摘要:

The chemical potential for a two-component Lennard-Jones fluid with associative interaction between opposite species promoting the formation of dimers is calculated using osmotic Monte Carlo (OMC) canonical ensemble simulations. Grand canonical Monte Carlo simulations also are performed to verify the accuracy of the OMC approach. The data from both methods agree very well for thermodynamic states with different degrees of dimerization. It follows that the OMC is a promising approach for the determination of the thermodynamics of and equilibria between associating and non-associating fluids and associating fluid mixtures.

ISSN:0026-8976    

DOI:10.1080/002689797169583

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Several anisotropic interaction potentials which describe the He–CO gaseous mixture were tested via the detailed analysis of recently measured thermal diffusion factors, as functions of both temperature and concentration. The calculations were carried out using the quantum treatment of collision dynamics via the infinite order sudden (IOS) coupling scheme between angular momenta, and further using two additional schemes of approximation to second order: the Chapman–Cowling and the Kihara expansions. The results made it possible to test the relative behaviour of the repulsive regions of the various interactions examined in this work, among which those given by the density functional approach for describing van der Waals forces turn out to give fairly realistic values for the anisotropy of the repulsive wall, but have difficulties in generating the well region correctly. These new calculations confirm also the reliability of an earlier multiproperty potential when treating transport properties at temperatures above 100 K, and therefore when examining features which do not depend much on the details of the attractive well.

ISSN:0026-8976    

DOI:10.1080/002689797169592

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The precision of several methods for computing the chemical potential by molecular simulation is investigated. The study does not apply molecular simulation to the analysis but instead works with models of the simulation process. These models enable the variance of the chemical potential to be computed accurately and very quickly and thereby permits the methods (freeenergy perturbation, expanded ensembles, thermodynamic integration, and histogram-distribution methods) to be optimized and compared over a range of densities. The study focuses exclusively on the hard-sphere model. This model is simple and well characterized; yet it exhibits the essential features that make the chemical potential calculation difficult; arguments are presented to support the broader applicability of the study. The severe asymmetry of particle insertion against particle deletion is highlighted, and it is shown that any staged free-energy perturbation method with a 'deletion' component is highly prone to systematic error. More generally this implies that such methods should always be staged in the direction of decreasing entropy. Other findings show that uniform sampling is not optimal for umbrellasampling and expanded-ensemble applications, although it remains a good rule of thumb for tuning these approaches. Among the techniques we study, optimally staged insertion and the distribution–histogram methods are the most efficient and precise. The latter is effective only when used in an interpolative fashion, and we identify it as the most likely route to further progress in the field.

ISSN:0026-8976    

DOI:10.1080/002689797169600

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Green's function quantum Monte Carlo (GF QMC) simulations of fermionic ensembles require the definition of a so-called spectral parameterWto yield trustworthy estimates of the fully correlated ground state energyE. In the present work we discuss the influence of the shift parameter,W, on the convergence behaviour of GF QMC simulations. As model systems we have considered some π molecules which are studied in the framework of the Pariser–Parr–Pople (PPP) Hamiltonian. The influence of theWparameter on the convergence of the GF QMC simulations in many-electron systems with an odd number of electronic permutations within one spin direction exceeds the influence observed in systems without such odd permutations. They do not occur in polyenes and Huckel annulenes with an electron count of (4n+ 2) (n= 0, 1, 2…). The GF QMC technique adopted as a computational tool has been developed to study π molecules with fermionic sign problems owing to odd electronic interchanges within one spin direction.

ISSN:0026-8976    

DOI:10.1080/002689797169619

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The temperature dependence of nuclear shieldings as well as isotope effects on shieldings and spin-rotation constants have been computationally investigated for H2, HF, F2, CO, and N2employing the coupled-cluster singles and doubles (CCSD) method augmented by a perturbative treatment of triple excitations (CCSD(T)) for the calculation of potential curves, shieldings and spin–rotation functions together with finite-element techniques for the solution of the rovibrational problem. Calculated and measured temperature dependence of the isotropic shieldings agrees for N2, while for CO and F2the computed temperature dependence is smaller than the experimental result. Isotropic shieldings have been deduced on the basis of our calculations from the measured spin-rotation constants for four isotopomers of H2and agree, as required by theory. However, calculated and measured temperature dependence of the isotope shifts between HD and D2differ by up to 10% which is larger than the estimated error bars for the experimental values. For HF and CO, calculated and measured isotope shifts agree, while for N2no experimental data for comparison are available. In case of spin–rotation constants, the calculated dependence on the rotational angular momentum quantum number are for both H2and F2in good agreement with the dependence deduced from measurements, while for HF not enough experimental data are available for a comparison.

ISSN:0026-8976    

DOI:10.1080/002689797169628

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The orbital amplitude difference function is used to assess the quality of Hartree–Fock orbitals obtained by invoking the algebraic approximation for the BF ground-state. Systematic sequences of even-tempered, spherical-harmonic Gaussian-type basis functions are used to generate orbitals for which the corresponding total Hartree–Fock energy approaches the 1 μEhlevel of accuracy. Exact orbitals are obtained from finite difference calculations using a grid based on spheroidal coordinates. The finite basis set approximations for the orbital are discretized. The accuracy of the discretization is assessed. For each occupied orbital a discretized representation of the orbital amplitude difference function is generated and analysed.

ISSN:0026-8976    

DOI:10.1080/002689797169637

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Anion polarizabilities in alkali halide crystals are analysed as a function of interionic separationR. The anion polarizability is treated as a function of the anion and cation radii, with its partial derivatives approximated by those with respect toRfor fixed cation and anion, respectively. With pressure derivatives of the ionic radii deduced from the crystal compressibility, assuming transferability among crystals, the polarizability derivatives with respect to ionic radius yield pressure derivatives of the polarizability that agree with experiment to within a factor of two. These results offer a useful means of predicting the pressure dependence of dielectric data.

ISSN:0026-8976    

DOI:10.1080/002689797169646

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Zero-field nuclear magnetic resonance spectroscopy of solids containing quadrupole nuclei usually results in broad spectral lines. This line-broadening is due mainly to the inhomogeneity of the electric field gradient (EFG) at the quadrupolar nuclear site. High resolution spectra of such solids can be obtained with the application of suitably designed multiple radiofrequency (RF) pulse sequences. The performance is reported for a periodic and cyclic phase alternated multiple RF pulse sequence (PAPS) in a spinI= 7/2 system in zero external magnetic field. Average Hamiltonian theory based on the Magnus expansion is used to solve the time-dependent Liouville–von Neumann equation of motion of the spin system under the effect of the PAPS sequence. Single transition operators are employed in the spin dynamics calculations. It is shown that the multiple pulse seqeunce θ1- [τ - θ2- 2τ - θ2- τ]N, where is θ1is thw preparation pulse, suppresses the EFG inhomogeneity to a maximum extent when θ2= 2θ1

ISSN:0026-8976    

DOI:10.1080/002689797169655

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Nine well studied (polar and non-polar) planar solutes have been used as probes to investigate the mechanism for orientational ordering in the nematic phase. Second-rank orientational order parameters have been derived from proton dipolar coupling constants, obtained from the NMR spectra of these molecules dissolved in a 55 wt% ZLI1132 + EBBA nematic mixture. The experimental data are interpreted in terms of an orienting potential experienced by molecules in a liquid crystal environment. A model for the orienting potential is presented which includes, in addition to short-range repulsion interactions described by the surface tensor model, long-range contributions treated according to a generalized reaction field approach. Least-squares fits of solvent dependent parameters to the experimental data have been performed to estimate the weight of the various contributions to the solute orientational order, and the results obtained in this way are discussed. Finally, the results obtained with the surface tensor model are compared with those from the size–shape potential suggested by other workers.

ISSN:0026-8976    

DOI:10.1080/002689797169664

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Cyclohexane, succinonitrile, cyclohexanol and pivalic acid were investigated in the plastic state by means of nuclear magnetic resonance (NMR) field-cycling relaxometry and NMR fieldgradient diffusometry. Measurements of the field-dependent relaxation timesT1(υ) of cyclohexane and succinonitrile are described assuming anisotropic molecular rotation on short timescales and translational diffusion on long timescales leading to a modulation of the intermolecular dipole–dipole coupling. Correlation times and anisotropy parameters are evaluated from the experimental results. The self-diffusion coefficients calculated from the field-cycling data are found to coincide with the values determined directly employing fieldgradient diffusometry. In the cases of pivalic acid and cyclohexanol, a more complicated relaxation dispersion is observed which indicates the presence of correlated motions between neighbouring molecules. For the cyclohexanol, the presence of a liquid component well below the freezing point is described quantitatively.

ISSN:0026-8976    

DOI:10.1080/002689797169673

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor