  The Pauli exclusion principle induces a repulsion between composite systems
of identical fermions such as colliding atomic nuclei. Our goal is to study how
heavy-ion fusion is impacted by this "Pauli repulsion". We propose a new
microscopic approach, the density-constrained frozen Hartree-Fock method, to
compute the bare potential including the Pauli exclusion principle exactly.
Pauli repulsion is shown to be important inside the barrier radius and
increases with the charge product of the nuclei. Its main effect is to reduce
tunnelling probability. Pauli repulsion is part of the solution to the
long-standing deep sub-barrier fusion hindrance problem.
