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Geometry-dependent distributed polarizability models have been constructed by fits to ab initio calculations at the coupled cluster level of theory with up to noniterative triple excitations in an augmented triple-zeta quality basis set for the water molecule in the field of a point charge. The investigated models include (i) charge-flow polarizabilities between chemically bonded atoms, (ii) isotropic or anisotropic dipolar polarizabilities on oxygen atom or on all atoms, and (iii) combinations of models (i) and (ii). For each model, the polarizability parameters have been optimized to reproduce the induction energy of a water molecule polarized by a point charge successively occupying a grid of points surrounding the molecule. The quality of the models is ascertained by examining their ability to reproduce these induction energies as well as the molecular dipolar and quadrupolar polarizabilities. The geometry dependence of the distributed polarizability models has been explored by changing bond lengths and HOH angle to generate 125 molecular structures (reduced to 75 symmetry-unique ones). For each considered model, the distributed polarizability components have been fitted as a function of the geometry by a Taylor expansion in monomer coordinate displacements up to the sum of powers equal to 4. [ABSTRACT FROM AUTHOR]

The dynamic polarizability and optical absorption spectrum of liquid water in the 6–15 eV energy range are investigated by a sequential molecular dynamics (MD)/quantum mechanical approach. The MD simulations are based on a polarizable model for liquid water. Calculation of electronic properties relies on time-dependent density functional and equation-of-motion coupled-cluster theories. Results for the dynamic polarizability, Cauchy moments, S(-2), S(-4), S(-6), and dielectric properties of liquid water are reported. The theoretical predictions for the optical absorption spectrum of liquid water are in good agreement with experimental information. [ABSTRACT FROM AUTHOR]

Classical nucleation theory pictures the homogeneous nucleation of a crystal as the formation of a spherical crystalline embryo, possessing the properties of the macroscopic crystal, inside a parent supercooled liquid. In this work we study crystal nucleation in moderately supercooled sulfur hexafluoride by umbrella sampling simulations. The nucleation free energy evolves from 5.2kBT at T=170 K to 39.1kBT at T=195 K. The corresponding critical nucleus size ranges from 40 molecules at T=170 K to 266 molecules at T=195 K. Both nucleation free energy and critical nucleus size are shown to evolve with temperature according to the equations derived from the classical nucleation theory. Inspecting the obtained nuclei we show, however, that they present quite anisotropic shapes in opposition to the spherical assumption of the theory. Moreover, even though the critical nuclei possess the structure of the stable bcc plastic phase, the only mechanically stable crystal phase for SF6 in the temperature range investigated, they are shown to be less ordered than the corresponding macroscopic crystal. Their crystalline order is nevertheless shown to increase regularly with their size. This is confirmed by a study of a nucleus growth from a critical size to a size of the order of 104 molecules. Similarly to the fact that it does not affect the temperature dependence of the nucleation free energy and of the critical nucleus size, the ordering of the nucleus with size does not affect the growth rate of the nucleus. [ABSTRACT FROM AUTHOR]

A method based on a lattice summation technique for treating long-range electrostatic interactions in hybrid quantum mechanics/molecular mechanics simulations is presented in this article. The quantum subsystem is studied at the semiempirical level, whereas the solvent is described by a two-body potential of molecular mechanics. Molecular dynamics simulations of a (quantum) chloride ion in (classical) water have been performed to test this technique. It is observed that the application of the lattice summations to solvent–solvent interactions as well as on solute–solvent ones has a significant effect on solvation energy and diffusion coefficient. Moreover, two schemes for the computation of the long-range contribution to the electrostatic interaction energy are investigated. The first one replaces the exact charge distribution of the quantum solute by a Mulliken charge distribution. The long-range electrostatic interactions are then calculated for this charge distribution that interacts with the solvent molecule charges. The second one is more accurate and involves a modified Fock operator containing long-range electron-charge interactions. It is shown here that both schemes lead to similar results, the method using Mulliken charges for the evaluation of long-range interactions being, however, much more computationally efficient. [ABSTRACT FROM AUTHOR]

We report on a molecular simulation study of the homogeneous nucleation of CO2 in the supercooled liquid at low pressure (P=5 MPa) and for degrees of supercooling ranging from 32% to 60%. In all cases, regardless of the degree of supercooling, the structure of the crystal nuclei is that of the Pa3 phase, the thermodynamically stable phase. For the more moderate degree of supercooling of 32%, the nucleation is an activated process and requires a method to sample states of high free energy. In this work, we apply a series of bias potentials, which promote the ordering of the centers of mass of the molecules and allow us to gradually grow crystal nuclei. The reliability of the results so obtained is assessed by studying the evolution of the nuclei in the absence of any bias potential, and by determining their probability of growth. We estimate that the size of the critical nucleus, for which the probability of growth is 0.5, is ≈240 molecules. Throughout the nucleation process, the crystal nuclei clearly exhibit a Pa3 structure, in apparent contradiction with Ostwald’s rule of stages. The other polymorphs have a much larger free energy. This makes their formation highly unlikely and accounts for the fact that the nucleation of CO2 proceeds directly in the stable Pa3 structure. [ABSTRACT FROM AUTHOR]

We report on a computer simulation study of the early stages of the crystallization of molecular nitrogen. First, we study how homogeneous nucleation takes place in supercooled liquid N2 for a moderate degree of supercooling. Using the umbrella sampling technique, we determine the free energy barrier of formation for a critical nucleus of N2. We show that, in accord with Ostwald’s rule of stages, the structure of the critical nucleus is predominantly that of a metastable polymorph (α-N2 for the state point investigated). We then monitor the evolution of several critical nuclei through a series of unbiased molecular dynamics trajectories. The growth of N2 crystallites is accompanied by a structural evolution toward the stable polymorph β-N2. The microscopic mechanism underlying this evolution qualitatively differs from that reported previously. We do not observe any dissolution or reorganization of the α-like core of the nucleus. On the contrary, we show that α-like and β-like blocks coexist in postcritical nuclei. We relate the structural evolution to a greater adsorption rate of β-like molecules on the surface and show that this transition actually starts well within the precritical regime. We also carefully investigate the effect of the system size on the height of the free energy barrier of nucleation and on the structure and size of the critical nucleus. [ABSTRACT FROM AUTHOR]

The introduction of a reaction field dipolar contribution in free energy calculations is considered. The method is applied to a series of polar compounds, namely TIP3P water, methanol, and acetamide. The hydration free energies computed with both spherical cutoff and reaction field geometries are critically compared. Although the estimated quantities appear to be very similar, the use of a reaction field correction offers a better representation of the liquid structure. In the case of methanol, different potential derived point charge models of increasing complexity are investigated. It is found that the sophisticated models, involving either lone pairs or bond center charges, do not lead to any significant improvement over the simple atomic charge distribution. This trend is correlated to the variable strength of the modeled hydrogen bonds in solution. [ABSTRACT FROM AUTHOR]

An approach based on finite perturbation theory is proposed for deriving models of distributed polarizabilities from quantum mechanically determined induction energies. It relies on the construction of a grid of points, over which the induction energy resulting from the interaction of a charge with the molecule of interest is evaluated at the desired level of approximation. Distributed polarizabilities of any order are then fitted by solving the normal equations of the least-squares problem, the solution of which provides an optimal description of induction effects. The method is probed by examining several models of distributed polarizabilities of increasing complexity in the case of water, methanol, acetonitrile, and benzene. At a reasonable order of multipole expansion, atomic polarizabilities derived using this approach are found to reproduce the corresponding molecular polarizabilities with an appropriate accuracy. The careful choice of the parameters to be fitted appears to be a key factor for obtaining physically realistic models. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report on a computer simulation study of the early stages of the crystallization of molecular nitrogen. First, we study how homogeneous nucleation takes place in supercooled liquid N(2) for a moderate degree of supercooling. Using the umbrella sampling technique, we determine the free energy barrier of formation for a critical nucleus of N(2). We show that, in accord with Ostwald's rule of stages, the structure of the critical nucleus is predominantly that of a metastable polymorph (alpha-N(2) for the state point investigated). We then monitor the evolution of several critical nuclei through a series of unbiased molecular dynamics trajectories. The growth of N(2) crystallites is accompanied by a structural evolution toward the stable polymorph beta-N(2). The microscopic mechanism underlying this evolution qualitatively differs from that reported previously. We do not observe any dissolution or reorganization of the alpha-like core of the nucleus. On the contrary, we show that alpha-like and beta-like blocks coexist in postcritical nuclei. We relate the structural evolution to a greater adsorption rate of beta-like molecules on the surface and show that this transition actually starts well within the precritical regime. We also carefully investigate the effect of the system size on the height of the free energy barrier of nucleation and on the structure and size of the critical nucleus.