Equations of a combined, liquid plus particle model are derived which accounts consistently for the short-range liquid properties of nuclei as well as for the long-range quasi-particle behavior. Macroscopic quantities are defined as statistical averages and their dynamics is determined by an Euler-type equation, which contains a term due to the interaction with the gas component, and also by the continuity equation. For the quasi-particle component the full quantal treatment is kept in the form of the Liouville equation with a time-dependent external potential related to the bulk density. It may include the residual quasi-particle interaction. It is suggested that the solution of the coupled dynamics is facilitated by the explicit use of an effective sharp nuclear surface as a dynamical variable.
The authors express their gratitude to Dr. R. Hasse for many stimulating discussions and help in preparing the manuscript.One of the authors (M.B.) is grateful to the Institute of Nuclear Research and the Academy of Science of the Ukrainian SSR for the hospitality extended to him during a visit. He also gratefully acknowledges a travel grant of the Deutsche Forschungsgemeinschaft.