摘要:

A general operator algebra formalism is proposed for describing the unitary time evolution of multilevel spin systems. The time-evolutional propagator of a multilevel spin system is decomposed completely into a product of a series of elementary propagators. Then the unitary time evolution of the system can be determined exactly through the decomposed propagator. This decomposition may be simplified with the help of the properties of the finite dimensional Liouville operator space and of its three operator subspaces, and the operator algebra structure of spin Hamiltonian of the system. The Liouville operator space contains the even-order multiple-quantum, the zero-quantum, and the longitudinal magnetization and spin order operator subspace, and moreover, each former subspace contains its following subspaces. The propagator can be decomposed readily and completely for a spin system whose Hamiltonian is a member of the longitudinal magnetization and spin order operator subspace. If the Hamiltonian of a spin system is a zero-quantum operator this decomposition may be implemented by making a zero-quantum unitary transformation on the Hamiltonian to convert it into the diagonalized Hamiltonian, while if the Hamiltonian is an even-order multiple-quantum operator the decomposition may be carried out by diagonalizing the Hamiltonian with an even-order multiple-quantum unitary transformation. When the Hamiltonian is a member of the Liouville operator space but not any element of its three subspaces the decomposition may be achieved first by making an odd-order multiple-quantum and then an even-order multiple-quantum unitary transformation to convert it into the diagonalized Hamiltonian. Parameter equations to determine the unknown parameters in the decomposed propagator are derived for the general case and approaches to solve the equations are proposed.

ISSN:0026-8976    

DOI:10.1080/00268970009483329

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Accurate SCF and MP2 quartic property hypersurfaces have been computed for the energy, dipole moments, quadrupole moments and polarizability tensors of the fluorinated methanes CF4, CHF3, CH2F2and CH3F, to establish accurate values of zero-point vibrational corrections to the properties. Using a consistent set of regeometries from density functional theory, these ZPVCs are coupled with accurate electrical properties computed using a range of correlated methods, especially BD and BD(T), and a number of purpose-built polarized basis sets, to obtain near definitive estimates of these properties that incorporate the effects of vibrational averaging. Careful attention has been paid to a critical comparison between these theoretical estimates and experimental measurements, and agreement between the two is shown to be exceptionally good. In particular, it is clear that in many instances more precise experimental results would be required in order to discriminate between different correlated results, or between the present results and those which may be obtained with larger basis sets. The work highlights the necessity to allow for the effects of zero-point vibrational averaging when comparing theory with experiment, or even when comparing different theoretical results with one another using experiment as a benchmark. It also points to the need for further precise experimental measurements of some of these properties.

ISSN:0026-8976    

DOI:10.1080/00268970009483330

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Measurements of spin-lattice relaxation time and complex permittivity have been performed for monocrystalline and polycrystalline pyridinium Perchlorate as a function of hydrostatic pressure and temperature. The influence of pressure on the reorientation of the pyridinium cation was analysed. The potential shape and heights of the energy barriers for this reorientation in all three phases have been determined. For phase I and III the pressure dependence of the activation enthalpy was obtained.

ISSN:0026-8976    

DOI:10.1080/00268970009483331

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The structure and thermodynamics of a dipolar hard-sphere fluid at a discretely polarized hard wall are investigated using Monte Carlo computer simulation. In contrast to a continuously polarized wall, the discrete case exhibits significantly enhanced adsorption of the fluid at the surface relative to that observed for an unpolarized wall. Significant orientational ordering of the first liquid layer is observed plus correlation with the wall polarization that extends over several layers. The relative potential and free energy are calculated as a function of the polarization angle using thermodynamic integration.

