  We have developed a method to derive the (approximate) quark contribution to
the fermion free energy of QCD on a lattice, at finite temperature and chemical
potential, with Kogut-Susskind fermions in the flavor basis. We show here the
expression at zero temperature. This result has been obtained at the lowest
order of the nilpotency expansion. At this order the well known "sign problem"
does not arise and the quark contribution to the action can be used as a
statistical weight in the Monte Carlo simulations.
