Variational density matrix theory I

By replacing the wave function by the coordinate space representation of the N-body density matrix, where N is the number of particles in the system, a generalization to finite temperatures of the Jastrow-Euler-Lagrange variational theory of the ground state of quantum fluids is achieved. Schematically, the Bloch equation for the density matrix may be replaced by an Euler-Lagrange equation in which the trial density satisfies the properties described. The authors have taken the simplest non-trivial choice for two many-body functions, whereby the factors of the trial density have the Jastrow form and the incoherence factor has a similar two-body structure. Because the simple trial density matrix cannot achieve the correct high temperature limit the separability approximation is restrictive, limiting the validity of the numerical results to low temperatures