摘要:

Vapour-liquid equilibrium data and critical properties were determined for a three-centre Lennard-Jones potential model (the Tildesley–Madden potential) for liquid carbon disulphide. The methods used are based on the Taylor expansion of the chemical potential calculated by Monte Carlo simulation in the canonical and in the isothermal–isobaric ensembles. Gibbs ensemble Monte Carlo simulations were also carried out for comparison. Phase coexistence data were obtained at rather low temperatures applying the Gibbs–Helmholtz equation for converting the chemical potential from higher temperature. The three methods for the calculation of the vapour–liquid equilibria yielded essentially the same results. At lower temperatures the agreement with experiment is better.

ISSN:0026-8976    

DOI:10.1080/002689796173390

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

We report measurements of the difference of the reduced second refractivity virial coefficient ΔB*(T) =BR(325.13 nm,T)/AR(325.13 nm,T)–BR(632.99 nm,T)/AR(632.99 nm,T) of xenon in the temperature range between 295 K and 1100 K.AR(λ,T) andBR(λ,T) are the first and second refractivity virial coefficients measured at λ = 325.13 nm and λ = 632.99 nm respectively. ΔB*(T) is found to decrease with increasing temperatureTas 107ΔB*(T)/m3mol-1= 12.6(6.8) × exp[-0.0038(13) × (T/K/)] with a standard deviation of 0.94 x 10-7m3mol-1. With the aid of one absolute value ofBR(λ,T) and calculations based on an extended dipole–induced-dipole model, a normal dispersion ofBR(λ,T) is found throughout the whole temperature range. This result is contrary to former room-temperature measurements, where an anomalous dispersion ofBR(λ,T) of xenon has been reported.

ISSN:0026-8976    

DOI:10.1080/002689796173408

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Discrete variable representation (DVR) calculations are performed for H+3. These calculations yield rotation–vibration wavefunctions which extend all the way to dissociation for the ground (J= 0) and first two (J= 1,2) rotationally excited states of the system. Approximately 10 million linestrengths are computed for transitions involving these states and infrared spectra in the near-dissociation region are synthesized. These spectra are analysed and the steps necessary to obtain a theoretical version of the Carrington–Kennedy (1984,J. chem. Phys.,81, 91) infrared H+3photodissociation spectrum are discussed.

ISSN:0026-8976    

DOI:10.1080/002689796173417

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Valleau's thermodynamic scaling Monte Carlo conceptis combined with the Gibbs ensemble method for phase coexistence calculations, and phase co-existence properties of several Hamiltonians (potential models) at many temperatures are obtained in a single simulation. The theoretical formulation of the method is presented in detail. An application to the (n– 6) Lennard-Jones family of potentials is demonstrated. The combined method is found to be more efficient than the conventional Gibbs ensemble method when properties of related potential models at many thermodynamic states are of interest. Possible extensions of the combined methodology to mixtures, intermolecular potential model development, and multisegment systems that require configurational-bias sampling are discussed.

ISSN:0026-8976    

DOI:10.1080/002689796173426

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Neutron diffraction measurements are reported of carbon dioxide in dense supercritical states distributed along three isotherms (313 K, 373 K and 473 K) and at various densities ranging from 8.7 molecules nm-3to 14.0 molecules nm-3. The intermolecular structure factors, obtained after subtraction of the intramolecular interference terms, are transformed torspace yielding the neutron weighted pair distribution functions. Throughout the supercritical region investigated, the structure of the fluid depends essentially upon the density. The experimental results are compared also with a model system of fluid CO2in the absence of orientational correlations as well as with existing molecular dynamics simulations. It is found that the orientational correlations persist in the supercritical region investigated and that the data are consistent with a predominance of T shaped local configurations at higher densities.

ISSN:0026-8976    

DOI:10.1080/002689796173435

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

For fluids of hard convex molecules, the ring integrals occurring in the diagrammatic expressions for the virial coefficients can be evaluated by direct integration in terms of a set of 2-point measures of 1-body molecular geometry. A detailed study is reported of the properties ofthe 2-point measures, presenting methods of calculation, and useful functional relations between different measures. In particular, the calculation of measures with one or two volume points is reduced to the calculation of 2-point measures with two surface points. This reduces the calculation of 2-point measures to integrals that are 3- dimensional in the absence of symmetry, and 2-dimensional for molecules with an axis of symmetry. Two sensitive tests of the accuracy of 2-point measures calculated numerically are derived. Integrals of 2-point measures, and the asymptotic forms of 2-point measures for smallrare both expressible in terms of 1-point measures, which are available in closed form for such typical models as spheroids, spherocylinders, and torocylinders.

ISSN:0026-8976    

DOI:10.1080/002689796173444

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The method of 2-point measures of molecular geometry is applied to the calculation of the third virial coefficients of fluids of hard convex molecules. For multicomponent fluids, existing semiempirical (SE) theories propose an expression in terms of the 1-point measures volumeV(i), surfaceS(i), and integral mean curvatureM(i) of the componentsi= A,B,C. The method of 2- point measures does not introduce these quantities explicitly. In the limit of C either much smaller or much larger than A and B, an expansion in the ratio of dimensions is obtained, in which the leading terms are expressible in terms of theV(i),S(i),andM(i). For C small, the three leading terms are of this form; the first two agree with the SE equations. For C large, only the leading order is expressible in terms of the 1-point measures; it agrees with the SE equations. The third virial coefficient is calculated numerically for prolate and oblate spheroids using the method of 2-point measures. The infinite set of measures is truncated at the lowest possible level required to yield exact results for hard spheres: all measures involving the curvature asymmetry are neglected. Results are compared with existing exact values obtained by Monte Carlo integration, and with the predictions of the SE theories.

ISSN:0026-8976    

DOI:10.1080/002689796173453

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Approximate calculations are reported on pyrene within the PPP model Hamiltonian using a novel restricted CI scheme which employs both molecular orbital and valence bond techniques. Also reported are detailed full CI results of the PPP model on 2,7-dihydropyrene obtained using the valence bond method. Spectral studies, charge and spin density calculations in ground and excited states, and ring current calculations in the ground state of the molecules are presented. In pyrene, the calculated excitation energies are in good agreement with experiment. The closed structure 𝛑-conjugated molecule pyrene appears to show smaller distortions from the ground state geometry compared with the open structure 𝛑-conjugated molecule 2,7-dihydropyrene. The ground state equilibrium structure of 2,7-dihydropyrene can be viewed as two hexatriene molecules connected by a vinyl crosslink, as is evident from bond order and ring current calculations. This is consistent with the only Kekulé resonant structure possible for this molecule.

ISSN:0026-8976    

DOI:10.1080/002689796173462

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Molecular dynamics simulations were performed on four systems: pure SPC/E water, and three solutes with different surface curvatures in water. The same intermolecular potential was employed throughout, but the radius of curvature was altered by inserting a hard ‘inner core’ into the solute. The radial distribution function, angular distribution, a bond-order parameter, a hydrogen bond analysis and a near-neighbour analysis were used to assess the static properties of water molecules in the first solvation shell and in the bulk phase, while their dynamic behaviour was examined using various velocity and reorientational autocorrelation functions. Water molecules in the solvation shell exhibit enhanced structuring, but this structuring is decreased when the curvature is decreased. The translational and rotational dynamics of water are slowed down, but the effect is independent of curvature.

ISSN:0026-8976    

DOI:10.1080/002689796173471

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Nanocrystals of Kr and Xe have been prepared by condensing the rare gas into the mesopores of dry silica gel with specified pore diameters of 22, 40, 60, and 100 Å, and their X-ray powder diffractograms are reported. In the small angle range the scattering intensity was reduced by 70-98% since the electron densities of these rare gas solids and of SiO2nearly match. This allows one to monitor the degree of pore filling. In the wide angle range some additional scattering intensity appears from the solid rare gases, and this was separated from the underlying silica gel diffractogram. All features of the solids in the pores are broadened considerably with respect to the patterns of the respective extended crystals. The broadening increases with decreasing pore size, the diffractogram of Kr in silica gel 22 being similar to those of amorphous solids. Diffraction patterns of Kr and Xe clusters calculated with appropriate geometries and degrees of disorder reproduce the experimental data. The dimensions of these clusters and the specified mean pore diameters of the respective silica gels match very well. The measurement of the X-ray contrast in the wide range should provide a powerful complementary approach to porosimetry.

ISSN:0026-8976    

DOI:10.1080/002689796173480

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor