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Values for the polarizabilities of both mono and multinuclear anions in aqueous solution at infinite dilution are derived. This is achieved by subtracting from each experimentally determined molar polarizability, the polarizability of the cation as well as the total change caused by this cation in the polarizabilities of neighbouring water molecules. Each cation polarizability was taken to be unchanged from its value known from previous work to be common to both the free ion and the ion in an ionic crystal. The required values of the total changes in the polarizabilities of water molecules influenced by cations are derived from the experimental molar polarizabilities by two different methods. The first of these yields results which are essentially identical to those derived from the second approach in which the value for Li + is fixed from ab initio quantum chemistry computations including electron correlation effects.The polarizability of each anion in aqueous solution is found to be considerably reduced from that of the free ion and to be close to that of the anion in its solid caesium salt.

A representation of the short-range repulsion energy in an ionic system is described which allows for the fact that an ion may be compressed by its neighbours. The total energy of the system is expressed in a pairwise additive form, but the interionic interactions have a many-body character. The form of this representation and the parameters required to represent MgO and CaO are obtained from recent ab-initio electronic structure calculations. The fact that the representation is transferable between crystals with different coordination number is demonstrated by direct comparison with ab-initio results on the different crystal types. Comparison with experimental results on the equation of state of different isomorphs and on the location of the pressure of the transition between them confirms the accuracy of the ab-initio results and of the potential derived from them in representing perfect crystal properties. A computationally efficient molecular dynamics (MD) scheme may be derived for this representation. The additional degrees of freedom which represent the varying ionic radii are constrained to their adiabatic values in the course of the simulation by an adaptation of Car and Parrinello’s method. The MD scheme is used to examine whether an ab-initio parameterized potential model which allows for the spherical compression of an oxide ion by its neighbours and for dipole polarization effects is a sufficiently good representation of the interactions in MgO to allow an accurate calculation of the phonon dispersion curves.

The effect on the P spectrum of exchange between bound and continuum electrons is discussed for decay of a many-electron atom R.esults of calculations of the exchange distortion of the P spectrum are presented using both a screened hydrogenic and a Hartree-Fock approximation for the electron wave functions. The results of the two approaches agree well and show that the inclusion of exchange leads to an enhancement of the P spectrum, particularly at low electron energy. This corrects an error in a previous calculation that found that exchange decreased the P intensity. For the low-energy P emitters Ru and 'Pu, the present calculation indicates that the enhancement due to exchange is of the order of several percent over much of the spectrum, becoming larger at very low electron energy. Exchange with 1s electrons dominates in the high-energy part of the spectrum, but exchange with ns (n~2) electrons becomes significant in the low-energy region. The inclusion of exchange leads to an increase in the phase-space integral by 6.4% for ' Ru and 7.5% for 'Pu. Results are presented for exchange effects in other P spectra, including those of ' C and S, in which experimentally measured distortions have been interpreted as evidence for a heavy antineutrino of mass 17 keV. The distortions due to exchange are found to be significantly smaller than the measured distortions.

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