A combined Car-Parrinello QM/MM implementation for ab initio molecular dynamics simulations of extended systems: Application to transition metal catalysis

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 ΔF⊄ = 14.8 kcal/mol by thermodynamic integration. This is in excellent agreement with the experimental termination barrier of ΔG‡ ≈ 16 kcal/mol. Without the bulky ligands, the analogous pure QM simulation provided a free energy barrier of ΔF⊄ = 9.8 kcal/mol.