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Atoms remaining as recognizably distinct constituents of bulk condensed phases can have properties modified from those of the isolated species. Dense helium bubbles at high pressures are a common form of radiation damage degrading the mechanical and electrical properties of host materials. Detailed knowledge is critical for predicting their long term performance. Modifications of the ground and first singlet excited states of confined compressed helium are investigated using an entirely non-empirical theory based on the results of ab initio self-consistent field calculations with corrections for the effects of electron correlation. For finite sized portions representing bulk condensed fcc and bcc phases of helium atoms, Hartree–Fock wavefunctions, energies, and charge distributions were computed as a function of different atomic densities using two models. The first model for the first excited state localizes the excitation on the central atom; in the second model, this is partially delocalized over the closest atomic neighbors. Total energies for the finite size portions are derived by adding the inter-atomic dispersive attractions and a density functional description of the short-range inter-atomic correlation energy. The experimental energy of the first allowed electronic transition increases with density being larger than in an isolated atom. The intra-atomic correlation energy does not contribute to this energy shift. The calculated energy shifts agree well with experiment for both bulk solid and liquid helium. The 2p orbital is increasingly compressed by density enhancement, thus generating the energy shifts. Consequently, calculations of the inelastic electron scattering cross sections are substantially incorrect if the compression of the final 1s2p state is not included. The character of the excitations is examined, and it is argued that these are of Frenkel rather than the Wannier type. [ABSTRACT FROM AUTHOR]

The cohesive energetics of the sixfold and eightfold coordinated phases of solid cubic cesium chloride are studied using a nonempirical fully ionic description. The Oxford Dirac-Fock program is used to compute the ion wave functions and rearrangement energies needed to convert a free anion into the compressed yet still spherically symmetric anions optimally adapted to crystal. The Relativistic Integrals Program (RIP) is used to compute uncorrelated two-body interionic interaction potentials which take full account of relativity and are exact for given ion wave functions. These potentials and the further contributions arising from electron correlation are reported. The OEMFS model for describing the environment of the anion in-crystal yields lattice energies and equilibrium interionic separations in good agreement with experiment and correctly predicts the eight-fold coordinated structure to lie lowest in energy. The "optimal" anion-anion short-range potentials, computed by using at each geometry the wave function optimal for that geometry, are compared with the "frozen" potential in which the same anion wave function is used to compute the entire potential. The "frozen" anion-anion potential in solid sodium chloride is reported. The basic idea in the ab initio perturbed ion (AIPI) method, that the wave functions of anions in-crystal remain spherically symmetric yet as strongly orthogonal as possible to those of neighboring ions, cannot be justified. It differs from the standard view in the RIP computations that an ion wave function only remains spherically symmetric whilst overlapping with those of neighboring ions, such overlaps generating the short-range interionic repulsions. The RIP predictions of crystal cohesive energetics are preferable to those of the AIPI method. The correlation energy used in AIPI calculations is theoretically objectionable for crystals and is numerically poor. For CsCl, the anion wave function, rearrangement energy, and closest... [ABSTRACT FROM AUTHOR]

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. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]