We use a numerical solution of the Schrodinger equation for a hydrogen atom at the center of an inert, impenetrable, spherical cavity, to predict the influence of isotropic compression on spectroscopic properties. The angular momentum sublevel degeneracy that occurs for the free atom is removed under isotropic compression. We account for this splitting on the basis of the underlying distribution functions. We find that the principal lines in the Lyman and Balmer series are blue-shifted relative to the free-atom frequencies, but, depending on the cavity radius, the intensities of these lines can be either enhanced or diminished, relative to the free atom intensities. We find that the excited-state lifetimes are all reduced by compression, with the reduction being especially large for the 2s level. We also find that nuclear volume effects on the electronic energy are greatly enhanced by isotropic compression.

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As part of their continuing study of the influence of the environment on the properties of atoms and molecules, the authors calculate the ground-state energies of three two-electron atoms and ions-H-, He and Li+-constrained at the centre of a hard spherical cavity. The calculations are performed using the techniques of quantum Monte Carlo (QMC); the particular QMC algorithm employed is unique in that the associated timestep error is quadratic, rather than linear, and thus yields highly accurate energies, a typical error bound being +or-0.0003 Hartree. This accuracy exceeds that previously obtained in studies employing self-consistent field Hartree-Fock and configuration interaction methods.

We present the generalization of Wertheim's theory of associated fluids to multicomponent liquid mixtures. For a model binary fluid mixture with sitesite coulombic interactions, the theory yields results in excellent agreement with Monte Carlo simulations. The simulations also provide pair distribution functions. IntroductionUsing perturbation theory [1], one can relate the thermodynamic properties of a system, for which the intermolecular pair potential is u(12), to those of a reference system where the potential is u0(12), by an expansion in powers either of the perturbational potential u1(12)= u(12)-Uo(12 ), or of some functional of ul such as the Mayer function f1(12) = exp (-ul(12)/kT) -1. The first-order, second-order, etc., perturbation terms then involve both ul (or fl ) and the distribution functions for the reference system. Such an approach is successful if ul is small, for then the perturbation series converges rapidly. In strongly associated liquids and liquid mixtures, however, u~ can be very large and the method fails [2]. Several alternative approaches, embodying varying degrees of sophistication and empiricism, are available to treat this problem. They are discussed in [2] and references therein. In this paper we focus on what seems to us to be the most promising route, based on ideas originating with Wertheim [2-7-1. Preliminary results have appeared in Part I [7].The basic premise of Wertheim's approach is that monomers and dimers (and higher aggregates of molecules, if these occur) should be treated as distinct chemical entities. However, his treatment is distinguished from earlier 'chemical' theories of association by a precise statistical-mechanical definition of what constitutes a monomer, dimer, etc. In contrast to these other theories, which are by essence

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