ISSN:0026-8976    

DOI:10.1080/00268970009483332

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The dependence of the self-diffusivityDon the reciprocal of the square of the Lennard-Jones sorbate parameter 1/σ2is known to exhibit two distinct regimes: (i) the linear regime, whereD∝ 1/σ2, and (ii) the anomalous regime, in which this proportionality does not hold. The existence of the anomalous regime has been attributed to what is termed the levitation effect. In the present work, the possibility of using the levitation effect to advantage in the separation of a multi-component mixture has been explored. The computer experiment is designed to mimic experimental conditions as far as possible. Two mixtures have been studied, one in which all three components belong to the linear regime, and another in which one of the three components belongs to the anomalous regime. Results suggest that good separation may be achieved at low temperatures, especially when one of the components of the mixture belongs to the anomalous regime in the dependence of self-diffusivity on the reciprocal of the squares of the sorbate size.

ISSN:0026-8976    

DOI:10.1080/00268970009483333

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Potential curves for the four Hund's case (a) basis electronic states (2Σ,2Πg,2Πu, and2Σ+) correlating with the I(2Pu) + I−(1Sg) dissociation limit have been calculated at the MRCISD level of theory using an all-electron double-zeta basis set augmented with an extensive set of polarization and diffuse functions complete throughiangular momentum functions. Transition moments for the dipole allowed transitions between these states are determined as a function of internuclear separation. The four Hund's case (a) curves are used to determine the six Hund's case (c) potential curves which arise when spin-orbit coupling is considered. The rovibrational eigenvalues for each of these six states are determined numerically, and standard spectroscopic constants are determined by fitting the energy of these levels to a Duham series. The results are compared with available experimental and theoretical information.

ISSN:0026-8976    

DOI:10.1080/00268970009483334

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The wetting of chemically heterogeneous surfaces is typically discussed in terms of Cassie's law (1948,Discuss. Faraday Soc.,3, 11), which defines the cosine of the contact angle in terms of an average over the different chemical regions, weighted by their relative surface coverage. This paper derives an exact formulation of Cassie's law, in the context of the wetting of chemically patterned substrates. Deviations from Cassie's law arise from quasi-two-dimensional fluid-mediated interactions, analogous to solvation forces.

ISSN:0026-8976    

DOI:10.1080/00268970009483335

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

A theoretical method previously introduced in order to study the structure and thermodynamics of a charged ellipsoidal fluid is here applied to the study of a dipolar ellipsoidal fluid. To enable the strong-coupling (large dipole) form of the direct correlation function to be written analytically, the model employs ellipsoids with charges distributed over their surfaces. This implies that the electric field at a large distance from one molecule results from many multipoles although the dipole is the dominant one. A geometricansatzfor the direct correlation function containing a small number of parameters is then constructed and the parameters determined through the means of a variational principle based upon the mean spherical approximation. From the resultant direct correlation function it is possible to determine thermodynamic properties, including the relative permittivity of the fluid and the electrostatic energy. Within the model no ferroelectric transition is observed. Further the resultant direct correlation function is in a suitable analytic form for applications to the structure of an inhomogeneous fluid. The model is applied to fluids of ellipsoids with elongations ranging between 1/5 (oblate) and 5 (prolate). The range of dipole moments and densities allows relative permitivities up to approximately 80. The results are consistent with other studies of hard dipolar fluids although the model is not precisely the same except in the special case of hard spheres.

ISSN:0026-8976    

DOI:10.1080/00268970009483336

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The Parsons-Lee theory has been modified to study the liquid-crystalline phase behaviour of the linear fused hard sphere chain fluids. The modification of the Parsons-Lee theory is based on the application of the so-called effective molecular volume instead of the real molecular volume. This alteration does not mean any change for the Parsons-Lee treatment of the hard convex bodies but does for the non-convex ones. The results of the modified Parsons-Lee theory show very good agreement with simulations not only for the location of the isotropic-nematic phase transition but for the equation of state.

ISSN:0026-8976    

DOI:10.1080/00268970009483337

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

(Molecular Physics, 1999,97, 1095–1103)

ISSN:0026-8976    

DOI:10.1080/00268970009483338

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor