ISSN:0026-8976    

DOI:10.1080/00268979600100831

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

A new configurational bias Monte Carlo technique is described for the simulation of peptides, focusing on the biologically relevant cases of linear and cyclic peptides. The approach leads to efficient, Boltzmann-weighted sampling of the torsional degrees of freedom in these biological molecules, a feat not possible with previous Monte Carlo and molecular dynamics methods.

ISSN:0026-8976    

DOI:10.1080/00268979600100841

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The macroscopic four-wave mixing susceptibility is separated into a geometrical and a molecular factor using vector coupling algebra. The geometrical factor for isotropic samples is given explicitly for collinear circularly polarized beams and for an arbitrary geometry of linearly polarized beams. The result for four independent beams is compared with the previously derived geometrical factor for ordinary Raman scattering and 2-laser coherent anti-Stokes Raman scattering. A sum rule is derived for checking the accuracy of published expressions. Isotropic samples are fully characterized by the isotropic, antisymmetric and symmetric components of the scattering tensor. It is shown that controlling all four beams allows the measurement of each component individually. This is not possible when fewer than four beams are controlled.

ISSN:0026-8976    

DOI:10.1080/00268979600100851

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

A self-consistent computer adapted theory to calculate the wavevector- and frequency-dependent dielectric constant of polar fluids with permanent dipoles is proposed. The dielectric tensor of the Stockmayer fluid is evaluated in a wide region of wavevectors and frequencies using molecular dynamics simulations. Wavevector-dependent relaxation time and longitudinal dipolaron frequency are calculated. Application of a generalized two-variable theory is discussed.

ISSN:0026-8976    

DOI:10.1080/00268979600100861

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Generalized Flory-dimer (GF-D) theory is extended to hard n-alkane fluids modelled as fused hard spheres constrained according to the rotational isomeric state approximation. Theoretical predictions for the second to the fifth virial coefficients for each conformer from n-butane to n-octane are compared with numerical results. Predictions for the second viral coefficient are in excellent agreement with recent simulation results; however, GF-D theory systematically overpredicts the third virial coefficient and underpredicts the fourth and the fifth virial coefficients. GF-D predictions for the compressibility factor for n-alkanes from n-butane to n-octane are compared with molecular dynamic simulation results for a closely related model. Despite the over- and underprediction of the individual virial coefficients, excellent agreement is observed between theory and simulation for the compressibility factors of n-butane and n-pentane using no adjustable parameters. However, in the cases of n-hexane, n-heptane and n-octane, GF-D theory slightly underpredicts the pressure at high volume fractions. Theoretical considerations suggest that probably the predictions of GF-D theory will deteriorate with increasing chain length.

ISSN:0026-8976    

DOI:10.1080/00268979600100871

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

A detailed examination of the spectroscopic properties of the lowest excited triplet metal-to-ligand charge transfer (3MLCT) states in the title systems is reported. The results are in accord with the C2/c single-crystal X-ray structure of [Zn(bpy)3](ClO4)2and [Ru(bpy)3](ClO4)2(bpy = 2,2′-bipyridine) which establishes one crystallographic site for the molecular cation. Two ligands are related by the twofold axis whereas the third ligand is unique. As a consequence, the partially deuterated complexes substitute in the lattice in two distinct ways. This leads to a superposition of spectra for thex= 1 andx= 2 systems. Spectroscopy of the series in both hosts can be described by using an exciton formalism with an excitation exchange interaction β between the two crystallographically equivalent metal-ligand subunits of ≈ 37 cm-1and ≈ 78 cm-1for levels I and II, respectively. These values of β are an order of magnitude larger than in the trigonal [Ru(bpy)3](PF6)2hosts. Thus partial deuteration does not lead to full localization of the lowest-energy mini-excitons in the title systems.

ISSN:0026-8976    

DOI:10.1080/00268979600100881

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

A study of model polyelectrolytes is reported based on both Monte Carlo and integral equation approaches. Solutions of macroions and counterions in water with asymmetries in charge of 20:1 and of 12:2, and asymmetries in size of 15:2 and of 10:1 are considered. The first results for highly asymmetrical electrolytes with divalent counterions are presented. The Monte Carlo results indicate strong clustering of divalent counterions around the macroion: the macroion-macroion pair correlation function is less structured than in the equivalent solution with monovalent counterions. The simulation results are used to assess the validity of the hypernetted chain (HNC) approximation and the recently developed two-density theory, based on Wertheim's formalism. The new theory yields better agreement with the simulation results for the 20:1 case than the conventional HNC approximation. Further, the theory provides good agreement with the Monte Carlo data for the range of concentrations where the ordinary HNC does not yield a convergent result. For divalent counterions (12:2 case), none of the theories presented is sufficiently accurate in the description of the structural and thermodynamic properties.

ISSN:0026-8976    

DOI:10.1080/00268979600100891

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Far-infrared absorption spectroscopy gives information on the charge fluctuations associated with hydrogen-bond dynamics in water. Here, we examine these fluctuations by calculating spectra from molecular dynamics simulations and comparing with experiment. Permanent and induced dipoles are included in the calculation of the system dipole moment. The induced dipoles are calculated on each atom using an iterative method that leads to self-consistency of the induced dipole contribution to the local field. The experimental far-infrared spectrum has two prominent bands, at ∼ 600 cm-1and ∼ 200 cm-1, due to hydrogen-bond libration and stretching, respectively. The librational band is found to arise from fluctuations of the positions of the permanent charges in the simulation, whereas the stretching band originates from induced dipole fluctuations. The use of the self-consistent method enhances the intensity of the stretching peak relative to a non-iterative method, producing improved agreement with experiment. The frequency of the librational band is influenced significantly by long-range electrostatic interactions in the potential function. In contrast, the induced dipole fluctuations are relatively insensitive to the long-range effects; the effective isotropy of the molecular polarizability tensor is found to lead to a decoupling of the induced dipole fluctuations from the dynamical intermolecular orientational correlations of the permanent dipoles.

ISSN:0026-8976    

DOI:10.1080/00268979600100901

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

In a glassy crystal, where the molecules cannot rotate, a question arises as to whether the frozen in rotational entropy should, or should not, be included in the free energy for the purpose of determining the stability of the crystal relative to another phase. We study fluids and crystals of two- and three-dimensional dimers, compare the location of the observed coexistence lines with the location predicted by each of the two possibilities and conlcude that the equilibrium vapour pressure of the glassy crystal is correctly predicted when the rotational entropy is included in the free energy of the glassy crystals.

ISSN:0026-8976    

DOI:10.1080/00268979600100911

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Monte Carlo simulations are reported of a polymer fluid confined between planar walls using the isotension ensemble, which allows one to perform simulations at constant chemical potential without the necessity of particle exchanges with a bulk polymer fluid. The pressure acting upon the walls and the free energy per unit area of the confined fluid as a function of separation between the walls are obtained, all at constant chemical potential. This is made possible by the use of the so-called free energy difference method. This method appears to be less susceptible to the difficulties that confront the simulation of confined fluids at high density in grand canonical ensemble simulations.

ISSN:0026-8976    

DOI:10.1080/00268979600100921

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor