The statistical mechanics of a liquid of two-state molecules. II. The competition between quantum mechanics and a condensed phase

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Abstract
In this paper we study the behavior of liquids whose molecules have explicitly quantum mechanical internal degrees of freedom, with the aim of finding out how such internal degrees of freedom affect, and are affected by, their environment. After a review of the way in which general problems of this kind can be treated with discretized path integrals, we specialize to a model system in which the intramolecular dynamics is that of a tunneling system. The model problem is then solved in the mean field and mean spherical approximations. The internal degrees of freedom are always found to become more classical because of the bath, and the intermolecular interactions are always found to be weakened because of the quantum mechanics—a situation we summarize as a competition between quantum mechanics and the condensed phase. The nature of the phase transition for our model system, and the implications of our findings for other types of quantal degrees of freedom, are discussed. In particular, it is pointed out how the observed disappearance of tunnel splitting in ammonia vapor provides an experimental confirmation of our ideas.