摘要:

Configurational bias Monte Carlo calculations in the Gibbs ensemble have been used to perform direct simulations of the vapour-liquid phase equilibria of three heptane isomers: n-heptane, 2-methylhexane, and 3-ethylpentane. The simulations were carried out using isotropic united-atom representations of the Lennard-Jones type for the CH3, CH2and CH groups. The aim of these calculations is to extend our force field, previously derived for linear alkanes, to branched alkanes by fitting new interaction parameters for methyl and ethyl branches.

ISSN:0026-8976    

DOI:10.1080/002689797172048

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

A simple procedure is given for determining model two- and three-body intermolecular potential energy functions from precise measurements of the speed of sound in the gas phase. The method is applied to the pure gases argon, methane and nitrogen and results are considered for propane and the mixture (methane + propane) obtained recently by similar methods. For the pair potential, the four-parameter model proposed by Maitland and Smith is used while the consequences of three-body forces were generally assumed to be represented adequately by the triple-dipole dispersion potential of Axilrod and Teller. The effect of including additional dispersion and exchange terms in the three-body potential was investigated for argon; each of these terms is significant but their effect on the third virial coefficient may be absorbed accurately in an effective triple-dipole potential. Three or, in some cases, all four of the parameters in the pair potential were optimized, together with the triple-dipole dispersion coefficient, in fits to the acoustic data. Ordinary second and third virial coefficients have been calculated from the model potential and the former are shown to be in excellent agreement with directly measured values. In the case of the third virial coefficient, some small discrepancies are noted; the resolution of these differences could have important implications for our understanding of many-body forces. The values of the dilute-gas viscosity predicted by the model pair potentials are examined also and, in the case of argon, found to be in good agreement with experimental values; for the other systems differences of several per cent were noted.

ISSN:0026-8976    

DOI:10.1080/002689797172057

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The practicablity and precision of the intermolecular potential was improved within the frame of an atom-atom approximation using 6-exp-1 type isotropic interatomic potentials without additional parameters. The improvement involves making the van der Waals parameters functions of the partial charges on constituent atoms supplement the classification of the parameter set, depending on atom type. The validity of the potential was confirmed with respect to crystal structures of polyacene (i.e., benzene, naphthalene, anthracene and tetracene) by molecular dynamics simulations.

ISSN:0026-8976    

DOI:10.1080/002689797172066

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Reliable results are reported for the various triple-dipole dispersion energy coefficients needed to evaluate the (exact) triple-dipole dispersion energy for all three-body interactions involving He, Ne, Ar, Kr, Xe, H2, N2, CO, O2and NO, and at least one molecule. A total of 645 unique coefficients, corresponding to 185 unique interactions, are considered.

ISSN:0026-8976    

DOI:10.1080/002689797172075

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Constrained anisotropic dipole oscillator strength techniques are used to obtain reliable values for a wide range of anisotropic and isotropic dipole properties of NO, including most anisotropic components of the dipole-dipole dispersion energy coefficients for the interaction of NO with NO, O2, H2, N2, CO, He, Ne, Ar, Kr and Xe. Some of the anisotropic constraints required for our calculations are obtained via dipole sum rules fromab initio, multi-reference configuration interaction wavefunctions for NO. The individual dipole properties of NO considered include the dipole oscillator strength sumsSk,k= 2,1,0(− 1/2)− 2,− 3,− 4,..., the logarithmic dipole sumsLkand mean excitation energiesIk, k = 2(− 1)− 2, and, as a function of wavelength, the dynamic polarizability and its anisotropy, the total depolarizaiton ratio, and the Rayleigh scattering cross-section. Our constrained dipole oscillator strength results are often the only reliable, and often the only available, values for many of the properties and dispersion energies considered.

