摘要:

Within the context of extended geminal models, the concepts of charge centroids and charge ellipsoids of the geminal one-electron densities, and an energy decomposition of the intermolecular potential, are introduced as tools of analysis. The intermolecular potential can be written within this framework as a sum of distortion energies of the subsystems plus the interaction energies between the distorted subsystems. The interaction energy is further partitioned into Coulombic, exchange and correlation contributions. The main results of calculations on the dimers (HF)2, H2O … HF and (H2O)2, using a [7s, 4p, 2d/4s, 2p] contracted Gaussian-type basis set, are as follows. First, the equilibrium geometry of the dimers are strongly correlated with the location in space of the lone-pair charge ellipsoids of the proton acceptor. Secondly, the intermolecular potential is dominated by the Coulombic interaction energy and the distortion energies. However, these contributions have different signs, and, although the intersystem exchange and correlation terms are considerably smaller in magnitude, the latter terms make substantial contributions to the intermolecular potential.

ISSN:0026-8976    

DOI:10.1080/00268979000101051

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The leading terms in the dispersion series for local dipole, quadrupole and octopole moments, pair quadrupole moment, electric field and electric field gradient at each nucleus are derived for a well-separated pair of H atoms.

ISSN:0026-8976    

DOI:10.1080/00268979000101061

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

A molecular-dynamical formulation of the Hartree-Fock (HF) method is presented. The simulated-annealing and simulated-quenching (steepest-descent) techniques are applied to the minimization of the HF energy functional and to the solution of the HF equations. In a test application to the helium atom with Gaussian basis sets both techniques show good performances but have two drawbacks: an excessive number of Fock-matrix calculations and a slowing-down behaviour close to convergence. We propose a new multistep algorithm for the simulated-quenching technique in which both drawbacks are removed: the Fock-matrix calculations are reduced to a minimum and the slowing down does not occur. The resulting program is several times faster than standard self-consistent-field HF programs.

ISSN:0026-8976    

DOI:10.1080/00268979000101071

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Equilibrium properties of inhomogeneous one-dimensional square-well fluids and their binary mixtures have been obtained exactly. Using the Laplace-transform technique for the evaluation of partition functions of one-dimensional systems with nearest-neighbour interactions, the canonical isobaric-isothermal and grand partition functions are determined. The molecular density distribution functions are determined from the grand partition function. The influence of nearest-neighbour interactions upon these functions is assessed. These systems can serve as simple models for describing selective adsorption of fluids in porous solids, and sample results are presented that show how molecular-size differences and attractive interactions determine selective adsorption.

ISSN:0026-8976    

DOI:10.1080/00268979000101081

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The contributions to the excess free energies of the orientational order-disorder phase transitions from the associated lattice distortions (spontaneous strains) in SF6and calcite have been calculated. These contributions are about one-third of the total excess free energies in both cases.

ISSN:0026-8976    

DOI:10.1080/00268979000101091

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

This paper discusses the mathematical formulation of the modelling of fluids of chain molecules as fluid mixtures of molecular conformations with the concentration of these conformations obtained through a constrained freeenergy minimization. Thermodynamic relationships can then be applied to obtain other properties, and an approximation is derived for the virial coefficients of chain molecules. Problems regarding the implementation of this approach through the virial equation at low densities and the generalized van der Waals equation of state at higher densities, and possible solutions to these, are also discussed. Feasibility studies for the case of n-octane show agreement with the experimental observations regarding the molecular shape in gas and liquid phases.

ISSN:0026-8976    

DOI:10.1080/00268979000101101

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

We have calculatedab initiothe three-dimensional potential-energy surface of the NH2molecule at 145 nuclear geometries spanning energy ranges of about 18 000 cm-1for the NH stretch and 12 000 cm-1for the bend. Theab initioconfiguration-interaction calculations were done using the multireference MRD-CI method. The calculated equilibrium configuration has NH bond lengthre= 1·0207 Å and bond angle α = 103·1°. The rotational-vibrational energies for14NH2,14NHD and14ND2were calculated variationally using the Morse-oscillator rigid-bender internal-dynamics Hamiltonian. For14NH2we calculate that υ1= 3267 (3219) cm-1, υ2= 1462 (1497) cm-1and υ3= 3283 (3301) cm-1, where experimental values are given in parentheses.

ISSN:0026-8976    

DOI:10.1080/00268979000101111

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

We report on neutron-diffraction measurements of liquid benzene under standard thermodynamic conditions. The measurements are taken at two different incident neutron wavelengths to cover an extendedQrange. The diffraction data are normalized and inelastic effects corrected, considering only the total sample scattering cross-section. To determine the molecular structure from the diffraction data, we have to introduce direction-dependent meansquare displacements for the carbon and deuterium atoms. An analysis is made of the pair distribution function in terms of a first-shell solid-like structure, and good agreement is found with previous x-ray measurements.

ISSN:0026-8976    

DOI:10.1080/00268979000101121

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

We calculate inelastic neutron-scattering intensities of methylene chloride for cold neutrons from molecular-dynamics (MD) simulations and compare them with experimental data. To obtain realistic scattering intensities, the effect of multiple scattering is taken into account by a Monte Carlo (MC) simulation, using the dynamic structure factor calculated from our MD simulations as input. The MD simulations of methylene chloride are performed with the same potentials as in Part I of this work. The dynamic structure factor is calculated using fast correlation algorithm (FCA), which is based on the fast-Fourier-transform (FFT) algorithm and the Wiener-Khinchin theorem for discrete functions.

ISSN:0026-8976    

DOI:10.1080/00268979000101131

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

We have carried out Monte Carlo (MC) simulations for a model of liquid n-butane in three thermodynamic states. In each case, both the flexible model in the limit when bond-length and bond-angle forces become infinite and the constrained model were studied. The different trajectories followed by each model lead to different ensemble averages for the intramolecular properties, such as the conformational distribution and intramolecular energy. The changes observed in the external configurational properties, such as the intermolecular energy and pressure are less than the statistical error. In all the cases we have found a weak shift of thetransconformational population with respect to the ideal-gas distribution (i.e. less than 3%). In addition, we have explored the possibilities of a purely repulsive model as a reference system for this liquid. As in the case of simple atomic liquids, the standard hard-core theoretical approaches are good starting points for obtaining the contribution of the external degrees of freedom to the thermodynamic and structural properties. However, repulsive forces shift the conformational equilibrium with respect to the ideal-gas values toward an increase ingaucheconformers. The influence of density on internal conformation is more pronounced in the repulsive model than observed for the total potential.

ISSN:0026-8976    

DOI:10.1080/00268979000101141

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor