摘要:

By two independent methods we localize in a simple way the tricritical points in the McMillan model of smecticAphase. We have derived analytical solutions for the smecticA-nematic second order phase transition line in the McMillan model. We have solved numerically the set of the consistency equations of the McMillan model and calculated the entropies of the transition. The tricritical points correspond to ΔS= 0. There is the excellent agreement between both methods of determining the tricritical points.

ISSN:0026-8976    

DOI:10.1080/00268978500101991

出版商:Taylor & Francis Group    

年代:1985

数据来源: Taylor

摘要:

In a crossed molecular beam experiment total differential cross sections and differential energy loss spectra have been measured for Ne + CH4at 88·8 meV. The energy loss spectra taken at centre-of-mass deflection angles from 45° to 145° show appreciable energy transfer in the backward direction with a maximal energy transfer corresponding to final rotational states ofj′ = 4. The total differential cross sections exhibit diffraction oscillations in the forward direction. These two data sets are compared with calculations performed in the accurate centrifugal sudden approximation both for theA- andT-symmetry of the CH4molecule. The non-spherical potential surface, which includes the anisotropicT3andT4is determined from accurate SCF-calculations and a damped dispersion contribution using the HFD-model. The general features of the experimental data are reproduced using a simple universal damping function. However, discrepancies concerning the isotropic potential well depth and the repulsive anisotropy remain. The comparison with other methane-rare gas interactions reveals an increase of the attractive minimum and a decrease of the relative repulsive anisotropy going from He to Ar.

ISSN:0026-8976    

DOI:10.1080/00268978500102001

出版商:Taylor & Francis Group    

年代:1985

数据来源: Taylor

摘要:

In two crossed beam experiments total differential and rotationally inelastic cross sections have been measured for He + CH4in the angular range from 3° to 70° in the c.m. system. At the low collision energy ofE= 34·1 meV state resolved cross sections for the 0–3, 1–2 and 1–3 transition have been observed. All cross sections show well resolved diffraction oscillations. The inelastic cross sections are much smaller than the elastic ones. At the collision energy ofE= 75·5 meV energy loss spectra have been measured, which are, at large angles, dominated by inelastic transitions. The data are compared with quantum coupled-states calculations based on a potential model of the Hartree-Fock-dispersion type, where high quality SCF-calculations are combined with damped dispersion coefficients. The gross features of the measurements are reproduced by adjusting one parameter in the damping function. However, discrepancies arise in predicting some of the inelastic cross sections at low energies.

ISSN:0026-8976    

DOI:10.1080/00268978500102011

出版商:Taylor & Francis Group    

年代:1985

数据来源: Taylor

摘要:

A new method of computing intermolecular dispersion energies, starting from the SCF wavefunctions of the separate systems, is discussed and illustrated. The method is based on the use of generalized frequency-dependent polarizabilities, calculated using TDHF (time-dependent Hartree-Fock) theory, and all ‘self-consistency’ effects are thus accounted for in the final expressions for the dispersion energy. No multipole expansions are required. The method is shown to be computationally efficient in test calculations of the dispersion interactions between helium atoms and between hydrogen fluoride molecules.

ISSN:0026-8976    

DOI:10.1080/00268978500102021

出版商:Taylor & Francis Group    

年代:1985

数据来源: Taylor

摘要:

Since the conformation of physical systems is often advantageously described with the help of generalized (i.e. curvilinear) coordinates, the following questions are raised and general answers given to them: (i) What are the components of the momentum operators, those of the adjoint momentum operators and those of the hermitian momentum operators? (ii) What is the general form of the hamiltonian operator? (iii) What do the general forms of the momentum and hamiltonian operators become when expressed in terms of quasi-momentum operators (e.g. angular momentum component operators)? (iv) How are the answers to the above questions affected by an arbitrary choice of normalization convention for the total wave-function? A complete set of formulae (whatever the independent choices of coordinates, quasi-momentum operators and normalization) is given and various ‘historical’ formulae are shown to be particular instances of the general formulae proposed.

ISSN:0026-8976    

DOI:10.1080/00268978500102031

出版商:Taylor & Francis Group    

年代:1985

数据来源: Taylor

摘要:

The form of the density profile at the liquid-gas interface of some model fluids is investigated using a simple free-energy functional. For fluids with attractive Yukawa potentials we find that the profile decays exponentially into the bulk regions with a decay length greater than the appropriate bulk correlation length; the two lengths become closer and diverge in the same fashion at the critical temperature. We contrast this decay with that found for fluids with attractive van der Waals potentials where the profile decays asymptotically asz-3. The results of numerical calculations suggest that the effects of van der Waals' potentials would be difficult to observe in experiments or in computer simulations of liquid-gas interfaces. We compare the results of our calculations for fluids with truncated Lennard-Jones potentials with those of recent simulations. The theory provides a rather good account of the interfacial width and surface tension as a function of temperature. Our results show that truncation of the potential has a significant effect on the surface properties. We comment briefly on attempts to infer capillary-wave contributions to interfacial quantities from comparison of theory and simulation.

ISSN:0026-8976    

DOI:10.1080/00268978500102041

出版商:Taylor & Francis Group    

年代:1985

数据来源: Taylor

摘要:

For low-pressure simulations of liquid-like states of pure substances, the statistical and systematic errors contained in the results for the compressibility factor are investigated. Calculations with particle numbers between 108 and 2048 are reported, giving a clear picture of the magnitude of the possible errors. Two state points have been considered, for which the total compressibility factor is much smaller than unity. The detected total error is found to amount to 15–30 per cent for convenient simulations with 500 particle and a cutoff sphere of 2·30σ. Furthermore the applicability of methods like the Baxter technique for continuation of the pair correlation function to largerfor the long range corrections neglected in the direct evaluation of the virial has been tested. For cutoff radii smaller than or equal to 2·30σ such a correction should be preferred to the usual correction term calculated on the basis ofg(r) = 1 (forrlarger than the cutoff sphere). When the potential is truncated at 3·3σ or at larger separations, the normal long-range correction suffices to avoid systematic errors of that type.

ISSN:0026-8976    

DOI:10.1080/00268978500102051

出版商:Taylor & Francis Group    

年代:1985

数据来源: Taylor

摘要:

Neutron diffraction measurements have been made on liquid DF at 293 K using 0·7 Å neutrons. The intramolecular distancerDFwas found to be 0·95 Å in satisfactory agreement with accepted values. The intermolecular contribution has been transformed to give the composite pair correlation functiondL(r) and exhibits a well-defined hydrogen-bond at a distance of 1·62 Å.

ISSN:0026-8976    

DOI:10.1080/00268978500102061

出版商:Taylor & Francis Group    

年代:1985

数据来源: Taylor

摘要:

The lifetimes of rotationally-mediated resonances in the scattering of HD from a flat Ag surface are calculated by the quantum-mechanical complex rotation method and by classical trajectories. Qualitatively, considerable similarity is found between the quantum and the classical descriptions of the nature of the trapped states and of the mechanism of their decay. Quantitatively, quantum lifetimes are shorter than the corresponding classical values by as much as a factor of 4 to 7. Several interesting properties of the resonance states and trends associated with the lifetimes are observed, and simple physical interpretations offered.

ISSN:0026-8976    

DOI:10.1080/00268978500102071

出版商:Taylor & Francis Group    

年代:1985

数据来源: Taylor

摘要:

The calculation of the van der Waals interaction energy between two helical molecules is considered. The molecules are built from spherical units and a unit-unit summation process is used to obtain the interaction. An analytical method involves the calculation of a double integral extended to the molecular shapes. This allows one to discuss the interaction for molecules with parallel axes as a function of the lengths of the helices and to point out the influence of the chirality of the structures on the potential energy surfaces. A second method introduces a summation programme and allows one to discuss directly the potential energy surfaces for molecules with nonparallel axes. Several previous theoretical predictions about this situation are verified.

ISSN:0026-8976    

DOI:10.1080/00268978500102081

出版商:Taylor & Francis Group    

年代:1985

数据来源: Taylor