ISSN:0026-8976    

DOI:10.1080/002689797172084

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

A variational procedure for rovibrational energy levels and wavefunctions of centrally connected tetra-atomic molecules is extended to include high rotational states, and in particular, J ⩽ 10 levels for the vibrational ground state of formaldehyde. It is very important to do this because it has made possible the calculation of the usual rotational spectroscopic constants which correspond to the forcefield and geometry. A direct comparison with the ‘observed’ spectroscopic constants is therefore possible. The geometry and forcefield are refined against 65J= 0 levels of H2CO, 6J= 0 levels of D2CO, 42J= 1, 70J= 2 and 98J= 3 levels of the ground and fundamentals of H2CO and D2CO, using an iterative scheme. The mean absolute error of theJ= 0 levels is 1·10 cm−1and that forJ≠ 0 is 0·005 cm−1, and the predicted geometry is CH = 1·10064 Å, CO = 1·20296 Å and HCO = 121·648°. Finally, the rotational constantsA,B, andCfor the ground state are 281956, 38846 and 34003 MHz, compared with the observed values 281971, 38836, and 34002 MHz. The centrifugal distortion constantsΔJ,ΔJK,ΔKandδJ, are 77, 1275, 18113 and 11 kHz compared with 75, 1291, 19422 and 10 kHz. These results underline the accuracy of the new quartic forcefield.

ISSN:0026-8976    

DOI:10.1080/002689797172093

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Brownian dynamics simulation is used to study the structure and small-deformation shear rheology of three-dimensional particle gels formed from a model of soft spherical particles incorporating a combination of flexible, irreversible bond formation and non-bonded interparticle interactions. An essential feature of the model is the restriction of angular reorganization of cross-linked aggregates during and after gelation. Numerical data relating to the fractal structure and porosity of the gels are compared with those from some related particle gel simulation models in two and three dimensions. Stress relaxation and frequency dependent rheological properties are determined as a function of the strength and nature of the interparticle interactions, and the results are discussed in relation to structural properties. A linear relationship is demonstrated between the high-frequency modulus and the number of bonds in the gel network.

ISSN:0026-8976    

DOI:10.1080/002689797172101

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

We present two new perturbation density functional theories to investigate non-uniform fluids of associating molecules. Each fluid molecule is modelled as a spherical hard core with four highly anisotropic square well sites placed in tetrahedral symmetry on the hard core surface. In one theory we apply the weighting from Tarazona's hard sphere density functional theory to Wertheim's bulk first-order perturbation theory. The other theory uses the inhomogeneous form of Wertheim's theory as a perturbation to Tarazona's hard-sphere density functional theory. Each theory approaches Tarazona's theory in the limit of zero association. We compare results from theory and simulation for density profiles, fraction of monomers, and adsorption of an associating fluid against a hard, smooth wall over a range of temperatures and densities. The non-uniform fluid theory which uses Tarazona's weighting of Wertheim's theory in the bulk is in good agreement with computer simulation results.

ISSN:0026-8976    

DOI:10.1080/002689797172110

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Inelastic neutron spectroscopy on pure light and heavy water and on mixtures is reported in this paper. The results are compared with calculations based on two models of the dynamic self-scattering law. Both models are connected to the mean kinetic energy of the atoms in water via an effective temperatureTE. For oxygen,TE≈ 450 K; for deuteron,TE≈ 1000 K; for the proton,TE≈ 1200 K. The models are intended as a base for inelasticity calculations for time-of-flight neutron diffraction data. A calculation of the differential cross-section for light and heavy water based on these models and effective temperatures is also presented and compared with measured data. In these calculations a new closed expression for the inelastic scattering was used; this is described in an appendix.

ISSN:0026-8976    

DOI:10.1080/002689797172129

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Molecular dynamics simulations of the aqueous solution of alanine dipeptide have been carried out for seven configurations characteristic of important regions of the Ramachandran plot. A hybrid quantum mechanical-molecular mechanical potential was used that describes the solute using the AM1 Hamiltonian and the solvent using the TIP3P model. The importance of differential solute polarization and the preferential stabilization of the extended structures αL, αRand β have been identified. The results are compared with experiment and with the predictions of theab initiopolarizable continuum model of solvation.

ISSN:0026-8976    

DOI:10.1080/002689797172138

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor