Comparison of ab Initio and DFT Methods for Studying Chain Propagation and Chain Termination Processes with Group 4 Polymerization Catalysts. 1. The ansa-Bis(cyclopentadienyl)zirconium Catalyst

In this paper we present a systematic comparison of the performance of different computational approaches to study the propagation and termination reactions of olefins with a prototype homogeneous group 4 ansa -metallocene catalyst. Chain propagation, β-H transfer to the monomer, and β-H elimination to the metal have been investigated for the H 2 Si(Cp) 2 ZrR + (R = ethyl, n -butyl) + C 2 H 4 system using ab initio and density functional theory (DFT) techniques. For all the species investigated, all the computational approaches we considered result in substantially similar geometries. A comparison of the DFT and Møller−Plesset theory (MP2) propagation and termination barriers with extrapolated coupled-cluster calculations with inclusion of single, double, and perturbatively connected triple excitation (CCSD(T)) values indicates that all the pure functionals considered underestimate the difference between termination and propagation by roughly 3−4 kcal/mol. In contrast, hybrid functionals are within 1 kcal/mol from extrapolated CCSD(T) values. For a comparison with experimental results inclusion of zero-point energy contributions and the use of an alkyl group longer than ethyl to simulate the growing chain in both termination reactions are mandatory.