摘要:

The Gibbs Ensemble Monte Carlo method has been used to simulate osmotic equilibria for Lennard-Jones mixtures. When the simulations are performed with two independent boxes, one containing solvent and the other a mixture of solute and solvent significantly negative osmotic pressures (Π) develop. Following a sugestion of Powleset al.(1997,Molec. Phys.,90, 665), we have extended these simulations to include a third box and the possibility of modelling three coexisting phases. The simulations show that the two phase equilibria with negative values of 77 are metastable and that the system spontaneously separates into three phases: pure solvent, dilute solute-solvent and dense solute-solvent with a resulting osmotic pressure that is normally small and positive.

ISSN:0026-8976    

DOI:10.1080/00268970009483347

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

In molecular dynamics simulations of reacting systems, the key step to determining the equilibrium constant and the reaction rate is the calculation of the free energy as a function of the reaction coordinate. Intuitively the derivative of the free energy is equal to the average force needed to constrain the reaction coordinate to a constant value, but the metric tensor effect of the constraint on the sampled phase space distribution complicates this relation. The appropriately corrected expression for the potential of mean constraint force method (PMCF) for systems in which only the reaction coordinate is constrained was published recently. Here we will consider the general case of a system with multiple constraints. This situation arises when both the reaction coordinate and the ‘hard’ coordinates are constrained, and also in systems with several reaction coordinates. The obvious advantage of this method over the established thermodynamic integration and free energy perturbation methods is that it avoids the cumbersome introduction of a full set of generalized coordinates complementing the constrained coordinates. Simulations of n-butane and n-pentane in vacuum illustrate the method.

ISSN:0026-8976    

DOI:10.1080/00268970009483348

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

We report results on the concentration dependence of the non-coincidence effect in thev(C=O) mode in N,N-dimethylformamide-CCl4mixtures. This study has been mainly addressed to the investigation of the experimentally observed sign inversion for this quantity in the low concentration range of chemical mixtures, which is also reported in the literature. We argue that the sign inversion of the non-coincidence effect in N,N-dimethylformamide-CCl4mixtures is an artefact originated by the presence of an intrinsic asymmetry which with dilution moves from the blue to the red side of the band. The Logan theory for the thermodynamic state dependence of the non-coincidence effect in chemical mixtures offers a valuable tool for the interpretation of the results in the whole concentration range. Different models are compared and discussed.

ISSN:0026-8976    

DOI:10.1080/00268970009483349

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Anab initiocalculation has been performed on the lowest seven doublet and six quartet Rydberg states of CIO at the CASSCF/MRCI level and with basis sets suitable for the extended molecular orbitals of such states (aug-cc-pVTZ with up to eleven extra Gaussian functions). Calculations on the quartet states reveal the energy ordering of Rydberg orbitals to be 4sσ, 4pπ, 4pσ;, 3dδ, 3dσ and 3dπ. The calculated doubletab initiopotential curves confirm experimental assignments of the C2Σ- and F2Σ- states but require reassignments for the symmetries of the D (2Δ), E (2Π) and H (2Δ) Rydberg states. These revisions are supported by spin-orbit coupling calculations that suggest the separation between the Ω components is small. In addition, a2Σ+state has been identified as the likely upper state for two previously unassigned vibronic bands recorded in absorption studies.

ISSN:0026-8976    

DOI:10.1080/00268970009483350

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

We report on an ellipsometric experimental study designed to explore the relevance of the wetting phase diagram predicted by liquid state physics of basic models, to the wide class of simple organic liquid films that adsorb from saturated vapour onto planar substrates at room temperature. The wetting properties are explored by measuring adsorption isotherms in the approach to saturation, in particular, for adsorption of n-hexane on a variety of specially constructed substrates (self-assembled monolayers) spanning a wide range of surface energy, and by carrying out the microscopic equivalent of contact angle experiments at saturation. We locate a wetting transition, which in our case is continuous, and then study its properties in detail. The general prediction of the wetting phase diagram, that wetting transitions should be ubiquitous in nature and readily located via control over the substrate field, is supported by our data, but the quantitative nature of the thick film adsorption regime is not in agreement with Lifshitz theory. This conclusion supports the work of a variety of earlier related studies, but contrasts with recent results for adsorption onto the surface of water. In addition, the correlation length determined from our complete wetting adsorption isotherms is mesoscopic, suggesting that equilibrium statistical mechanics of simple models of inhomogeneous fluids cannot explain the data.

ISSN:0026-8976    

DOI:10.1080/00268970009483351

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The locked echo is the result of the application, to an inhomogeneously-broadened system, of a pulse sequence consisting of a short π/2 pulse, a free evolution period of length r and a long pulse with high turning angle (HTA). In this paper the nature and features of the signal detected after such a pulse sequence are reported upon by analysing the underlying physical mechanisms in the framework of the density operator formalism. It is found that the total signal does not contain any contribution from Free Induction Decay (FID)-like or anti-echo signals but comprises a group of several distinct echoes: the true simultaneous locked echo, arising from locked magnetization, and two or three non-simultaneous, oscillatory echoes similar to those observed in the single-pulse experiment. Properties of these echoes are deduced from the structure of the density operator at the end of the experiment. Extensive numerical simulations provide independent evidence of the correctness of the developed theory, display the variety of patterns shown by the locked echo when experimental conditions are changed, and also permit one to investigate the locked echo shape beyond the approximations introduced in the theory.

ISSN:0026-8976    

DOI:10.1080/00268970009483352

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

In previous work we have studied the solution of the Ornstein-Zernike equation with a general multiyukawa closure. Here the direct correlation function is expressed by a rapidly converging sum of M (complex) exponentials. For a simple fluid the mathematical problem of solving the Ornstein-Zernike equation is equivalent to finding the solution of a linear algebraic equation of orderM.The solution for the arbitrary case is given in terms of a scaling matrix Γ. For only one component this matrix is diagonal and the general solution using the properties ofM-dimensionalSOMLie group is given. In the Mean Spherical Approximation (MSA) the excess entropy is obtained and expressed as a sum of 1-dimensional integrals of algebraic functions. We remark that the general solution of theMexponents-1 component case was found in our early work (Blum, L., and Hoye, J. S.,1978,J. stat. Phys.,19, 317) in implicit form. The present explicit solution agrees completely with the early one. Other thermodynamic properties such as the energy equation of state are also obtained, explicitly for 2 and 3 exponentials. The analytical solution of theeffectiveMSA is also obtained from the simple variational form for the Helmholtz excess free energy δA(1)where(2)where both the excess energyΔE(Γ)and the excess entropy ΔS(Γ) are functionals of Γ, which opens interesting possibilities that are discussed elsewhere. We remark that this is a non-trivial property, which is certainly true for the MSA (Chandler, D., and Andersen, H. C., 1972,J. chem. Phys.,57, 1930). It implies cross-derivative properties for the closure equations, which have been verified in all cases.

ISSN:0026-8976    

DOI:10.1080/00268970009483353

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor