The process of inserting cavities in water is studied with the aim of a better description of some of the terms necessary in continuum quantum mechanical models. Free-energy changes for the formation of soft and hard spherical cavities in TIP4P water have been computed by Monte Carlo (MC) simulation with statistical perturbation theory, up to a radius of 6 Angstrom. The free-energy change for the formation of a hard sphere, Delta G(cav), is obtained combining the Delta G(sol) of a soft repulsive sphere with the Delta G corresponding to the process of transforming the soft sphere into a hard one. Two definitions of hard-sphere repulsive potentials have been considered, one only based on the distance of oxygens from the center of the cavity, while the other also excludes hydrogens from the same region. Differences in free energies are significant. The cubic polynomial expression Delta G(cav), obtained by extrapolating the exact scaled particle theory (SPT) expression for very small excluding cavities, gives results in agreement with MC, with effective ''hard-sphere'' diameter for water larger than 2.77 Angstrom. The SPT prediction is compared with other treatments based on surface tension. It is shown that a properly chosen surface and an ''effective'' surface tension of water lead to a good agreement with MC Delta G(cav) without curvature or microscopic corrections. The ''effective'' surface tension of water turns out to be very close to the experimental value. Some different simple ways to extend SPT expression to nonspherical cavities have been compared, for a limited number of nonspherical convex cavities modelled as n interlocking spheres, meant to mimic n-alkanes in the all-staggered conformation. Entropy changes for soft cavities have been computed with two methods, i.e., combining free energy and enthalpy computations and by finite difference methods. Discrepancies between SPT predictions and MC results are significant. The calculated probability distributions of relevant angles of first hydration shell waters are consistent with orientations where no O-H or O-lone pair vector points towards the cavity. Their variation when the cavity size increases is mostly quantitative and only the broadening of the bands observed for the largest cavities might indicate the early stage of the transition to hydration patterns peculiar to an infinite hydrophobic surface. (C) 1997 American Institute of Physics
Molecular dynamics atomistic simulations of solid and liquid benzene have been performed, employing a model intermolecular potential derived from quantum mechanical calculations. The ab initio database includes approximately 200 geometries of the benzene dimer with interaction energies computed at the MP2 level of theory. The accuracy of the modeled force field results is satisfactory. The thermodynamic and structural properties, calculated in the condensed phases, are compared with experimental data and previous simulation results. Single particle and collective dynamical properties are also investigated through the calculation of translational and rotational diffusion coefficients, reorientational dynamics, and viscosities. The agreement of these data with experimental measurements confirms the reliability of the proposed force field.
Bulk phase atomistic computer simulations of 4-n-pentyl-4'-cyanobiphenyl (5CB) were performed with a specific force field obtained from ab initio and DFT calculations. The intermolecular potential was previously derived through the fragmentation reconstruction method (FRM), developed in our group. The description of some intramolecular interactions, like the torsional potential between the phenyl rings and at the aryl-alkyl linkage, is achieved through accurate DFT studies. Lengthy ( approximately 40 ns) molecular dynamics (MD) simulations were then carried out at constant atmospheric pressure and different temperatures. The system was stable in the experimental crystalline structure up to 285 K, where the early stage of the melting process appears with the loss of positional order. At higher temperatures (between 290 and 305 K) a kinetically stable, orientationally ordered phase is obtained. This nematic phase was reached starting with three initial configurations, differing in their orientational order parameter. The calculated values of thermodynamic and structural properties of each phase were in fairly good agreement with the relevant experimental data.
Lengthy molecular dynamics (MD) simulations were performed at constant atmospheric pressure and different temperatures for the series of the 4-n-alkyl-4'-cyanobiphenyls (nCB) with n = 6, 7, and 8. The accurate atomistic force field (Bizzarri, M.; Cacelli, I.; Prampolini, G; Tani, A. J. Phys. Chem. A 2004, 108, 10336), successfully employed to reproduce thermodynamic and transport properties of the 5CB molecule, has here been extended to higher homologues. Nematic and isotropic phases were found for all members of the series, and also, a smectic phase was (tentatively) identified for 8CB at 1 atm and 300 K. Transition temperatures reproduce the experimental values within +/-10 K. Also, structural properties as second and fourth rank orientational order parameters are in good agreement with the corresponding experimental quantities. This means that the well-known odd-even effect, observed for many properties along the nCB series, is well reproduced, despite the narrow range of oscillations, e.g., in clearing temperatures. A detailed analysis of the correlation between molecular properties and odd-even effects is presented.
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