Correlation Energy of a High-Density Gas: Plasma Coordinates

The model Hamiltonian of Sawada which describes electron correlation at high density is examined. It is shown that the set of scattering modes for momentum transfers below a certain $q_{max}$ is not complete. It is completed by the plasma mode. ${\left(q_{max}\right)}^{-1\mathrm{}}$ is the natural Debye length of the theory.