A. Terry Amos

Atom—atom potential parameters for van der Waals complexes of aromatics and rare-gas atoms

Abstract The structure of the dispersion terms in the interaction for van der Waals complexes of aromatics and rare-gas atoms is examined and found to contain important three-body terms. Using calculations for benzene with a sequence of rare gases, the errors in approximating the three-body π-electron dispersion energy by atom—atom terms are found to be generally significant, but small for certain symmetrical positions (including the equilibrium position). For these positions, the total interaction coefficients are compared with empirically determined values using a Lennard-Jones model. Agreement is good after scaling to allow for higher-order terms.

Intermolecular forces using spatial eigenfunctions

Abstract A formulation is presented for the determination of intermolecular forces using spatial eigenfunctions, rather than wavefunctions which contain spin. This formulation, along with a new perturbation theoretic procedure, should simplify the calculation of these forces. Also, the role of the Pauli principle in requiring wavefunctions to be eigenfunctions of S 2 is discussed briefly.

Electronic spectral shifts of large van der Waals molecules

Abstract Experimental values for the red spectral shifts in the fluorescence excitation spectra of large van der Waals complexes of aromatic molecules and rare gas atoms have been used, in conjunction with calculated equilibrium distances between the aromatic and rare gas atom, to estimate differences between the dispersion forces in the ground and excited states. Theoretical red-shifts calculated by a formalism given by Longuet-Higgins and Pople are an order of magnitude too large and this discrepancy is traced to incorrect assumptions concerning values chosen for average excitation energies. Modifications of the theory for a van der Waals complex are explored. A simpler and more easily used formula to correlate the magnitudes of the spectral shifts is derived and tested.

Solvent shifts in van der waals complexes of perylene

Abstract The solvation and structure of vdW complexes formed in supersonic free-jet expansions of perylene with rare gas atoms and organic “solvent” molecules, using helium as carrier gas, have been investigated. The complexes were probed and analysed by combining laser-induced fluorescence spectroscopy with approximate summed pair-potential calculations. Theoretical examination of the bathochromic spectral shifts for the isolated 1:1 vdW complexes is consistent between solvent-shift theory and equilibrium configuration calculations. Analysis of the spectral shifts assumes they arise solely through the change in dispersion energies on electronic excitation and that this difference can be estimated using average excitation energies.

A point-charge representation of frost-model wave functions

A point-charge representation of Frost-model wave functions is derived from a symmetry-adapted perturbation theoretic expansion. The new point-charge model is simpler than those suggested previously yet gives good estimates of first-order molecular properties. The treatment can easily be extended to deal with second-order properties and, when this is done, formulae similar to those of the Drude theory are obtained. Using these formulae, theoretical expressions for the refractive indices of methane, ethane and water are computed and are in reasonable accord with experiment for the two hydrocarbons but less satisfactory for water.

Configuration interaction methods for improving unrestricted Hartree-Fock spin densities

Limited Configuration Interaction wave functions based on Unrestricted Hartree-Fock natural orbitals are found to be easy to compute and to give much more satisfactory spin densities than are provided by techniques currently in use.

First and second-order properties of charge-localized and Frost-localized orbitals

Two methods of localization of Frost-model molecular orbitals are considered, both of which are extremely easy to apply, requiring the evaluation of only one-election integrals. The chargelocalization criterion can be used with any single-determinant wavefunction, but when applied to the Frost model it yields similar orbitals to those got by orthonormalizing the basis of floating spherical gaussians, while maintaining their localized feature. Properties of charge-localized orbitals are calculated for NH3, H2O, CH4, C2H6.

Long-Range Molecular Interaction Coefficients Computed from Frost-Model Wave Functions

The average long-range interaction energy between two molecules can be written as an inverse asymptotic series in the intermolecular separation distanceR. Using Frost-model wave functions, the dispersion coefficients of the first three (R−6,R−8,R−10 terms in the series are obtained. Coefficients of three- and four-body non-additive interaction energies are also calculated and the form of the dispersion interaction when retardation effects are included is examined.

Dipole sums and intermolecular interaction coefficients derived from refractive index data

The old problem of using refractive index measurements to obtain estimates of dipole sums and interaction coefficients is reconsidered. It is found that a consistent numerical procedure for fitting the measured values to a two-term representation of the frequency dependent polarizability enables satisfactory values to be obtained.

Some rules forS(-2k) dipole sums?

Stieltjes conditions and the ratio test provide necessary but not sufficient conditions onS(-2k) dipole sums. If the dipole sums are accurate the associated [n, n −1] Padé approximant provides a better representation of α (Ω), the frequency-dependent dipole polarizability, than a truncated series expression and, in addition, should bound α(Ω) below. It is shown how constraints on the dipole sums effect the form of the [2,1] Padé approximant and an additional constraint is derived that ensures the analyticity of the approximant on 0≤ Ω < Ω1. There then follows a discussion of the reliability of available literature dipole sum values for small molecules containing H, C, N and O.