  A scheme is presented that enables a description of a paramagnetic Mott
insulator in terms of free fermions. The main idea is to view the physical
fermions as a part of a multi-band system and to allow for a correlation
between the physical fermions and the auxiliary ones. Technically this is
implemented through a non-linear canonical transformation, which is
conveniently formulated in terms of Majorana fermions. The transformed
Hamiltonian is in the next stage approximated with a free fermion theory. The
approximation step is variational and provides an upper bound on the ground
state energy at zero or the Free energy at finite temperature.
