摘要:

The commonly used exchange-correlation functionals of density functional theory and their potentials are examined numerically following the first such investigation by Perdew. They are also investigated for Ne and Kr. Their behaviour for large gradients of the density and for large distances is not satisfactory. In particular, the correct asymptoticr-1behaviour is difficult to achieve. Following van Leeuwen and Baerends, this is linked to the energy ϵmaxof the highest occupied orbital arising from the Kohn-Sham equations. This deficiency is linked also with the poor prediction of molecular polarizabilities.

ISSN:0026-8976    

DOI:10.1080/00268979600100011

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

To improve our understanding of the liquid-gas phase separation occurring in ordinary molten salts a variety of model ionic fluids is investigated extending from the restricted primitive model (RPM) (composed of hard ions with equal size) to model reactive fluids in which a chemical reaction between ionic and covalent species takes place. In searching to improve the existing linearized theories for the RPM, it is shown that the mean-field approach of Debye-Hückel (DH) theory is very sensitive to successive improvements (ion pairing, hard sphere contribution, dielectric constant), whereas the mean-spherical approximation (MSA) exhibits, under similar conditions, less sensitive behaviour. In particular, it is shown that the good agreement obtained by M. E. Fisher and Y. Levin (1993,Phys. Rev. Lett.,71, 3826) with computer simulation data in implementing the ion-dipole interaction into DH theory with ion pairing worsens significantly if the dielectric medium composed of ion pairs is properly taken into account in a self-consistent way. However, our improved MSA incorporating at the same time ion pairing, ionic solvation and dipole-dipole contributions shows only qualitative agreement with the available simulation data.

ISSN:0026-8976    

DOI:10.1080/00268979600100021

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

A site-site intermolecular potential for the prediction of vapour-liquid coexistence curves for the homologous series of alkanols is tested for applicability by means of Gibbs-ensemble Monte Carlo simulations. For lower alkanols up to propanol, a literature parameter set (fitted to describe liquid densities at ambient temperature) performs quite satisfactorily. Mere extrapolation to higher alkanols, however, does not yield satisfactory results. A more sensible route to derive site parameter values for higher alkanols is found in combining parameter sets for alkanes and methanol, which are optimized for coexistence property predictions.

ISSN:0026-8976    

DOI:10.1080/00268979600100031

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

X-Ray diffraction measurements of liquid water at high temperatures and pressures (up to 200 °C and 200 MPa) are reported. A diffractometer with a transmission geometry and a cylindrical sample holder made of beryllium were used, with a specially designed high pressure unit. The data reduction procedures are described in detail. The analysis of the structure functions and radial distribution functions show that the O-O distances shorten due to pressure but they are elongated with increasing temperature. The average contraction rate is 0·02 pm MPa-1. The average coordination number of the first neighbour molecules does not change significantly with increasing temperature but slightly increases with increasing pressure. The density dependence of the distances and coordination are discussed. The expansion of water is proportional to the expansion of first neighbour distances in the medium density region only (0·95-1·01 g cm-3), while the coordination number produces a minimum in the medium density region. This is an indication of coexistence of unchanged H bonds with deformed or disrupted H bonds in the pressure and temperature region studied.

ISSN:0026-8976    

DOI:10.1080/00268979600100041

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

A theory for an electrode-associative ionic fluid interface is proposed. It takes into account the effect of interionic pairing as well as packing and electrostatics. The theory is based on the use of Wertheim's two-density formalism, i.e., the ions are represented as free and associated into dimers. Dimerization is made possible by the attractive interaction between sites embedded in each ion. The results for ionic density profiles calculated in a generalized exponential approximation are given. Analytical expressions for the differential capacitance, potential drop and adsorption coefficient calculated in the mean spherical approximation are presented.

ISSN:0026-8976    

DOI:10.1080/00268979600100051

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

A1H single crystal NMR study on strontium nitroprusside Sr[Fe(CN)5NO]·4H2O has demonstrated that water molecules are not static in the unit cell but undergo librations. The dipolar coupling tensors have been measured at 287 K and 120 K for three types of water molecule, W1, W2 and W3, existing in the unit cell of SrNP. The tensors are motionally averaged and the results are interpreted in terms of librational motion of the water molecules. The models for such motions are proposed and the angles of libration are calculated for each of the water molecules. At 120 K the asymmetry parameters of the tensors are still different from zero indicating that even at this temperature the water molecules in SrNP are characterized by great mobility. Despite the dipolar-dipolar (DD) tensors not being axially symmetric the unique eigenvectors still give reliable information about the average proton-proton directions within the unit cell of SrNP. It was found that the directions of the proton-proton vector given by the largest eigenvector of the DD tensor for W1 are quite different at 286 K and at 120 K. This result is interpreted as being due to a rearrangement of the W1 water molecule, which takes place at the phase transition at 198 K.

ISSN:0026-8976    

DOI:10.1080/00268979600100061

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Thermodynamic properties of mixtures of dipolar and non-polar linear molecules are obtained fromNpTmolecular dynamics (MD) simulations and are compared with previously reported results fromNVTMD simulations, and from a perturbation theory for polar non-spherical molecular fluids. The interactions between the particles of the mixtures are all of the same two-centre Lennard-Jones (2CLJ) type, of elongationL= 0·505 with an additional axial point dipole on the polar molecules. For these mixturesNpTMD simulations were performed for various dipole moments and compositions for a statepoint previously investigated byNVTMD simulations. We show that the approximation used to calculate excess Gibbs energies at constant pressure from theNVTensemble gives correct results even if it is more computationally demanding. Additionally, we present results for the excess enthalpy and the excess volume, which are in good agreement with perturbation theory.

ISSN:0026-8976    

DOI:10.1080/00268979600100071

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

A Monte Carlo simulation study is made of the phase diagram of the restricted primitive model and of the solid-liquid and solid-solid phase coexistence curves in particular. At low temperatures, there is liquid-bcc coexistence and with increasing density there is bcc-fcc coexistence. These coexistence curves end in a triple point (liquid-bcc-fcc) above which only liquid-fcc coexistence is observed.

ISSN:0026-8976    

DOI:10.1080/00268979600100081

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Differences in thermodynamic properties are investigated by means of constant pressure-constant temperature molecular dynamics simulations for three isomeric pairs of fluorinated ethanes: CHF2CHF2(HFC-134) and CF3CH2F (HFC-134a); CF3CH3(HFC-143) and CHF2CH2F (HFC-143a); and CH2FCH2F (HFC-152) and CHF2CH3(HFC-152a). These semi-rigid molecules have interaction centres at the atomic sites and internal rotation about the C-C bond. Intermolecular interactions consist of repulsion-dispersion and Coulombic parts. Molecular dynamics simulations were performed at the normal boiling points and the accord between simulated and experimental values of the potential energy and molar volume is very good. A great difference was found between the repulsion-dispersion and Coulombic contributions to the potential energy for every isomeric pair. A detailed analysis of the repulsion-dispersion and Coulombic interactions was carried out and was discussed in relation to the thermodynamic properties. A comparison of the radial distribution functions, autocorrelation functions and self-diffusion coefficients for these substances is also presented.

ISSN:0026-8976    

DOI:10.1080/00268979600100091

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The structure of liquidN-methylformamide (NMF) has been investigated using synchrotron radiation at 77 keV and 95 keV. The use of high energy photons has several advantages, in this case in particular the large accessible momentum transfer range, the low absorption and the direct comparability with neutron diffraction. The range of momentum transfer covered is 0·6 Å-1<Q< 24·0 Å-1. Neutron diffraction data on the same sample in the same momentum transfer range have been published previously. In that study two differently isotope-substituted species were investigated. In order to compare neutron and photon diffraction data properly reverse Monte Carlo (RMC) simulations have been performed. Some modifications had to be added to the standard RMC code introducing different constraints for inter- and intra-molecular distances as these distances partly overlap in liquid NMF. RMC simulations having only the neutron data as input were carried out in order to test the quality of the X-ray data. The photon structure factor calculated from the RMC configurations is found to agree well with the present experimental data, while it deviates considerably from earlier X-ray work using low energy photons (17 keV). Finally, there is comment on whether the different interaction mechanisms of neutrons and photons can be used to access directly the electronic structure in the liquid. Evidence is presented that the elastic self-scattering part of liquid NMF is changed with respect to the independent atom approximation. This modification can be accounted for by a simple charged atoms model.

ISSN:0026-8976    

DOI:10.1080/00268979600100101

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor