum simulations.

The phase diagram of water has been calculated from the TIP4PQ/2005 model, an empirical rigid non-polarisable model. The path integral Monte Carlo technique was used, permitting the incorporation of nuclear quantum effects. The coexistence lines were traced out using the Gibbs-Duhem integration method, once having calculated the free energies of the liquid and solid phases in the quantum limit, which were obtained via thermodynamic integration from the classical value by scaling the mass of the water molecule. The resulting phase diagram is qualitatively correct, being displaced to lower temperatures by 15-20 K. It is found that the influence of nuclear quantum effects is correlated to the tetrahedral order parameter.

Force field parametrization and molecular dynamics simulation of flexible POSS-linked (NHC; phosphine) Ru catalytic complexes.

In recent years, N-heterocyclic carbene (NHC) or phospine groups have been put forward as candidate catalysts ligands for olefin metathesis reactions to be performed using multistep methods. Some of these proposed ligands contain polyhedral oligomeric silsesquioxane (POSS) structures linked to NHC rings by means of alkyl chains. Some important properties for the prediction of catalytic activity, such as the theoretically defined buried volume, are related to the conformational characteristics of these complex ligands that can be studied through molecular dynamics simulations. However, the chemical structure of resulting catalytic complexes usually contains atoms or groups that are not included in the common forcefields used in simulations. In this work we focus on complexes formed by a catalytic metal center (Ru) with both phospine and POSS-linked NHC groups. The central part of the complexes contain atoms and groups that have bonds, bond angles, and torsional angles whose parameters have not been previously evaluated and included in existing force fields. We have performed basic ab initio quantum mechanical calculations based on the density functional theory to obtain energies for this central section. The force field parameters for bonds, bond angles, and torsional angles are then calculated from an analysis of energies calculated for the equilibrium and different locally deformed structures. Nonbonded interactions are also conveniently evaluated. From subsequent molecular dynamics simulations, we have obtained results that illustrate the conformational characteristics most closely connected with the catalytic activity.

A quantum propagator for path-integral simulations of rigid molecules.

The expression for the quantum propagator for rigid tops, proposed by Müser and Berne [Phys. Rev. Lett. 77, 2638 (1996)], has been extended to asymmetric tops. Path-integral Monte Carlo simulations are provided that show that the quantum propagator proposed in this work exactly reproduces the rotational energy of free asymmetric tops as evaluated from the partition function. This propagator can subsequently be used in path-integral simulations of condensed phases if a rigid molecular model is used.

Can gas hydrate structures be described using classical simulations?

Quantum path-integral simulations of the hydrate solid structures have been performed using the recently proposed TIP4PQ/2005 model. By also performing classical simulations using this model, the impact of the nuclear quantum effects on the hydrates is highlighted; nuclear quantum effects significantly modify the structure, densities, and energies of the hydrates, leading to the conclusion that nuclear quantum effects are important not only when studying the solid phases of water but also when studying the hydrates. To analyze the validity of a classical description of hydrates, a comparison of the results of the TIP4P/2005 model (optimized for classical simulations) with those of TIP4PQ/2005 (optimized for path-integral simulations) was undertaken. A classical description of hydrates is able to correctly predict the densities at temperatures above 150 K and the relative stabilities between the hydrates and ice I(h). The inclusion of nuclear quantum effects does not significantly modify the sequence of phases found in the phase diagram of water at negative pressures, namely, I(h)-->sII-->sH. In fact the transition pressures are little affected by the inclusion of nuclear quantum effects; the phase diagram predictions for hydrates can be performed with reasonable accuracy using classical simulations. However, for a reliable calculation of the densities below 150 K, the sublimation energies, the constant pressure heat capacity, and the radial distribution functions, the incorporation of nuclear quantum effects is indeed required.

Quantum contributions in the ice phases: the path to a new empirical model for water-TIP4PQ/2005.

With a view to a better understanding of the influence of atomic quantum delocalization effects on the phase behavior of water, path integral simulations have been undertaken for almost all of the known ice phases using the TIP4P/2005 model in conjunction with the rigid rotor propagator proposed by Muser and Berne [Phys. Rev. Lett. 77, 2638 (1996)]. The quantum contributions then being known, a new empirical model of water is developed (TIP4PQ/2005) which reproduces, to a good degree, a number of the physical properties of the ice phases, for example, densities, structure, and relative stabilities.

Radial distribution functions and densities for the SPC/E, TIP4P and TIP5P models for liquid water and ices Ih, Ic, II, III, IV, V, VI, VII, VIII, IX, XI and XII.

Monte Carlo computer simulation studies have been undertaken for virtually all of the ice phases as well as for liquid water for three of the most popular model potentials; namely SPC/E, TIP4P and TIP5P. Densities have been calculated for specific thermodynamic state points and compared to experimental results. The SPC/E and TIP4P models overestimate the solid densities by about 2%. The TIP5P model overestimates the solid densities by about 5-10%. The structural pair correlation functions between oxygen-oxygen, hydrogen-hydrogen and oxygen-hydrogen atoms were also obtained from the simulations. (These are available as ESIt). It has been found that SPC/E and TIP4P structural predictions are rather similar, with the only exception of ice II for which differences are visible between these two models. Predictions from the TIP5P are clearly different from those of the other models, especially for ices Ih and II. For the higher density ices structural differences between the models are rather small. Experimental data would be highly desirable to test the structural predictions of the different models of water. This is especially true for ice II. We have also found that the oxygen-oxygen correlation function of high density amorphous (HDA) water presents the same broad features as those exhibited by ice XII.

Formation of high density amorphous ice by decompression of ice VII and ice VIII at 135 K.

Monte Carlo computer simulations of ice VII and ice VIII phases have been undertaken using the four-point transferable intermolecular potential model of water. By following thermodynamic paths similar to those used experimentally, ice is decompressed resulting in an amorphous phase. These phases are compared to the high density amorphous phase formed upon compression of ice Ih and are found to have very similar structures. By cooling liquid water along the water/Ih melting line a high density amorphous phase was also generated.

Characterization of the order-disorder transition of a charged hard-sphere model.

Monte Carlo simulations at constant pressure are used to characterize the structure of the restricted primitive model in the tetragonal-ordered solid phase. A method to estimate the location of the order-disorder transition and the densities of the coexistence phases is discussed. The results support the weakly first-order character of the transition.

Scaling laws for the equation of state of flexible and linear tangent hard sphere chains.

The influence that molecular flexibility has on the phase diagram and equation of state of hard sphere chains is examined. In the isotropic phase the equation of state is insensitive to flexibility; rigid chains display the same equation of state as flexible chains. However, with the onset of liquid crystalline phases for rigid molecules this similarity disappears. Differences are also apparent between the rigid and flexible models in the solid phase. Wertheim's thermodynamic perturbation theory has been extended to describe the solid phase of fully flexible chains and excellent agreement with simulation results is seen. A scaling is proposed that, when applied to the fully flexible model, reproduces simulation results for a linear rigid model. It is shown that for the fully flexible model the compressibility factor for the fluid and solid phases scale with the number of monomers m. The compressibility factor for the linear model scales with m in the isotropic fluid, and becomes independent of m in the nematic, smectic, and solid phases.