  The Dirac-Fock (DF) model replaces the Hartree-Fock (HF) approximation in
quantum chemistry when relativistic effects cannot be neglected. Since the
Dirac operator is not bounded from below, the notion of ground state is
problematic in this model, and several definitions have been proposed in the
literature. We give a new definition for the ground state of the DF energy,
inspired of Lieb's relaxed variational principle for HF. Our definition and
existence proof are simpler and more natural than in previous works on DF, but
remains more technical than in the nonrelativistic case. One first needs to
construct a set of physically admissible density matrices that satisfy a
certain nonlinear fixed-point equation: we do this by introducing an iterative
procedure, described in an abstract context. Then the ground state is found as
a minimizer of the DF energy on this set.
