摘要:

The distribution of fluctuations of the total dipole moment in polar liquids is characterized. The functional form of the probability distribution function,P(g), wheregis proportional to the square of the total moment and its average value is directly related to the static dielectric constant, is empirically determined. A formal justification of the functional form is then developed and implications to the calculation of the dielectric constant in computer simulations are discussed.

ISSN:0026-8976    

DOI:10.1080/00268979300102231

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The Gay-Berne potential is proving to be of considerable value in computer simulation studies of liquid crystals. However, the parameters employed in the potential were chosen by comparison with that for a line of four Lennard-Jones centres; they may not, therefore, be appropriate for mesogenic molecules. To see if this is the case we have estimated the parameters in the Gay-Berne potential by comparison with the site-site potential constructed forp-terphenyl, which has a molecular structure typical of many mesogens. Unlike the Gay-Berne potential the site-site potential is biaxial, and a method for projecting out this biaxiality is proposed. The resultant uniaxial model has then been used to obtain values for the parameters occurring in the Gay-Berne potential. These are found to differ significantly from those proposed originally; they are, we believe, more appropriate for investigations of the behaviour of liquid crystals. Our own molecular dynamics simulations, based on the new parametrization, reveals the existence of isotropic, nematic and smectic A phases. The results, in common with those from previous studies, indicate that the structure of the isotropic and nematic phases of the Gay-Berne mesogen are dominated by short range anisotropic repulsive forces. In marked contrast, the stability of the smectic A phase is found to be critically dependent upon the anisotropy in the attractive forces.

ISSN:0026-8976    

DOI:10.1080/00268979300102241

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

In this paper we present a computer simulation study of thin film behaviour performed using the Lebwohl-Lasher liquid crystal model. In this, we study systems with a slab geometry ofLparallel planes. We have measured the shifts in the bulk transition temperature that result from variation both ofLand of an aligning surface field ϵs. From these, we have used a Kelvin-like equation to calculate the nematic-isotropic surface tension. We have also investigated the nature of the shifted bulk transition in these thin film systems. From detailed comparison of energy distribution functions, we have found that the ϵs= 0 system has a critical thickness, 8 <Lc< 16, below which there is no bulk transition. We note that mean field theories fail to predict this critical point and suggest that its cause is related to the pretransitional divergence in orientational correlations seen at the bulk 3d transition.

ISSN:0026-8976    

DOI:10.1080/00268979300102251

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Results are reported for a molecular dynamics simulation study of a flexible model mesogen composed of seven tangential spheres. We follow the ‘rattling spheres’ method by which bonded atoms are constrained to lie within narrow potential wells. The dynamics of the system is that of a hard sphere fluid with added constraints. The phase diagram of our model system is calculated as a function of density and shows the presence of three fluid phases. These are assigned to be isotropic, nematic and smectic-A phases. Results are reported for the orientational order, radial distribution functions, structure factors and single-particle structural data within these phases. A small but significant change in shape is measured as the nematic phase is entered from the isotropic liquid. This is interpreted in terms of a quenching of allowed conformations parallel to the director.

ISSN:0026-8976    

DOI:10.1080/00268979300102261

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Suggested mechanisms responsible for liquid-crystalline ordering include non-spherical excluded volume effects, anisotropic attraction forces and flexibility. It has been shown using hard-core models that non-spherical excluded volume effects are the essential factor and can qualitatively explain the phenomenology of the problem. However, the simulation of hard-core models is technically demanding. A simpler and more direct alternative is to use a model with a soft-core site-site potential. We employ here a system of molecules composed of a few (11) atoms, constrained to form a multilinear molecule, and in mutual interaction via a continuous repulsive site-site potential of the formr-12. Our results show that such a model is capable of exhibiting nematic and smectic liquid-crystal phases.

ISSN:0026-8976    

DOI:10.1080/00268979300102271

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Isotherms and IR spectra were measured for CO2-MZSM-5 (M = Li+, Na+, K+, Rb+, Cs+) adsorption systems. The observed adsorption characteristics were well approximated by a physical adsorption where the van der Waals force and an electrostatic interaction force are operating: the degree of irreversible adsorption (chemisorption) was less than 10% of the total adsorption. Although the adsorbed CO2molecule interacts with both the cation and the pore wall, only the CO2-cation interaction can convert the IR inactive ν1vibration into an IR active state. The adsorption model proposed enables us to calculate the initial heat of adsorption as well as the molecular orientation angle of CO2against the cation site.

ISSN:0026-8976    

DOI:10.1080/00268979300102281

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Homonuclear clustersXmof heavy group V atoms (X= As, Sb) up tom= 6 have been studied with valenceab initioself consistent field/configuration integration calculations using energy-adjusted pseudopotentials. Several structures have been investigated and results are given for bond lengths (Re), atomization energies (De) and vertical ionization potentials of the ground states. Comparison with experimental and other theoretical values is made where possible.

ISSN:0026-8976    

DOI:10.1080/00268979300102291

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

As in earlier investigations on group V clusters, homonuclear clustersXmof heavy group VI atoms (X= Se, Te) up tom= 6 have been studied with valenceab initioself consistent field/configuration integration calculations using energy-adjusted pseudopotentials. Several structures have been investigated and results are given for bond lengths (Re), atomization energies (De) and vertical ionization potentials of the ground states. Comparison with experimental and other theoretical values is made where possible, and differences from the corresponding group V clusters are discussed.

ISSN:0026-8976    

DOI:10.1080/00268979300102301

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

A molecular theory of circular and axial birefringence and dichroism induced by circularly polarized, linearly polarized or unpolarized light in media composed either of chiral or of achiral molecules is presented. The general analytical expressions for all molecular contributions are given in terms of linear and nonlinear susceptibilities and discussed for some specific cases. The estimations of order of magnitude and comparison with other types of birefringence show that the optically induced birefringence may be noticeable in liquids and solutions by advanced laser and optoelectronic techniques.

ISSN:0026-8976    

DOI:10.1080/00268979300102311

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Non-adiabatic calculations on the positronium ion have been performed using laboratory-fixed hyperspherical coordinates and numerical wavefunctions. Convergence properties show that more than 98% of the true energy can be recovered using modest basis sets by truncating the radial domain and carrying out self-consistent field iterations solely onK= 0 levels. It seems likely that such procedures will enable more complex systems to be studied without recourse to extremely large basis sets.

ISSN:0026-8976    

DOI:10.1080/00268979300102321

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor