We test a new implementation of the combined quantum mechanics and molecular mechanics (QM/MM) methodology applied within the Car-Parrinello framework to perform ab initio molecular dynamics simulations of extended systems. The novel method is applied to determine the free energy barrier of the chain termination process in a nickel diimine based ethylene polymerization catalyst of the type (ArN=C(R)-C(R)=NAr)Ni-(II)-R’(+), where R=Me and Ar=2,6-C6H3(i-Pr)(2). In this combined QM/MM ab initio molecular dynamics simulation, the Ni diimine core was treated at the Becke88-Perdew86 DFT level while the large substituted aryl rings were treated by the AMBER molecular mechanics force field. A 39 000 time step slow growth simulation of the termination process at 300 K has been performed providing a free energy barrier of Delta F-double dagger=14.8 kcal/mol by thermodynamic integration. This is in excellent agreement with the experimental termination barrier of Delta G(double dagger)approximate to 16 kcal/mol. Without the bulky ligands, the analogous pure QM simulation provided a free energy barrier of Delta F-double dagger=9.8 kcal/mol.